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Title: Beginners' botany
Author: Bailey, L. H. (Liberty Hyde)
Language: English
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BEGINNERS’ BOTANY
[Illustration: (Sigil)]
THE MACMILLAN COMPANY
NEW YORK · BOSTON · CHICAGO
SAN FRANCISCO
MACMILLAN & CO., LIMITED
LONDON · BOMBAY · CALCUTTA
MELBOURNE
THE MACMILLAN CO. OF CANADA, LTD.
TORONTO
[Illustration: BOUQUET OF BEARDED WHEAT]
BEGINNERS’ BOTANY
BY
L. H. BAILEY
[Illustration: (Colophon)]
_AUTHORIZED BY THE MINISTER OF EDUCATION
FOR ONTARIO_
TORONTO
THE MACMILLAN CO. OF CANADA, LIMITED
1921
COPYRIGHT, 1921
BY THE MACMILLAN CO. OF CANADA LTD.
PREFACE
In all teaching of plants and animals to beginners, the plants
themselves and the animals themselves should be made the theme,
rather than any amount of definitions and of mere study in books.
Books will be very useful in guiding the way, in arranging the
subjects systematically, and in explaining obscure points; but if
the pupil does not know the living and growing plants when he has
completed his course in botany, he has not acquired very much that is
worth the while.
It is well to acquaint the beginner at first with the main features
of the entire plant rather than with details of its parts. He should
at once form a mental picture of what the plant is, and what are some
of its broader adaptations to the life that it leads. In this book,
the pupil starts with the entire branch or the entire plant. It is
sometimes said that the pupil cannot grasp the idea of struggle for
existence until he knows the names and the uses of the different
parts of the plant. This is an error, although well established in
present-day methods of teaching.
Another very important consideration is to adapt the statement of any
fact to the understanding of a beginner. It is easy, for example,
to fall into technicalities when discussing osmosis; but the minute
explanations would mean nothing to the beginner and their use would
tend to confuse the picture which it is necessary to leave in the
pupil’s mind. Even the use of technical forms of expression would
probably not go far enough to satisfy the trained physicist. It
is impossible ever to state the last thing about any proposition.
All knowledge is relative. What is very elementary to one mind may
be very technical and advanced to another. It is neither necessary
nor desirable to safeguard statements to the beginner by such
qualifications as will make them satisfactory to the critical expert
in science. The teacher must understand that while accuracy is always
essential, the _degree of statement_ is equally important when
teaching beginners.
The value of biology study lies in the work with the actual objects.
It is not possible to provide specimens for every part of the work,
nor is it always desirable to do so; for the beginning pupil may
not be able to interest himself in the objects, and he may become
immersed in details before he has arrived at any general view or
reason of the subject. Great care must be exercised that the pupil is
not swamped. Mere book work or memory stuffing is useless, and it may
dwarf or divert the sympathies of active young minds.
The present tendency in secondary education is away from the formal
technical completion of separate subjects and toward the developing
of a workable training in the activities that relate the pupil to his
own life. In the natural science field, the tendency is to attach
less importance to botany and zoology as such, and to lay greater
stress on the processes and adaptations of life as expressed in
plants and animals. Education that is not applicable, that does not
put the pupil into touch with the living knowledge and the affairs
of his time, may be of less educative value than the learning of a
trade in a shop. We are beginning to learn that the ideals and the
abilities should be developed out of the common surroundings and
affairs of life rather than imposed on the pupil as a matter of
abstract unrelated theory.
It is much better for the beginning pupil to acquire a real
conception of a few central principles and points of view respecting
common forms that will enable him to tie his knowledge together and
organize it and apply it, than to familiarize himself with any number
of mere facts about the lower forms of life which, at the best, he
can know only indirectly and remotely. If the pupil wishes to go
farther in later years, he may then take up special groups and phases.
CONTENTS
CHAPTER
I. NO TWO PLANTS OR PARTS ARE ALIKE 1
II. THE STRUGGLE TO LIVE 4
III. SURVIVAL OF THE FIT 7
IV. PLANT SOCIETIES 9
V. THE PLANT BODY 15
VI. SEEDS AND GERMINATION 20
VII. THE ROOT—THE FORMS OF ROOTS 32
VIII. THE ROOT—FUNCTION AND STRUCTURE 38
IX. THE STEM—KINDS AND FORMS—PRUNING 49
X. THE STEM—ITS GENERAL STRUCTURE 59
XI. LEAVES—FORM AND POSITION 73
XII. LEAVES—STRUCTURE AND ANATOMY 86
XIII. LEAVES—FUNCTION OR WORK 92
XIV. DEPENDENT PLANTS 106
XV. WINTER AND DORMANT BUDS 111
XVI. BUD PROPAGATION 121
XVII. HOW PLANTS CLIMB 129
XVIII. THE FLOWER—ITS PARTS AND FORMS 133
XIX. THE FLOWER—FERTILIZATION AND POLLINATION 144
XX. FLOWER-CLUSTERS 155
XXI. FRUITS 163
XXII. DISPERSAL OF SEEDS 172
XXIII. PHENOGAMS AND CRYPTOGAMS 176
XXIV. STUDIES IN CRYPTOGAMS 182
INDEX 205
BEGINNERS’ BOTANY
CHAPTER I
NO TWO PLANTS OR PARTS ARE ALIKE
[Illustration: FIG. 1.—NO TWO BRANCHES ARE ALIKE. (Hemlock.) ]
If one compares _any two plants_ of the same kind ever so closely, it
will be found that they _differ from each other_. The difference is
apparent in size, form, colour, mode of branching, number of leaves,
number of flowers, vigour, season of maturity, and the like; or, in
other words, all plants and animals _vary from an assumed or standard
type_.
If one compares _any two branches or twigs_ on a tree, it will be
found that they differ in size, age, form, vigour, and in other ways
(Fig. 1).
If one compares _any two leaves_, it will be found that they are
unlike in size, shape, colour, veining, hairiness, markings, cut
of the margins, or other small features. In some cases (as in Fig.
2) the differences are so great as to be readily seen in a small
black-and-white drawing.
If the pupil extends his observation to animals, he will still
find the same truth; for probably _no two living objects are exact
duplicates_. If any person finds two objects that he thinks to be
exactly alike, let him set to work to discover the differences,
remembering that _nothing in nature is so small or apparently trivial
as to be overlooked_.
[Illustration: FIG. 2.—NO TWO LEAVES ARE ALIKE.]
=Variation=, or differences between organs and also between
organisms, is one of the most significant facts in nature.
SUGGESTIONS.—The first fact that the pupil should acquire about
plants is that no two are alike. The way to apprehend this great
fact is to see a plant accurately and then to compare it with
another plant of the same species or kind. In order to direct and
concentrate the observation, it is well to set a certain number of
attributes or marks or qualities to be looked for. =1.= Suppose any
two or more plants of corn are compared in the following points,
the pupil endeavouring to determine whether the parts exactly
agree. See that the observation is close and accurate. Allow no
guesswork. Instruct the pupil to measure the parts when size is
involved.
(1) Height of the plant.
(2) Does it branch? How many secondary stems or “suckers” from one
root?
(3) Shade or colour.
(4) How many leaves.
(5) Arrangement of leaves on stem.
(6) Measure length and breadth of six main leaves.
(7) Number and position of ears; colour of silks.
(8) Size of tassel, and number and size of its branches.
(9) Stage of maturity or ripeness of plant.
(10) Has the plant grown symmetrically, or has it been crowded by
other plants or been obliged to struggle for light or room?
(11) Note all unusual or interesting marks or features.
(12) Always make note of comparative vigour of the plants.
NOTE TO TEACHER.—The teacher should always insist on _personal work_
by the pupil. Every pupil should _handle and study the object by
himself_. Books and pictures are merely guides and helps. So far as
possible, study the plant or animal _just where it grows naturally_.
=Notebooks.=—Insist that the pupils make full notes and preserve
these notes in suitable books. Note-taking is a powerful aid in
organizing the mental processes, and in insuring accuracy of
observation and record. The pupil should draw what he sees, even
though he is not expert with the pencil. The drawing should not be
made for looks, but to aid the pupil in his orderly study of the
object; it should be a means of self-expression.
CHAPTER II
THE STRUGGLE TO LIVE
Every plant and animal is _exposed to unfavourable conditions_. It is
obliged to contend with these conditions in order to live.
No two plants or parts of plants are identically exposed to the
conditions in which they live. The large branches in Fig. 1 probably
had more room and a better exposure to light than the smaller ones.
Probably no two of the leaves in Fig. 2 are equally exposed to light,
or enjoy identical advantages in relation to the food that they
receive from the tree.
[Illustration: FIG. 3.—A BATTLE FOR LIFE.]
Examine any tree to determine under what advantages or disadvantages
any of the limbs may live. Examine similarly the different plants in
a garden row (Fig. 3); or the different bushes in a thicket; or the
different trees in a wood.
The plant meets its conditions by _succumbing to them_ (that is, by
dying), or by _adapting itself to them_.
The tree _meets the cold_ by ceasing its active growth, hardening
its tissues, dropping its leaves. Many herbaceous or soft-stemmed
plants meet the cold by dying to the ground and withdrawing all life
into the root parts. Some plants meet the cold by dying outright and
providing abundance of seeds to perpetuate the kind next season.
[Illustration: FIG. 4.—THE REACH FOR LIGHT OF A TREE ON THE EDGE OF A
WOOD.]
Plants _adapt themselves to light_ by growing toward it (Fig. 4); or
by hanging their leaves in such position that they catch the light;
or, in less sunny places, by expanding their leaf surface, or by
greatly lengthening their stems so as to overtop their fellows, as do
trees and vines.
The adaptations of plants will afford a fertile field of study as we
proceed.
=Struggle for existence= and =adaptation to conditions= are among the
most significant facts in nature.
The sum of all the conditions in which a plant or an animal is
placed is called its =environment=, that is, its surroundings. The
environment comprises the conditions of climate, soil, moisture,
exposure to light, relation to food supply, contention with other
plants or animals. _The organism adapts itself to its environment, or
else it weakens or dies._ Every weak branch or plant has undergone
some hardship that it was not wholly able to withstand.
SUGGESTIONS.—The pupil should study any plant, or branch of a
plant, with reference to the position or condition under which it
grows, and compare one plant or branch with another. With animals,
it is common knowledge that every animal is alert to avoid or to
escape danger, or to protect itself. =2.= It is well to begin with
a branch of a tree, as in Fig. 1. Note that no two parts are alike
(Chap. I). Note that some are large and strong and that these
stand farthest toward light and room. Some are very small and
weak, barely able to live under the competition. Some have died.
The pupil can easily determine which of the dead branches perished
first. He should take note of the position or place of the branch
on the tree, and determine whether the greater part of the dead
twigs are toward the centre of the tree top or toward the outside
of it. Determine whether accident has overtaken any of the parts.
=3.= Let the pupil examine the top of any thick old apple tree, to
see whether there is any struggle for existence and whether any
limbs have perished. =4.= If the pupil has access to a forest, let
him determine why there are no branches on the trunks of the old
trees. Examine a tree of the same kind growing in an open field.
=5.= A row of lettuce or other plants sown thick will soon show
the competition between plants. Any fence-row or weedy place will
also show it. Why does the farmer destroy the weeds among the corn
or potatoes? How does the florist reduce competition to its lowest
terms? what is the result?
CHAPTER III
THE SURVIVAL OF THE FIT
The plants that most perfectly meet their conditions are able to
persist. _They perpetuate themselves._ Their offspring are likely
to inherit some of the attributes that enabled them successfully
to meet the battle of life. _The fit_ (those best adapted to their
conditions) _tend to survive_.
Adaptation to conditions depends on the fact of variation; that is,
if plants were perfectly rigid or invariable (all exactly alike) they
could not meet new conditions. Conditions are necessarily new for
every organism. _It is impossible to picture a perfectly inflexible
and stable succession of plants or animals._
=Breeding.=—_Man is able to modify plants and animals._ All our
common domestic animals are very unlike their original ancestors.
So all our common and long-cultivated plants have varied from their
ancestors. Even in some plants that have been in cultivation less
than a century the change is marked: compare the common black-cap
raspberry with its common wild ancestor, or the cultivated blackberry
with the wild form.
[Illustration: FIG. 5.—DESIRABLE AND UNDESIRABLE TYPES OF COTTON
PLANTS. Why?]
By choosing seeds from a plant that pleases him, the breeder may be
able, under given conditions, to produce numbers of plants with
more or less of the desired qualities; from the best of these, he
may again choose; and so on until the race becomes greatly improved
(Figs. 5, 6, 7). This process of continuously choosing the most
suitable plants is known as =selection=. A somewhat similar process
proceeds in wild nature, and it is then known as =natural selection=.
[Illustration: FIG. 6.—FLAX BREEDING.
_A_ is a plant grown for seed production;
_B_ for fibre production. Why?]
[Illustration: FIG. 7.—BREEDING.
_A_, effect from breeding from smallest grains (after four years),
average head; _B_, result from breeding from the plumpest and
heaviest grains (after four years), average head.]
SUGGESTIONS.—=6.= Every pupil should undertake at least one simple
experiment in selection of seed. He may select kernels from
the best plant of corn in the field, and also from the poorest
plant,—having reference not so much to mere incidental size and
vigour of the plants that may be due to accidental conditions in
the field, as to the apparently constitutional strength and size,
number of ears, size of ears, perfectness of ears and kernels,
habit of the plant as to suckering, and the like. The seeds may
be saved and sown the next year. Every crop can no doubt be very
greatly improved by a careful process of selection extending over
a series of years. Crops are increased in yield or efficiency in
three ways: better general care; enriching the land in which they
grow; attention to breeding.
CHAPTER IV
PLANT SOCIETIES
In the long course of time in which plants have been accommodating
themselves to the varying conditions in which they are obliged to
grow, _they have become adapted to every different environment_.
Certain plants, therefore, may live together or near each other,
all enjoying the same general conditions and surroundings. These
aggregations of plants that are adapted to similar general conditions
are known as =plant societies=.
Moisture and temperature are the leading factors in determining
plant societies. The great geographical societies or aggregations
of the plant world may conveniently be associated chiefly with the
moisture supply, as: _wet-region societies_, comprising aquatic and
bog vegetation (Fig. 8); _arid-region societies_, comprising desert
and most sand-region vegetation; _mid-region societies_, comprising
the mixed vegetation in intermediate regions (Fig. 9), this being the
commonest type. Much of the characteristic scenery of any place is
due to its plant societies. Arid-region plants usually have small and
hard leaves, apparently preventing too rapid loss of water. Usually,
also, they are characterized by stiff growth, hairy covering, spines,
or a much-contracted plant body, and often by large underground parts
for the storage of water.
Plant societies may also be distinguished with reference to latitude
and temperature. There are _tropical societies_, _temperate-region
societies_, _boreal_ or _cold-region societies_. With reference
to altitude, societies might be classified as _lowland_ (which
are chiefly wet-region), _intermediate_ (chiefly mid-region),
_subalpine_ or _mid-mountain_ (which are chiefly boreal), _alpine_ or
_high-mountain_.
The above classifications have reference chiefly to great
geographical floras or societies. But there are _societies within
societies_. There are small societies coming within the experience of
every person who has ever seen plants growing in natural conditions.
There are roadside, fence-row, lawn, thicket, pasture, dune,
woods, cliff, barn-yard societies. _Every different place has its
characteristic vegetation._ Note the smaller societies in Figs. 8 and
9. In the former is a water-lily society and a cat-tail society. In
the latter there are grass and bush and woods societies.
[Illustration: FIG. 8.—A WET-REGION SOCIETY.]
=Some Details of Plant Societies.=—Societies may be composed of
_scattered and intermingled plants_, or of dense _clumps_ or _groups
of plants_. Dense clumps or groups are usually made up of one kind
of plant, and they are then called =colonies=. Colonies of most
plants are transient: after a short time other plants gain a foothold
amongst them, and an intermingled society is the outcome. Marked
exceptions to this are grass colonies and forest colonies, in which
one kind of plant may hold its own for years and centuries.
[Illustration: FIG. 9.—A MID-REGION SOCIETY.]
In a large newly cleared area, plants usually _first establish
themselves in dense colonies_. Note the great patches of nettles,
jewel-weeds, smart-weeds, clot-burs, fire-weeds in recently cleared
but neglected swales, also the fire-weeds in recently burned areas,
the rank weeds in the neglected garden, and the ragweeds and
May-weeds along the recently worked highway. The competition amongst
themselves and with their neighbours finally breaks up the colonies,
and _a mixed and intermingled flora is generally the result_.
In many parts of the world the _general tendency of neglected areas
is to run into forest_. All plants rush for the cleared area. Here
and there bushes gain a foothold. Young trees come up; in time these
shade the bushes and gain the mastery. Sometimes the area grows to
poplars or birches, and people wonder why the original forest trees
do not return; but these forest trees may be growing unobserved
here and there in the tangle, and in the slow processes of time the
poplars perish—for they are short-lived—and the original forest may
be replaced. Whether one kind of forest or another returns will
depend partly on the kinds that are most seedful in that vicinity and
which, therefore, have sown themselves most profusely. Much depends,
also, on the kind of undergrowth that first springs up, for some
young trees can endure more or less shade than others.
[Illustration: FIG. 10.—OVERGROWTH AND UNDERGROWTH IN THREE
SERIES,—trees, bushes, grass.]
Some plants _associate_. They grow together. This is possible largely
because they diverge or differ in character. Plants associate in two
ways: _by growing side by side_; _by growing above or beneath_. In
sparsely populated societies, plants may grow alongside each other.
In most cases, however, there is _overgrowth_ and _undergrowth_: one
kind grows beneath another. Plants that have become adapted to shade
are usually undergrowths. In a cat-tail swamp, grasses and other
narrow-leaved plants grow in the bottom, but they are usually unseen
by the casual observer. Note the undergrowth in woods or under trees
(Fig. 10). Observe that in pine and spruce forests there is almost no
undergrowth, partly because there is very little light.
On the same area the societies may _differ at different times of
the year_. There are spring, summer, and fall societies. The knoll
which is cool with grass and strawberries in June may be aglow with
goldenrod in September. If the bank is examined in May, look for
the young plants that are to cover it in July and October; if in
September, find the dead stalks of the flora of May. What succeeds
the skunk cabbage, hepaticas, trilliums, phlox, violets, buttercups
of spring? What precedes the wild sunflowers, ragweed, asters, and
goldenrod of fall?
=The Landscape.=—To a large extent the _colour of the landscape_
is determined by the character of the plant societies. Evergreen
societies remain green, but the shade of green varies from season to
season; it is bright and soft in spring, becomes dull in midsummer
and fall, and assumes a dull yellow-green or a black-green in winter.
Deciduous societies vary remarkably in colour—from the dull browns
and grays of winter to the brown greens and olive-greens of spring,
the staid greens of summer, and the brilliant colours of autumn.
The _autumn colours_ are due to intermingled shades of green,
yellow and red. The coloration varies with the kind of plant, the
special location, and the season. Even in the same species or kind,
individual plants differ in colour; and this individuality usually
distinguishes the plant year by year. That is, an oak which is maroon
red this autumn is likely to exhibit that range of colour every year.
The autumn colour is associated with the natural maturity and death
of the leaf, but it is most brilliant in long and open falls—largely
because the foliage ripens more gradually and persists longer in such
seasons. It is probable that the autumn tints are of no utility to
the plant. _Autumn colours are not caused by frost._ Because of the
long, dry falls and the great variety of plants, the autumnal colour
of the American landscape is phenomenal.
=Ecology.=—The study of the relationships of plants and animals to
each other and to seasons and environments is known as =ecology=
(still written _œcology_ in the dictionaries). It considers the
habits, habitats, and modes of life of living things—the places in
which they grow, how they migrate or are disseminated, means of
collecting food, their times and seasons of flowering, producing
young, and the like.
SUGGESTIONS.—One of the best of all subjects for school instruction
in botany is the study of plant societies. It adds definiteness
and zest to excursions. =7.= Let each excursion be confined to one
or two societies. Visit one day a swamp, another day a forest,
another a pasture or meadow, another a roadside, another a weedy
field, another a cliff or ravine. Visit shores whenever possible.
Each pupil should be assigned a bit of ground—say 10 or 20 ft.
square—for special study. He should make a list showing (1) how
many kinds of plants it contains, (2) the relative abundance of
each. The lists secured in different regions should be compared. It
does not matter greatly if the pupil does not know all the plants.
He may count the kinds without knowing the names. It is a good plan
for the pupil to make a dried specimen of each kind for reference.
The pupil should endeavour to discover why the plants grow as they
do. Note what kinds of plants grow next each other; and which are
undergrowth and which overgrowth; and which are erect and which
wide-spreading. _Challenge every plant society._
CHAPTER V
THE PLANT BODY
=The Parts of a Plant.=—Our familiar plants are made up of several
distinct parts. The most prominent of these parts are _root, stem,
leaf, flower, fruit, and seed. Familiar plants differ wonderfully
in size and shape_,—from fragile mushrooms, delicate waterweeds and
pond-scums, to floating leaves, soft grasses, coarse weeds, tall
bushes, slender climbers, gigantic trees, and hanging moss.
=The Stem Part.=—In most plants there is a _main central part or
shaft_ on which the other or _secondary parts_ are borne. This main
part is the =plant axis=. Above ground, in most plants, the main
plant axis bears the _branches_, _leaves_, and _flowers_; below
ground, it bears the _roots_.
The rigid part of the plant, which persists over winter and which
is left after leaves and flowers are fallen, is the =framework= of
the plant. The framework is composed of both root and stem. When
the plant is dead, the framework remains for a time, but it slowly
decays. The dry winter stems of weeds are the framework, or skeleton
of the plant (Figs. 11 and 12). The framework of trees is the most
conspicuous part of the plant.
=The Root Part.=—The root bears the stem at its apex, but otherwise
it normally _bears only root-branches_. The stem, however, _bears
leaves, flowers, and fruits_. Those living surfaces of the plant
which are most exposed to light are _green or highly coloured_. The
root tends to grow _downward_, but the stem tends to grow _upward
toward light and air_. The plant is anchored or fixed in the soil by
the roots. Plants have been called “earth parasites.”
=The Foliage Part.=—The _leaves precede the flowers_ in point of
time or life of the plant. _The flowers always precede the fruits
and seeds._ Many plants die when the seeds have matured. The whole
mass of leaves of any plant or any branch is known as its _foliage._
In some cases, as in crocuses, the flowers seem to precede the
leaves; but the leaves that made the food for these flowers grew the
preceding year.
=The Plant Generation.=—The course of a plant’s life, with all the
events through which the plant naturally passes, is known as the
plant’s =life-history=. The life-history embraces various stages,
or epochs, as _dormant seed_, _germination_, _growth_, _flowering_,
_fruiting_. Some plants run their course in a few weeks or months,
and some live for centuries.
[Illustration: FIG. 11.—PLANT OF A WILD SUNFLOWER.]
[Illustration: FIG. 12.—FRAMEWORK OF FIG. 11.]
The entire life period of a plant is called a =generation=. It is the
whole period from birth to normal death, without reference to the
various stages or events through which it passes.
A generation begins with _the young seed_, not with germination. _It
ends with death_—that is, when no life is left in any part of the
plant, and only the seed or spore remains to perpetuate the kind. In
a bulbous plant, as a lily or an onion, the generation does not end
until the bulb dies, even though the top is dead.
When the generation is of only one season’s duration, the plant is
said to be =annual=. When it is of two seasons, it is =biennial=.
Biennials usually bloom the second year. When of three or more
seasons, the plant is =perennial=. Examples of annuals are pigweed,
bean, pea, garden sunflower; of biennials, evening primrose, mullein,
teasel; of perennials, dock, most meadow grasses, cat-tail, and all
shrubs and trees.
=Duration of the Plant Body.=—Plant structures which are more or
less soft and which die at the close of the season are said to be
=herbaceous=, in contradistinction to being =ligneous= or =woody=.
A plant which is herbaceous to the ground is called an =herb=; but
an herb may have a woody or perennial root, in which case it is
called an =herbaceous perennial=. Annual plants are classed as herbs.
Examples of herbaceous perennials are buttercups, bleeding heart,
violet, water-lily, Bermuda grass, horse-radish, dock, dandelion,
goldenrod, asparagus, rhubarb, many wild sunflowers (Figs. 11, 12).
Many herbaceous perennials have _short generations_. They become weak
with one or two seasons of flowering and gradually die out. Thus,
red clover usually begins to fail after the second year. Gardeners
know that the best bloom of hollyhock, larkspur, pink, and many other
plants, is secured when the plants are only two or three years old.
Herbaceous perennials which die away each season to bulbs or tubers,
are sometimes called =pseud-annuals= (that is, _false annuals_). Of
such are lily, crocus, onion, potato, and bull nettle.
True annuals reach old age the first year. Plants which are normally
perennial _may become annual in a shorter-season climate by being
killed by frost_, rather than by dying naturally at the end of a
season of growth. They are climatic annuals. Such plants are called
=plur-annuals= in the short-season region. Many tropical perennials
are plur-annuals when grown in the north, but they are treated as
true annuals because they ripen sufficient of their crop the same
season in which the seeds are sown to make them worth cultivating, as
tomato, red pepper, castor bean, cotton. Name several vegetables that
are planted in gardens with the expectation that they will bear till
frost comes.
[Illustration: FIG. 13.—A SHRUB OR BUSH. Dogwood osier.]
Woody or ligneous plants usually live longer than herbs. Those that
remain low and produce several or many similar shoots from the base
are called =shrubs=, as lilac, rose, elder, osier (Fig. 13). Low and
thick shrubs are =bushes=. Plants that produce one main trunk and
a more or less elevated head are =trees= (Fig. 14). All shrubs and
trees are perennial.
Every plant makes an effort _to propagate, or to perpetuate its
kind_; and, as far as we can see, this is the end for which the plant
itself lives. _The seed or spore is the final product of the plant._
[Illustration: FIG. 14.—A TREE. The weeping birch.]
SUGGESTIONS.—=8.= The teacher may assign each pupil to one plant in
the school yard, or field, or in a pot, and ask him to bring out
the points in the lesson. =9.= The teacher may put on the board
the names of many common plants and ask the pupils to classify
into annuals, pseud-annuals, plur-annuals (or climatic annuals),
biennials, perennials, herbaceous perennials, ligneous perennials,
herbs, bushes, trees. Every plant grown on the farm should be so
classified: wheat, oats, corn, buckwheat, timothy, strawberry,
raspberry, currant, tobacco, alfalfa, flax, crimson clover, hops,
cowpea, field bean, sweet potato, peanut, radish, sugar-cane,
barley, cabbage, and others. Name all the kinds of trees you know.
CHAPTER VI
SEEDS AND GERMINATION
The seed contains a _miniature plant_, or =embryo=. The embryo
usually has three parts that have received names: the stemlet, or
=caulicle=; the seed-leaf, or =cotyledon= (usually 1 or 2); the
bud, or =plumule=, lying between or above the cotyledons. These
parts are well seen in the common bean (Fig. 15), particularly
when the seed has been soaked for a few hours. One of the large
cotyledons—comprising half of the bean—is shown at _R_. The caulicle
is at _O_. The plumule is shown at _A_. The cotyledons are attached
to the caulicle at _F_: _this point may be taken as the first node or
joint_.
[Illustration: FIG. 15.— PARTS OF THE BEAN.
_R_, cotyledon; _O_, caulicle; _A_, plumule; _F_, first node.]
=The Number of Seed-leaves.=—All plants having _two seed-leaves_
belong to the group called =dicotyledons=. Such seeds in many
cases split readily in halves, _e.g._ a bean. Some plants have
only _one_ seed-leaf in a seed. They form a group of plants called
=monocotyledons=. Indian corn is an example of a plant with only one
seed-leaf: a grain of corn does not split into halves as a bean does.
Seeds of the pine family contain more than two cotyledons, but for
our purposes they may be associated with the dicotyledons, although
really forming a different group.
These two groups—the dicotyledons and the monocotyledons—represent
two great natural divisions of the vegetable kingdom. The
dicotyledons contain the woody bark-bearing trees and bushes
(except conifers), and most of the herbs of temperate climates
except the grasses, sedges, rushes, lily tribes, and orchids. The
flower-parts are usually in fives or multiples of five, the leaves
mostly netted-veined, the bark or rind distinct, and the stem often
bearing a pith at the centre. The monocotyledons usually have the
flower-parts in threes or multiples of three, the leaves long and
parallel-veined, the bark not separable, and the stem without a
central pith.
[Illustration: FIG. 16.—EXTERNAL PARTS OF BEAN. ]
Every seed is _provided with food_ to support the germinating
plant. Commonly this food is starch. The food may be stored _in the
cotyledons_, as in bean, pea, squash; or _outside the cotyledons_, as
in castor bean, pine, Indian corn. When the food is outside or around
the embryo, it is usually called =endosperm=.
=Seed-coats; Markings on Seed.=—The embryo and endosperm are inclosed
within a covering made of two or more layers and known as the
=seed-coats=. Over the point of the caulicle is a minute hole or a
thin place in the coats known as the =micropyle=. This is the point
at which the pollen-tube entered the forming ovule and through which
the caulicle breaks in germination. The micropyle is shown at _M_ in
Fig. 16. The scar where the seed broke from its funiculus (or stalk
that attached it to its pod) is named the =hilum=. It occupies a
third of the length of the bean in Fig. 16. The hilum and micropyle
are always present in seeds, but they are not always close together.
In many cases it is difficult to identify the micropyle in the
dormant seed, but its location is at once shown by the protruding
caulicle as germination begins. Opposite the micropyle in the
bean (at the other end of the hilum) is an elevation known as the
=raphe=. This is formed by a union of the funiculus, or seed-stalk,
with the seed-coats, and through it food was transferred for the
development of the seed, but it is now functionless.
Seeds differ wonderfully in size, shape, colour, and other
characteristics. They also vary in longevity. These characteristics
are _peculiar to the species or kind_. Some seeds maintain life
only a few weeks or even days, whereas others will “keep” for ten
or twenty years. In special cases, seeds have retained vitality
longer than this limit, but the stories that live seeds, several
thousand years old, have been taken from the wrappings of mummies are
unfounded.
=Germination.=—The embryo is not dead; it is only dormant. _When
supplied with moisture, warmth, and oxygen (air), it awakes and
grows: this growth is_ =germination=. The embryo lives for a time on
the stored food, but gradually the plantlet secures a foothold in the
soil and gathers food for itself. _When the plantlet is finally able
to shift for itself, germination is complete._
=Early Stages of Seedling.=—The germinating seed first _absorbs
water, and swells_. The starchy matters gradually become soluble.
The seed-coats are ruptured, the caulicle and plumule emerge. During
this process the seed _respires freely, throwing off carbon dioxide_
(CO_{2}).
[Illustration: FIG. 17.—PEA. Grotesque forms assumed when the roots
cannot gain entrance to the soil. ]
The caulicle usually elongates, and from its lower end roots are
emitted. The elongating caulicle is known as the =hypocotyl= (“below
the cotyledons”). That is, the hypocotyl is that part of the stem
of the plantlet lying between the roots and the cotyledon. _The
general direction of the young hypocotyl, or emerging caulicle,
is downwards._ As soon as roots form, it becomes fixed and its
subsequent growth tends to raie the cotyledons above the ground, as
in the bean. When cotyledons rise into the air, germination is said
to be =epigeal= (“above the earth”). Bean and pumpkin are examples.
When the hypocotyl does not elongate greatly and the cotyledons
remain under ground, the germination is =hypogeal= (“beneath the
earth”). Pea and scarlet runner bean are examples (Fig. 48). When
the germinating seed lies on a hard surface, as on closely compacted
soil, the hypocotyl and rootlets may not be able to secure a foothold
and they assume grotesque forms (Fig. 17). Try this with peas and
beans.
The first internode (“between nodes”) above the cotyledons is the
=epicotyl=. It elevates the plumule into the air, and _the plumule
leaves expand into the first true leaves of the plant_. These first
true leaves, however, may be very unlike the later leaves in shape.
[Illustration: FIG. 18.—COTYLEDONS OF GERMINATING BEAN SPREAD APART
TO SHOW ELONGATING CAUDICLE AND PLUMULE. ]
[Illustration: FIG. 19.—GERMINATION OF BEAN.]
=Germination of Bean.=—The common bean, as we have seen (Fig. 15),
has cotyledons that occupy all the space inside the seed-coats.
When the hypocotyl, or elongated caulicle, emerges, the plumule
leaves have begun to enlarge, and to unfold (Fig. 18). The hypocotyl
elongates rapidly. One end of it is held by the roots. The other
is held by the seed-coats in the soil. It therefore takes the form
of a loop, and the central part of the loop “comes up” first (_a_,
Fig. 19). Presently the cotyledons come out of the seed-coats, and
the plant straightens and the cotyledons expand. These cotyledons,
or “halves of the bean,” persist for some time (_b_, Fig. 19). They
often become green and probably perform some function of foliage.
Because of its large size, the Lima bean shows all these parts well.
[Illustration: FIG. 20.—SPROUTING OF CASTOR BEAN. ]
=Germination of Castor Bean.=—In the castor bean the hilum and
micropyle are at the smaller end (Fig. 20). The bean “comes up”
with a loop, which indicates that the hypocotyl greatly elongates.
On examining germinating seed, however, it will be found that the
cotyledons are contained inside a fleshy body, or sac (_a_, Fig. 21).
This sac is the endosperm. Against its inner surface the thin, veiny
cotyledons are very closely pressed, absorbing its substance (Fig.
22). The cotyledons increase in size as they reach the air (Fig. 23),
and become functional leaves.
[Illustration: FIG. 21.—GERMINATION OF CASTOR BEAN.
Endosperm at _a_.]
[Illustration: FIG. 22.—CASTOR BEAN.
Endosperm at _a_, _a_; cotyledons at _b_.]
[Illustration: FIG. 23.—GERMINATION COMPLETE IN CASTOR BEAN. ]
=Germination of Monocotyledons.=—Thus far we have studied
dicotyledonous seeds; we may now consider the monocotyledonous
group. Soak kernels of corn. Note that the micropyle and hilum are
at the smaller end (Fig. 24). Make a longitudinal section through
the narrow diameter; Fig. 25 shows it. The single cotyledon is
at _a_, the caulicle at _b_, the plumule at _p_. The cotyledon
remains in the seed. The food is stored both in the cotyledon and as
endosperm, chiefly the latter. The emerging shoot is the plumule,
with a sheathing leaf (_p_, Fig. 26). The root is emitted from the
tip of the caulicle, _c_. The caulicle is held in a sheath (formed
mostly from the seed-coats), and some of the roots escape through
the upper end of this sheath (_m_, Fig. 26). The epicotyl elongates,
particularly if the seed is planted deep or if it is kept for a time
confined. In Fig. 27 the epicotyl has elongated from _n_ to _p_. The
true plumule-leaf is at _o_, but other leaves grow from its sheath.
In Fig. 28 the roots are seen emerging from the two ends of the
caulicle-sheath, _c_, _m_; the epicotyl has grown to _p_; the first
plumule-leaf is at _o_.
[Illustration: FIG. 24.—SPROUTING INDIAN CORN.
Hilum at _h_; micropyle at _d_.]
[Illustration: FIG. 25.—KERNEL OF INDIAN CORN.
Caulicle at _b_; cotyledon at _a_; plumule at _p_.]
[Illustration: FIG. 26.—INDIAN CORN.
Caulicle at _c_; roots emerging at _m_; plumule at _p_.]
[Illustration: FIG. 27.—INDIAN CORN.
_o_, plumule; _n_ to _p_, epicotyl.]
In studying corn or other fruits or seeds, the pupil should note
how the seeds are arranged, as on the cob. Count the rows on a corn
cob. Odd or even in number? Always the same number? The silk is the
style: find where it was attached to the kernel. Did the ear have any
coverings? Explain. Describe colours and markings of kernels of corn;
and of peas, beans, castor bean.
[Illustration: FIG. 28.—GERMINATION IS COMPLETE.
_p_, top of epicotyl; _o_, plumule-leaf; _m_, roots; _c_, lower
roots.]
=Gymnosperms.=—The seeds in the pine cone, not being inclosed in a
seed vessel, readily fall out when the cone dries and the scales
separate. Hence it is difficult to find cones with seeds in them
after autumn has passed (Fig. 29). The cedar is also a gymnosperm.
Remove a scale from a pine cone and draw it and the seeds as they lie
in place on the upper side of the scale. Examine the seed, preferably
with a magnifying glass. Is there a hilum? The micropyle is at the
bottom or little end of the seed. Toss a seed upward into the air.
Why does it fall so slowly? Can you explain the peculiar whirling
motion by the shape of the wing? Repeat the experiment in the wind.
Remove the wing from a seed and toss it and an uninjured seed into
the air together. What do you infer from these experiments?
[Illustration: FIG. 29.—CONES OF HEMLOCK (ABOVE), WHITE PINE, PITCH
PINE. ]
[Illustration: FIG. 30.—MUSKMELON SEEDLINGS, with the unlike
seed-leaves and true leaves.]
SUGGESTIONS.—Few subjects connected with the study of plant-life
are so useful in schoolroom demonstrations as germination. The
pupil should prepare the soil, plant the seeds, water them, and
care for the plants. =10.= Plant seeds in pots or shallow boxes.
The box should not be very wide or long, and not over four inches
deep. Holes may be bored in the bottom so it will not hold water.
Plant a number of squash, bean, corn, pine, or other seeds about
an inch deep in damp sand or pine sawdust in this box. The depth
of planting should be two to four times the diameter of the seeds.
Keep the sand or sawdust moist but not wet. If the class is large,
use several boxes, that the supply of specimens may be ample. Cigar
boxes and chalk boxes are excellent for individual pupils. It is
well to begin the planting of seeds at least ten days in advance
of the lesson, and to make four or five different plantings at
intervals. A day or two before the study is taken up, put seeds to
soak in moss or cloth. The pupil then has a series from swollen
seeds to complete germination, and all the steps can be made out.
Dry seeds should be had for comparison. If there is no special
room for laboratory, nor duplicate apparatus for every pupil, each
experiment may be assigned to a committee of two pupils to watch
in the schoolroom. =11.= Good seeds for study are those detailed
in the lesson, and buckwheat, pumpkin, cotton, morning glory,
radish, four o’clock, oats, wheat. It is best to use familiar
seeds of farm and garden. Make drawings and notes of all the
events in the germination. Note the effects of unusual conditions,
as planting too deep and too shallow and different sides up. For
hypogeal germination, use the garden pea, scarlet runner, or Dutch
case-knife bean, acorn, horse-chestnut. Squash seeds are excellent
for germination studies, because the cotyledons become green and
leafy and germination is rapid. Onion is excellent, except that
it germinates too slowly. In order to study the root development
of germinating plantlets, it is well to provide a deeper box with
a glass side against which the seeds are planted. =12.= Observe
the germination of any common seed about the house premises. When
elms, oaks, pines, or maples are abundant, the germination of their
seeds may be studied in lawns and along fences. =13.= When studying
germination the pupil should note the differences in shape and
size between cotyledons and plumule leaves, and between plumule
leaves and the normal leaves (Fig. 30). Make drawings. =14=. Make
the tests described in the introductory experiments with bean,
corn, the castor bean, and other seed for starch and proteids.
Test flour, oatmeal, rice, sunflower, four o’clock, various nuts,
and any other seeds obtainable. Record your results by arranging
the seeds in three classes, 1. Much starch (colour blackish or
purple), 2. Little starch (pale blue or greenish), 3. No starch
(brown or yellow). =15.= _Rate of growth of seedlings as affected
by differences in temperature._ Pack soft wet paper to the depth
of an inch in the bottom of four glass bottles or tumblers. Put
ten soaked peas or beans into each. Cover each securely and set
them in places having different temperatures that vary little. (A
furnace room, a room with a stove, a room without stove but reached
by sunshine, an unheated room not reached by the sun). Take the
temperatures occasionally with the thermometer to find difference
in temperature. The tumblers in warm places should be covered very
tightly to prevent the germination from being retarded by drying
out. Record the number of seeds which sprout in each tumbler within
1 day, 2 days, 3 days, 4 days, etc. =16.= _Is air necessary for
the germination and growth of seedlings?_ Place damp blotting
paper in the bottom of a bottle and fill it three-fourths full of
soaked seeds, and close it tightly with a rubber stopper or oiled
cork. Prepare a “check experiment” by having another bottle with
all conditions the same except that it is covered loosely that
air may have access to it, and set the bottles side by side (why
keep the bottles together?). Record results as in the preceding
experiment. =17.= _What is the nature of the gas given off by
germinating seeds?_ Fill a tin box or large-necked bottle with dry
beans or peas, then add water; note how much they swell. Secure two
fruit jars. Fill one of them a third full of beans and keep them
moist. Allow the other to remain empty. In a day or two insert a
lighted splinter or taper into each. In the empty jar the taper
burns: it contains oxygen. In the seed jar the taper goes out: the
air has been replaced by carbon dioxide. The air in the bottle
may be tested for carbon dioxide by removing some of it with a
rubber bulb attached to a glass tube (or a fountain-pen filler)
and bubbling it through lime water. =18.= _Temperature._ Usually
there is a perceptible rise in temperature in a mass of germinating
seeds. This rise may be tested with a thermometer. =19.= _Interior
of seeds._ Soak seeds for twenty-four hours and remove the coat.
Distinguish the embryo from the endosperm. Test with iodine. =20.=
_Of what utility is the food in seeds?_ Soak some grains of corn
overnight and remove the endosperm, being careful not to injure
the fleshy cotyledon. Plant the incomplete and also some complete
grains in moist sawdust and measure their growth at intervals.
(Boiling the sawdust will destroy moulds and bacteria which might
interfere with the experiment.) Peas or beans may be sprouted on
damp blotting paper; the cotyledons of one may be removed, and this
with a normal seed equally advanced in germination may be placed
on a perforated cork floating in water in a jar so that the roots
extend into the water. Their growth may be observed for several
weeks. =21.= _Effect of darkness on seeds and seedlings._ A box may
be placed mouth downward over a smaller box in which seedlings are
growing. The empty box should rest on half-inch blocks to allow
air to reach the seedlings. Note any effects on the seedlings of
this cutting off of the light. Another box of seedlings not so
covered may be used as a check. Lay a plank on green grass and
after a week note the change that takes place beneath it. =22.=
_Seedling of pine._ Plant pine seeds. Notice how they emerge. Do
the cotyledons stay in the ground? How many cotyledons have they?
When do the cotyledons get free from the seed-coat? What is the
last part of the cotyledon to become free? Where is the growing
point or plumule? How many leaves appear at once? Does the new
pine cone grow on old wood or on wood formed the same spring with
the cone? Can you always find partly grown cones on pine trees
in winter? Are pine cones when mature on two-year-old wood? How
long do cones stay on a tree after the seeds have fallen out? What
is the advantage of the seeds falling before the cones? =23.=
_Home experiments._ If desired, nearly all of the fore-going
experiments may be tried at home. The pupil can thus make the
drawings for the notebook at home. A daily record of measurements
of the change in size of the various parts of the seedling should
also be made. =24.= _Seed-testing._—It is important that one know
before planting whether seeds are good, or able to grow. A simple
seed-tester may be made of two plates, one inverted over the other
(Fig. 31). The lower plate is nearly filled with clean sand, which
is covered with cheese cloth or blotting paper on which the seeds
are placed. Canton flannel is sometimes used in place of sand and
blotting paper. The seeds are then covered with another blotter or
piece of cloth, and water is applied until the sand and papers are
saturated. Cover with the second plate. Set the plates where they
will have about the temperature that the given seeds would require
out of doors, or perhaps a slightly higher temperature. Place 100
or more grains of clover, corn, wheat, oats, rye, rice, buckwheat,
or other seeds in the tester, and keep record of the number that
sprout. The result will give a percentage measure of the ability
of the seeds to grow. Note whether all the seeds sprout with equal
vigour and rapidity. Most seeds will sprout in a week or less.
Usually such a tester must have fresh sand and paper after each
test, for mould fungi are likely to breed in it. If canton flannel
is used, it may be boiled. If possible, the seeds should not touch
one another.
[Illustration: FIG. 31.—A HOME-MADE SEED-TESTER.]
NOTE TO TEACHER.—With the study of germination, the pupil will need
to begin dissecting.
=For dissecting=, one needs a lens for the examination of the
smaller parts of plants and animals. It is best to have the lens
mounted on a frame, so that the pupil has both hands free for
pulling the part in pieces. An ordinary pocket lens may be mounted
on a wire in a block as in Fig. A. A cork is slipped on the top of
the wire to avoid injury to the face. The pupil should be provided
with two dissecting needles (Fig. B), made by securing an ordinary
needle in a pencil-like stick. Another convenient arrangement is
shown in Fig. C. A small tin dish is used for the base. Into this
a stiff wire standard is soldered. The dish is filled with solder
to make it heavy and firm. Into a cork slipped on the standard, a
cross wire is inserted, holding on the end a jeweller’s glass. The
lens can be moved up and down and sidewise. This outfit can be made
for about seventy-five cents. Fig. D shows a convenient hand-rest
or dissecting-stand to be used under this lens. It may be 16 in.
long, 4 in. high, and 4 or 5 in. broad.
Various kinds of dissecting microscopes are on the market, and
these are to be recommended when they can be afforded.
[Illustration: _A._—IMPROVISED STAND FOR LENS.]
[Illustration: _B._—DISSECTING NEEDLE ½ natural size.]
[Illustration: _C._—DISSECTING GLASS.]
[Illustration: _D._—DISSECTING STAND.]
Instructions for the use of the compound microscope, with which
some schools may be equipped, cannot be given in a brief space;
the technique requires careful training. Such microscopes are not
needed unless the pupil studies cells and tissues.
CHAPTER VII
THE ROOT—THE FORMS OF ROOTS
=The Root System.=—The offices of the root are _to hold the plant in
place_, and _to gather food_. Not all the food materials, however,
are gathered by the roots.
[Illustration: FIG. 32.—TAP-ROOT SYSTEM OF ALFALFA.]
[Illustration: FIG. 33.—TAP-ROOT OF THE DANDELION.]
The entire mass of roots of any plant is called its =root system=.
The root system may be annual, biennial or perennial, herbaceous or
woody, deep or shallow, large or small.
=Kinds of Roots.=—A strong leading central root, which runs directly
downwards, is a =tap-root=. The tap-root forms an axis from
which the side roots may branch. The side or spreading roots are
usually smaller. Plants that have such a root system are said to be
_tap-rooted_. Examples are red clover, alfalfa, beet, turnip, radish,
burdock, dandelion, hickory (Figs. 32, 33).
A =fibrous root system= is one that is composed of many nearly equal
slender branches. The greater number of plants have fibrous roots.
Examples are many common grasses, wheat, oats, corn. The buttercup in
Fig. 34 has a fibrous root system. Many trees have a strong tap-root
when very young, but after a while it ceases to extend strongly
and the side roots develop until finally the tap-root character
disappears.
[Illustration: FIG. 34.—A BUTTERCUP PLANT, with fibrous roots.]
=Shape and Extent of the Root System.=—The depth to which roots
extend depends on the _kind of plant_, and the _nature of the soil_.
Of most plants the roots extend far _in all directions_ and lie
comparatively _near the surface_. The roots usually radiate from a
common point just beneath the surface of the ground.
_The roots grow here and there in search of food_, often extending
much farther in all directions than the spread of the top of the
plant. Roots tend to spread farther in poor soil than in rich soil,
for the same size of plant. _The root has no such definite form as
the stem has._ Roots are usually very crooked, because they are
constantly turned aside by obstacles. Examine roots in stony soil.
_The extent of root surface is usually very large_, for the feeding
roots are fine and very numerous. An ordinary plant of Indian corn
may have a total length of root (measured as if the roots were placed
end to end) of several hundred feet.
The fine feeding roots are _most abundant in the richest part of the
soil_. They are attracted by the food materials. Roots often will
completely surround a bone or other morsel. When roots of trees are
exposed, observe that most of them are horizontal and lie near the
top of the ground. Some roots, as of willows, extend far _in search
of water_. They often run into wells and drains, and into the margins
of creeks and ponds. Grow plants in a long narrow box, in one end of
which the soil is kept very dry and in the other moist: observe where
the roots grow.
[Illustration: FIG. 35.—THE BRACING BASE OF A FIELD PINE.]
=Buttresses.=—With the increase in diameter, the upper roots often
protrude above the ground and become _bracing buttresses_. These
buttresses are usually largest in trees which always have been
exposed to strong winds (Fig. 35). Because of growth and thickening,
the roots elevate part of their diameter, and the washing away of the
soil makes them to appear as if having risen out of the ground.
=Aërial Roots.=—Although roots usually grow underground, _there
are some that naturally grow above ground_. These usually occur on
climbing plants, the roots becoming _supports_ or fulfilling the
office of tendrils. These aërial roots _usually turn away from the
light_, and therefore enter the crevices and dark places of the wall
or tree over which the plant climbs. The trumpet creeper (Fig. 36),
true or English ivy, and poison ivy climb by means of roots.
[Illustration: FIG. 36.—AËRIAL ROOTS OF TRUMPET CREEPER OR TECOMA. ]
[Illustration: FIG. 37.—AËRIAL ROOTS OF AN ORCHID.]
In some plants all the roots are aërial; that is, _the plant grows
above ground_, and the roots gather food from the air. Such plants
usually grow on trees. They are known as _epiphytes_ or _air-plants_.
The most familiar examples are some of the tropical orchids which are
grown in glass-houses (Fig. 37). Rootlike organs of dodder and other
parasites are discussed in a future chapter.
Some plants bear aërial roots, that may _propagate the plant_ or may
_act as braces_. They are often called =prop-roots=. The roots of
Indian corn are familiar (Fig. 38). Many ficus trees, as the banyan
of India, send out roots from their branches; when these roots reach
the ground they take hold and become great trunks, thus spreading
the top of the parent tree over large areas. The mangrove tree of
the tropics grows along seashores and sends down roots from the
overhanging branches (and from the fruits) into the shallow water,
and thereby gradually marches into the sea. The tangled mass behind
catches the drift, and soil is formed.
[Illustration: FIG. 38.—INDIAN CORN, showing the brace roots at _oo_.]
=Adventitious Roots.=—Sometimes roots grow from the stem or other
unusual places as the result of some accident to the plant, being
located without known method or law. They are called =adventitious=
(chance) =roots=. Cuttings of the stems of roses, figs, geraniums,
and other plants, when planted, send out adventitious roots and
form new plants. The ordinary roots, or soil roots, are of course
not classed as adventitious roots. The adventitious roots arise on
occasion, and not as a normal or regular course in the growth of the
plant.
=No two roots are alike=; that is, they vary among themselves as
stems and leaves do. _Each kind of plant has its own form or habit
of root_ (Fig. 39). Carefully wash away the soil from the roots of
any two related plants, as oats and wheat, and note the differences
in size, depth, direction, mode of branching, number of fibrils,
colour, and other features. The character of the root system often
governs the treatment that the farmer should give the soil in which
the plant or crop grows.
[Illustration: FIG. 39.—ROOTS OF BARLEY AT _A_ AND CORN AT _B_.
Carefully trace the differences.]
Roots differ not only in their form and habit, but also in colour
of tissue, character of bark or rind, and other features. It is
excellent practice to _try to identify different plants by means of
their roots_. Let each pupil bring to school two plants with the
roots very carefully dug up, as cotton, corn, potato, bean, wheat,
rye, timothy, pumpkin, clover, sweet pea, raspberry, strawberry, or
other common plants.
=Root Systems of Weeds.=—Some weeds are pestiferous because they seed
abundantly, and others because their underground parts run deep or
far and are persistent. Make out the root systems in the six worst
weeds in your locality.
CHAPTER VIII
THE ROOT.—FUNCTION AND STRUCTURE
=The function of roots is twofold=,—to provide _support or anchorage_
for the plant, and to _collect and convey food_ materials. The first
function is considered in Chapter VII; we may now give attention in
more detail to the second.
[Illustration: FIG. 40.—WHEAT GROWING UNDER DIFFERENT SOIL
TREATMENTS. Soil deficient in nitrogen; commercial nitrogen applied
to pot 3 (on right). ]
The feeding surface of the roots is _near their ends_. As the roots
become old and hard, they serve only as _channels through which food
passes_ and as _hold-fasts or supports_ for the plant. The root hold
of a plant is very strong. Slowly pull upwards on some plant, and
note how firmly it is anchored in the soil.
=Roots have power to choose their food=; that is, they do not absorb
all substances with which they come in contact. They do not take
up great quantities of useless or harmful materials, even though
these materials may be abundant in the soil; but they may take up a
greater quantity of some of the plant-foods than the plant can use to
advantage. _Plants respond very quickly to liberal feeding_,—that is,
to the application of plant-food to the soil (Fig 40). The poorer the
soil, the more marked are the results, as a rule, of the application
of fertilizers. Certain substances, as common salt, will kill the
roots.
[Illustration: FIG. 41.—NODULES ON ROOTS OF RED CLOVER.]
=Roots absorb Substances only in Solution.=—Substances cannot be
taken in solid particles. These materials are in solution in the
soil water, and the roots themselves also have the power to dissolve
the soil materials to some extent by means of substances that they
excrete. The materials that come into the plant through the roots are
_water and mostly the mineral substances_, as compounds of potassium,
iron, phosphorus, calcium, magnesium, sulphur, and chlorine. These
mineral substances compose the ash when the plant is burned. The
carbon is derived from the air through the green parts. Oxygen is
derived from the air and the soil water.
[Illustration: FIG. 42.—NODULES ON VETCH.]
=Nitrogen enters through the Roots.=—All plants must have nitrogen;
yet, although about four-fifths of the air is nitrogen, plants are
not able, so far as we know, to take it in through their leaves.
It enters through the roots in combination with other elements,
chiefly in the form of nitrates (certain combinations with oxygen
and a mineral base). The great family of leguminous plants, however
(as peas, beans, cowpea, clover, alfalfa, vetch), _use the nitrogen
contained in the air in the soil_. They are able to utilize it
through the _agency of nodules_ on their roots (Figs. 41, 42).
These nodules contain bacteria, which appropriate the free or
uncombined nitrogen and pass it on to the plant. The nitrogen
becomes incorporated in the plant tissue, so that these crops are
high in their nitrogen content. Inasmuch as nitrogen in any form is
expensive to purchase in fertilizers, the use of leguminous crops to
plough under is a very important agricultural practice in preparing
the land for other crops. In order that leguminous crops may acquire
atmospheric nitrogen more freely and thereby thrive better, _the land
is sometimes sown or inoculated with the nodule-forming bacteria_.
[Illustration: FIG. 43.—TWO KINDS OF SOIL THAT HAVE BEEN WET AND THEN
DRIED. The loamy soil above remains loose and capable of growing
plants; the clay soil below has baked and cracked. ]
=Roots require moisture= in order to serve the plant. The soil water
that is valuable to the plant is not the free water, but the _thin
film of moisture which adheres to each little particle of soil_. The
finer the soil, the greater the number of particles, and therefore
the greater is the quantity of film moisture that it can hold. This
moisture surrounding the grains may not be perceptible, yet the plant
can use it. _Root absorption may continue in a soil which seems to be
dust dry._ Soils that are very hard and “baked” (Fig. 43) contain
very little moisture or air,—not so much as similar soils that are
granular or mellow.
=Proper Temperature for Root Action.=—_The root must be warm in order
to perform its functions._ Should the soil of fields or greenhouses
be much colder than the air, the plant suffers. When in a warm
atmosphere, or in a dry atmosphere, plants need to absorb much water
from the soil, and the roots must be warm if the root-hairs are
to supply the water as rapidly as it is needed. _If the roots are
chilled, the plant may wilt or die._
[Illustration: FIG. 44.—ROOT-HAIRS OF THE RADISH. ]
=Roots need Air.=—Corn on land that has been flooded by heavy
rains loses its green colour and turns yellow. _Besides diluting
plant-food, the water drives the air from the soil, and this
suffocation of the roots is very soon apparent in the general ill
health of the plant._ Stirring or tilling the soil aërates it. Water
plants and bog plants have adapted themselves to their particular
conditions. They get their air either by special surface roots, or
from the water through stems and leaves.
=Rootlets.=—_Roots divide into the thinnest and finest fibrils: there
are roots and there are rootlets._ The smallest rootlets are so
slender and delicate that they break off even when the plant is very
carefully lifted from the soil.
_The rootlets, or fine divisions, are clothed with the_ =root-hairs=
(Figs. 44, 45, 46). _These root-hairs attach to the soil particles,
and a great amount of soil is thus brought into actual contact with
the plant._ These are very _delicate prolonged surface cells of the
roots_. They are borne for a short distance just back of the tip of
the root.
[Illustration: FIG. 45.—CROSS-SECTION OF ROOT, enlarged, showing
root-hairs.]
_Rootlet and root-hair differ._ The rootlet is a _compact cellular
structure. The root-hair is a delicate tubular cell_ (Fig. 45),
_within which is contained living matter (protoplasm); and the
protoplasmic lining membrane of the wall governs the entrance of
water and substances in solution._ Being long and tube-like, these
root-hairs are especially adapted for taking in the largest quantity
of solutions; and they are the principal means by which plant-food
is absorbed from the soil, although the surfaces of the rootlets
themselves do their part. Water plants do not produce an abundant
system of root-hairs, and such plants depend largely on their
rootlets.
[Illustration: FIG. 46.—ROOT-HAIR, much enlarged, in contact with
the soil particles (_s_). Air-spaces at _a_; water-films on the
particles, as at _w_. ]
The root-hairs are very small, often invisible. They, with the
young roots, are usually broken off when the plant is pulled up.
They are best seen when seeds are germinated between layers of dark
blotting paper or flannel. On the young roots they will be seen as a
mould-like or gossamer-like covering. _Root-hairs soon die_: they do
not grow into roots. New ones form as the root grows.
=Osmosis.=—The water with its nourishment goes through the thin walls
of the root-hairs and rootlets by the process of osmosis. If there
are two liquids of different density on the inside and outside of
an organic (either vegetable or animal) membrane, the liquids tend
to mix through the membrane. The _law of osmosis_ is that _the most
rapid flow is toward the denser solution_. The protoplasmic lining
of the cell wall is such a membrane. The soil water being a weaker
solution than the sap in the roots, the flow is into the root. A
strong fertilizer sometimes causes a plant to wither, or “burns it.”
Explain.
=Structure of Roots.=—The root that grows from the lower end of the
caulicle is the _first_ or =primary root. Secondary roots= branch
from the primary root. Branches of secondary roots are sometimes
called =tertiary roots=. Do the secondary roots grow from the cortex,
or from the central cylinder of the primary root? Trim or peel
the cortex from a root and its branches and determine whether the
branches still hold to the central cylinder of the main root.
=Internal Structure of Roots.=—A section of a root shows that it
consists of a _central cylinder_ (see Fig. 45) surrounded by a layer.
This layer is called the =cortex=. The outer layer of cells in the
cortex is called the =epidermis=, and some of the cells of the
epidermis are prolonged and form the delicate root-hairs. The cortex
resembles the bark of the stem in its nature. The central cylinder
contains many tube-like canals, or “vessels” that convey water and
food (Fig. 45). Cut a sweet potato across (also a radish and a
turnip) and distinguish the central cylinder, cortex, and epidermis.
Notice the hard cap on the tip of roots. Roots differ from stems in
having no real pith.
[Illustration: FIG. 47.—GROWING POINT OF ROOT OF INDIAN CORN.
_d_, _d_, cells which will form the epidermis;
_p_, _p_, cells that will form bark;
_e_, _e_, endodermis;
_pl_, cells which will form the axis cylinder;
_i_, initial group of cells, or growing point proper;
_c_, root-cap.]
=Microscopic Structure of Roots.=—Near the end of any young root or
shoot the cells are found to differ from one another more or less,
according to the distance from the point. _This differentiation
takes place in the region just back of the growing point._ To study
growing points, use the hypocotyl of Indian corn which has grown
about one-half inch. Make a longitudinal section. Note these points
(Fig. 47): (_a_) the tapering root-cap beyond the growing point;
(_b_) the blunt end of the root proper and the rectangular shape of
the cells found there; (_c_) the group of cells in the middle of the
first layers beneath the root-cap,—this group is the growing point;
(_d_) study the slight differences in the tissues a short distance
back of the growing point. There are four regions: the =central
cylinder=, made up of several rows of cells in the centre (_pl_);
the =endodermis=, (_e_) composed of a single layer on each side
which separates the central cylinder from the bark; the =cortex=, or
inner bark, (_e_) of several layers outside the endodermis; and the
=epidermis=, or outer layer of bark on the outer edges (_d_). Make
a drawing of the section. If a series of the cross-sections of the
hypocotyl should be made and studied by the pupil beginning near the
growing point and going upward, it would be found that these four
tissues become more distinctly marked, for at the tip the tissues
have not yet assumed their characteristic form. The central cylinder
contains the ducts and vessels which convey the sap.
[Illustration: FIG. 48.—THE MARKING OF THE STEM AND ROOT. ]
=The Root-cap.=—Note the form of the root-cap shown in the
microscopic section drawn in Fig. 47. Growing cells, and especially
those which are forming tissue by subdividing, are very delicate and
are easily injured. The cells forming the root-cap are older and
tougher and are suited for pushing aside the soil that the root may
penetrate it.
=Region of most Rapid Growth.=—The roots of a seedling bean may be
marked at equal distances by waterproof ink or by bits of black
thread tied moderately tight. The seedling is then replanted and
left undisturbed for two days. When it is dug up, the region of most
rapid growth in the root can be determined. Give a reason why _a root
cannot elongate throughout its length_,—whether there is anything to
prevent a young root from doing so.
[Illustration: FIG. 49.—THE RESULT.]
In Fig. 48 is shown a germinating scarlet runner bean with a short
root upon which are marks made with waterproof ink; and the same root
(Fig. 49) is shown after it has grown longer. Which part of it did
not lengthen at all? Which part lengthened slightly? Where is the
region of most rapid growth?
=Geotropism.=—Roots turn toward the earth, even if the seed is
planted with the micropyle up. This phenomenon is called =positive
geotropism=. Stems grow away from the earth. This is =negative
geotropism=.
[Illustration: FIG. 50.—THE GRASP OF A PLANT ON THE PARTICLES OF
EARTH. A grass plant pulled in a garden.]
[Illustration: FIG. 51.—PLANT GROWING IN INVERTED POT.]
[Illustration: FIG. 52.—HOLES IN SOIL MADE BY ROOTS, now decayed.
Somewhat magnified.]
SUGGESTIONS (Chaps. VII and VIII).—=25.= _Tests for food._ Examine
a number of roots, including several fleshy roots, for the presence
of food material, making the tests used on seeds. =26.= _Study of
root-hairs._ Carefully germinate radish, turnip, cabbage, or other
seed, so that no delicate parts of the root will be injured. For
this purpose, place a few seeds in packing-moss or in the folds
of thick cloth or of blotting paper, being careful to keep them
moist and warm. In a few days the seed has germinated, and the root
has grown an inch or two long. Notice that, except at a distance
of about a quarter of an inch behind the tip, the root is covered
with minute hairs (Fig. 44). They are actually hairs; that is,
root-hairs. Touch them and they collapse, they are so delicate.
Dip one of the plants in water, and when removed the hairs are
not to be seen. The water mats them together along the root and
they are no longer evident. Root-hairs are usually destroyed when
a plant is pulled out of the soil, be it done ever so carefully.
They cling to the minute particles of soil (Fig. 46). The hairs
show best against a dark background. =27.= On some of the blotting
papers, sprinkle sand; observe how the root-hairs cling to the
grains. Observe how they are flattened when they come in contact
with grains of sand. =28.= _Root hold of plant._ The pupil should
also study the root hold. Let him carefully pull up a plant. If a
plant grows alongside a fence or other rigid object, he may test
the root hold by securing a string to the plant, letting the string
hang over the fence, and then adding weights to the string. Will
a stake of similar size to the plant and extending no deeper in
the ground have such firm hold on the soil? What holds the ball of
earth in Fig. 50? =29.= _Roots exert pressure._ Place a strong bulb
of hyacinth or daffodil on firm-packed earth in a pot; cover the
bulb nearly to the top with loose earth; place in a cool cellar;
after some days or weeks, note that the bulb has been raised out
of the earth by the forming roots. All roots exert pressure on the
soil as they grow. Explain. =30.= _Response of roots and stems to
the force of gravity, or geotropism._ Plant a fast-growing seedling
in a pot so that the plumule extends through the drain hole and
suspend the pot with mouth up (_i.e._ in the usual position). Or
use a pot in which a plant is already growing, cover with cloth or
wire gauze to prevent the soil from falling, and suspend the pot in
an inverted position (Fig. 51). Notice the behaviour of the stem,
and after a few days remove the soil and observe the position of
the root. =31.= If a pot is laid on one side, and changed every two
days and laid on its opposite side, the effect on the root and stem
will be interesting. =32.= If a fleshy root is planted wrong end
up, what is the result? Try it with pieces of horse-radish root.
=33.= By planting radishes on a slowly revolving wheel the effect
of gravity may be neutralized. =34.= _Region of root most sensitive
to gravity._ Lay on its side a pot containing a growing plant.
After it has grown a few days, wash away the earth surrounding the
roots. Which turned downward most decidedly, the tip of root or the
upper part? =35.= _Soil texture._ Carefully turn up soil in a rich
garden or field so that you have unbroken lumps as large as a hen’s
egg. Then break these lumps apart carefully with the fingers and
determine whether there are any traces or remains of roots (Fig.
52). Are there any pores, holes, or channels made by roots? Are
the roots in them still living? =36.= Compare another lump from a
clay bank or pile where no plants have been growing. Is there any
difference in texture? =37.= Grind up this clay lump very fine, put
it in a saucer, cover with water, and set in the sun. After a time
it will have the appearance shown in the lower saucer in Fig. 43.
Compare this with mellow garden soil. In which will plants grow
best, even if the plant-food were the same in both? Why? =38.= _To
test the effect of moisture_ on the plant, let a plant in a pot or
box dry out till it wilts; then add water and note the rapidity
with which it recovers. Vary the experiment in quantity of water
applied. Does the plant call for water sooner when it stands in
a sunny window than when in a cool shady place? Prove it. =39.=
Immerse a potted plant above the rim of the pot in a pail of water
and let it remain there. What is the consequence? Why? =40.= _To
test the effect of temperature on roots._ Put one pot in a dish of
ice water, and another in a dish of warm water, and keep them in a
warm room. In a short time notice how stiff and vigorous is the one
whose roots are warm, whereas the other may show signs of wilting.
=41.= _The process of osmosis._ Chip away the shell from the large
end of an egg so as to expose the uninjured membrane beneath for
an area about as large as a ten cent piece. With sealing-wax,
chewing-gum, or paste, stick a quill about three inches long to the
smaller end of the egg. After the tube is in place, run a hat pin
into it so as to pierce both shell and membrane; or use a short
glass tube, first scraping the shell thin with a knife and then
boring through it with the tube. Now set the egg upon the mouth
of a pickle jar nearly full of water, so that the large end with
the exposed membrane is beneath the water. After several hours,
observe the tube on top of the egg to see whether the water has
forced its way into the egg and increased its volume so that part
of its contents are forced up into the tube. If no tube is at
hand, see whether the contents are forced through the hole which
has been made in the small end of the egg. Explain how the law of
osmosis is verified by your result. If the eggshell contained only
the membrane, would water rise into it? If there were no water in
the bottle, would the egg-white pass down into the bottle? =42.=
_The region of most rapid growth._ The pupil should make marks with
waterproof ink (as Higgins’ ink or indelible marking ink) on any
soft growing roots. Place seeds of bean, radish, or cabbage between
layers of blotting paper or thick cloth. Keep them damp and warm.
When stem and root have grown an inch and a half long each, with
waterproof ink mark spaces exactly one-quarter inch apart (Figs.
48, 49). Keep the plantlets moist for a day or two, and it will
be found that on the stem some or all of the marks are more than
one-quarter inch apart; on the root the marks have not separated.
The root has grown beyond the last mark.
CHAPTER IX
THE STEM—KINDS AND FORMS; PRUNING
=The Stem System.=—The stem of a plant is the part that _bears the
buds, leaves, flowers, and fruits_. Its office is _to hold these
parts up to the light and the air_; and through its tissues the
various food materials and the life-giving fluids _are distributed to
the growing and working parts_.
The entire mass or fabric of stems of any plant is called its =stem
system=. It comprises the trunk, branches, and twigs, but not the
stalks of leaves and flowers that die and fall away. The stem system
may be herbaceous or woody, annual, biennial, or perennial; and it
may assume many sizes and shapes.
=Stems are of Many Forms.=—The general way in which a plant grows is
called its =habit=. The habit is the _appearance or general form_.
Its habit may be open or loose, dense, straight, crooked, compact,
straggling, climbing, erect, weak, strong, and the like. The roots
and the leaves are _the important functional or working parts_; the
stem merely connects them, and its form is exceedingly variable.
[Illustration: FIG. 53.—STRICT SIMPLE STEM OF MULLEIN.]
[Illustration: FIG. 54.—STRICT UPRIGHT STEM OF NARROW-LEAVED DOCK.]
=Kinds of Stems.=—_The stem may be so short as to be scarcely
distinguishable._ In such cases the crown of the plant—that part just
at the surface of the ground—bears the leaves and the flowers; but
this crown is really a very short stem. The dandelion, Fig. 33, is
an example. Such plants are often said to be =stemless=, however, in
order to distinguish them from plants that have long or conspicuous
stems. _These so-called stemless plants die to the ground every year._
[Illustration: FIG. 55.—TRAILING STEM OF WILD MORNING GLORY
(_Convolvulus arvensis_).]
Stems are =erect= when they grow straight up (Figs. 53, 54). They
are =trailing= when they run along on the ground, as melon, wild
morning-glory (Fig. 55). They are =creeping= when they run on
the ground and take root at places, as the strawberry. They are
=decumbent= when they flop over to the ground. They are =ascending=
when they lie mostly or in part on the ground but stand more or less
upright at their ends; example, a tomato. They are =climbing= when
they cling to other objects for support (Figs. 36, 56).
[Illustration: FIG. 56.—A CLIMBING PLANT (a twiner).]
Trees in which the main trunk or the “leader” continues to grow from
its tip are said to be =excurrent= in growth. _The branches are borne
along the sides of the trunk_, as in common pines (Fig. 57) and
spruces. Excurrent means _running out_ or _running up_.
Trees in which the main trunk does not continue are said to be
=deliquescent=. _The branches arise from one common point or from
each other._ The stem is lost in the branches. The apple tree, plum
(Fig. 58), maple, elm, oak, China tree, are familiar examples.
Deliquescent means _dissolving_ or _melting away_.
[Illustration: FIG. 57.—EXCURRENT TRUNK. A pine.]
[Illustration: FIG. 58.—DELIQUESCENT TRUNK OF PLUM TREE.]
=Each kind of plant has its own peculiar habit or direction of
growth.= Spruces always grow to a single stem or trunk, pear trees
are always deliquescent, morning-glories are always trailing or
climbing, strawberries are always creeping. We do not know why each
plant has its own habit, but the habit is in some way _associated
with the plant’s genealogy or with the way in which it has been
obliged to live_.
The stem may be =simple= or =branched=. A simple stem usually grows
from the terminal bud, and side branches either do not start, or, if
they start, they soon perish. Mulleins (Fig. 53) are usually simple.
So are palms.
_Branched stems may be of very different habit and shape._ Some stem
systems are narrow and erect; these are said to be _strict_ (Fig.
54). Others are _diffuse_, _open_, _branchy_, _twiggy_.
=Nodes and Internodes.=—The parts of the stem at which buds grow
are called =nodes= or =joints= and the spaces between the buds are
=internodes=. The stem at nodes is usually enlarged, and the pith is
usually interrupted. The distance between the nodes is influenced by
the vigour of the plant: how?
[Illustration: FIG. 59.—RHIZOME OR ROOTSTOCK.]
=Stems vs. Roots.=—Roots sometimes grow above ground (Chap. VII); so,
also, _stems sometimes grow underground_, and they are then known as
=subterranean stems=, =rhizomes=, or =rootstocks= (Fig. 59).
=Stems normally bear leaves and buds, and thereby are they
distinguished from roots=; usually, also, they contain a pith. The
leaves, however, may be reduced to mere scales, and the buds beneath
them may be scarcely visible. Thus the “eyes” on a white potato
are cavities with a bud or buds at the bottom (Fig. 60). Sweet
potatoes have no evident “eyes” when first dug (but they may develop
adventitious buds before the next growing season). The white potato
is a stem: the sweet potato is probably a root.
=How Stems elongate.=—_Roots elongate by growing near the tip. Stems
elongate by growing more or less throughout the young or soft part_
or “between joints” (Figs. 48, 49). But any part of the stem soon
reaches a limit beyond which it cannot grow, or becomes “fixed”; and
the new parts beyond elongate until they, too, become rigid. When
a part of the stem once becomes fixed or hard, it never increases
in length: that is, _the trunk or woody parts never grow longer or
higher; branches do not become farther apart or higher from the
ground_.
[Illustration: FIG. 60.—SPROUTS ARISING FROM THE BUDS, or eyes, of a
potato tuber.]
=Stems are modified in form= by the particular or incidental
conditions under which they grow. _The struggle for light_ is the
chief factor in determining the shape and the direction of any limb
(Chap. II). This is well illustrated in any tree or bush that grows
against a building or on the margin of a forest (Fig. 4). In a very
dense thicket the innermost trees shoot up over the others or they
perish. Examine any stem and endeavour to determine why it took its
particular form.
[Illustration: FIG. 61.—CRACKING OF THE BARK ON AN ELM BRANCH.]
=The stem is cylindrical=, _the outer part being bark and the inner
part being wood or woody tissue_. In the dicotyledonous plants, the
bark is usually easily separated from the remainder of the cylinder
at some time of the year; in monocotyledonous plants the bark is not
free. Growth in thickness takes place inside the covering and not on
the very outside of the plant cylinder. It is evident, then, that
the covering of _bark must expand in order to allow of the expansion
of the woody cylinder within it_. The tissues, therefore, must be
under constant pressure or tension. It has been determined that the
pressure within a growing trunk is often as much as fifty pounds to
the square inch. The lower part of the limb in Fig. 61 shows that the
outer layers of bark (which are long since dead, and serve only as
protective tissue) have reached the limit of their expanding capacity
and have begun to split. The pupil will now be interested in the bark
on the body of an old elm tree (Fig. 62); and he should be able to
suggest one reason why stems remain cylindrical, and why the old bark
becomes marked with furrows, scales, and plates.
[Illustration: FIG. 62.—PIECE OF BARK FROM AN OLD ELM TRUNK.]
Most woody plants increase in diameter by the addition of an _annual
layer_ or “_ring_” on the outside of the woody cylinder, underneath
the bark. The monocotyledonous plants comprise very few trees and
shrubs in temperate climates (the palms, yuccas, and other tree-like
plants are of this class), and they do not increase greatly in
diameter and they rarely branch to any extent.
=Bark-bound Trees.=—If, for any reason, the bark should become so
dense and strong that the trunk cannot expand, the tree is said to be
“bark-bound.” Such condition is not rare in orchard trees that have
been neglected. When good tillage is given to such trees, they may
not be able to overcome the rigidity of the old bark, and, therefore,
do not respond to the treatment. Sometimes the parts with thinner
bark may outgrow in diameter the trunk or the old branches below
them. The remedy is to _release the tension_. This may be done either
by softening the bark (by washes of soap or lye), or by separating
it. The latter is done by slitting the bark-bound part (in spring),
thrusting the point of a knife through the bark to the wood, and
then drawing the blade down the entire length of the bark-bound
part. The slit is scarcely discernible at first, but it opens with
the growth of the tree, filling up with new tissue beneath. Let the
pupil consider the ridges which he now and then finds on trees, and
determine whether they have any significance—whether the tree has
ever been released, or injured by natural agencies.
[Illustration: FIG. 63.—PROPER CUTTING OF A BRANCH. The wound will
soon be “healed.”]
=The Tissue covers the Wounds and “heals” them.=—This is seen in Fig.
63, in which a ring of tissue rolls out over the wound. This ring
of healing tissue forms most rapidly and uniformly when the wound
is smooth and regular. Observe the healing on broken and splintered
limbs; also the difference in rapidity of healing between wounds on
strong and weak limbs. There is a difference in the rapidity of the
healing process in different kinds of trees. Compare the apple tree
and the peach. This tissue may in turn become bark-bound, and the
healing may stop. On large wounds it progresses more rapidly the
first few years than it does later. This roll or ring of tissue is
called a =callus=.
[Illustration: FIG. 64.—ERRONEOUS PRUNING.]
=The callus grows from the living tissue of the stem= just about the
wound. It cannot cover long dead stubs or very rough broken branches
(Fig. 64). Therefore, in pruning _the branches should be cut close
to the trunk_ and made even and smooth; _all long stubs must be
avoided_. The seat of the wound should be close to the living part of
the trunk, for the stub of the limb that is severed has no further
power in itself of making healing tissue. The end of the remaining
stub is merely covered over by the callus, and usually remains a dead
piece of wood sealed inside the trunk (Fig. 65). If wounds do not
heal over speedily, germs and fungi obtain foothold in the dying wood
and _rot sets in_. Hollow trees are those in which the decay-fungi
have progressed into the inner wood of the trunk; _they have been
infected_ (Fig. 66).
[Illustration: FIG. 65.—KNOT IN A HEMLOCK LOG.]
=Large wounds should be protected= with a covering of paint, melted
wax, or other adhesive and lasting material, to keep out the germs
and fungi. A covering of sheet iron or tin may keep out the rain, but
it will not exclude the germs of decay; in fact, it may provide the
very moist conditions that such germs need for their growth. Deep
holes in trees should be treated by having all the decayed parts
removed down to the clean wood, the surfaces painted or otherwise
sterilized, and the hole filled with wax or cement.
[Illustration: FIG. 66.—A KNOT HOLE, and the beginning of a hollow
trunk.]
=Stems and roots are living=, and they should not be wounded or
mutilated unnecessarily. Horses should never be hitched to trees.
Supervision should be exercised over persons who run telephone,
telegraph, and electric light wires, to see that they do not mutilate
trees. Electric light wires and trolley wires, when carelessly strung
or improperly insulated, may kill trees (Fig. 67).
[Illustration: FIG. 67.—ELM TREE KILLED BY A DIRECT CURRENT FROM AN
ELECTRIC RAILROAD SYSTEM.]
SUGGESTIONS.—_Forms of stems._ =43.= Are the trunks of trees ever
perfectly cylindrical? If not, what may cause the irregularities?
Do trunks often grow more on one side than the other? =44.= Slit a
rapidly growing limb, in spring, with a knife blade, and watch the
result during the season. =45.= Examine the woodpile, and observe
the variations in thickness of the annual rings, and especially
of the same ring at different places in the circumference.
Cross-sections of horizontal branches are interesting in this
connection. =46.= Note the enlargement at the base of a branch,
and determine whether this enlargement or bulge is larger on long,
horizontal limbs than on upright ones. Why does this bulge develop?
Does it serve as a brace to the limb, and is it developed as the
result of constant strain? =47.= _Strength of stems._ The pupil
should observe the fact that a stem has wonderful strength. Compare
the proportionate height, diameter, and weight of a grass stem with
those of the slenderest tower or steeple. Which has the greater
strength? Which the greater height? Which will withstand the most
wind? Note that the grass stem will regain its position even if
its top is bent to the ground. Note how plants are weighted down
after a heavy rain and how they recover themselves. =48.= Split a
cornstalk and observe how the joints are tied together and braced
with fibres. Are there similar fibres in stems of pigweed, cotton,
sunflower, hollyhock?
[Illustration: FIG. 68.—POTATO. What are roots, and what stems? Has
the plant more than one kind of stem? more than two kinds? Explain.]
CHAPTER X
THE STEM—ITS GENERAL STRUCTURE
There are two main types of stem structure in flowering plants, the
differences being based on the arrangement of bundles or strands of
tissue. These types are _endogenous_ and _exogenous_ (page 20). It
will require patient laboratory work to understand what these types
and structures are.
=Endogenous, or Monocotyledonous Stems.=—Examples of endogenous stems
are all the grasses, cane-brake, sugar-cane, smilax or green-brier,
palms, banana, canna, bamboo, lilies, yucca, asparagus, all the
cereal grains. For our study, a cornstalk may be used as a type.
[Illustration: FIG. 69.—CROSS-SECTION OF CORNSTALK, showing the
scattered fibro-vascular bundles. Slightly enlarged.]
A piece of _cornstalk_, either green or dead, should be in the hand
of each pupil while studying this lesson. Fig. 69 will also be of
use. Is there a swelling at the nodes? Which part of the internode
comes nearest to being perfectly round? There is a grooved channel
running along one side of the internode: how is it placed with
reference to the leaf? with reference to the groove in the internode
below it? What do you find in each groove at its lower end? (In a
dried stalk only traces of this are usually seen.) Does any bud on a
cornstalk besides the one at the top ever develop? Where do suckers
come from? Where does the ear grow?
[Illustration: FIG. 70.—DIAGRAM TO SHOW THE COURSE OF FIBRO-VASCULAR
BUNDLES IN MONOCOTYLEDONS.]
Cut a cross-section of the stalk between the nodes (Fig. 69). Does
it have a distinct bark? The interior consists of soft “pith” and
tough woody parts. The wood is found in _strands_ or _fibres_. Which
is more abundant? Do the fibres have any definite arrangement?
Which strands are largest? Smallest? The firm smooth _rind_ (which
cannot properly be called a bark) consists of small wood strands
packed closely together. Grass stems are hollow cylinders; and
the cornstalk, because of the lightness of its contents, is also
practically a cylinder. Stems of this kind are admirably adapted for
providing a strong support to leaves and fruit. This is in accordance
with the well-known law that a hollow cylinder is much stronger than
a solid cylinder of the same weight of material. Cut a thin slice of
the inner soft part and hold it up to the light. Can you make out a
number of tiny compartments or cells? These cells consist of a tissue
called _parenchyma_, the tissue from which when young all the other
tissues arise and differentiate. The numerous walls of these cells
may serve to brace the outer wall of the cylinder; but their chief
function in the young stalk is to give origin to other cells. When
alive they are filled with cell sap and protoplasm.
Trace the _woody strands_ through the nodes. Do they ascend
vertically? Do they curve toward the rind at certain places? Compare
their course with the strands shown in Fig. 70. _The woody strands
consist chiefly of tough fibrous cells that give rigidity and
strength to the plant, and of long tubular interrupted canals that
serve to convey sap upward from the root and to convey food downward
from the leaves to the stem and the roots._
[Illustration: FIG. 71.—DIAGRAM OF WOOD STRANDS OR FIBRO-VASCULAR
BUNDLES IN A ROOT, showing the wood (_x_) and bast (_p_) separated.]
Monocotyledons, as shown by fossils, existed before dicotyledons
appeared, and it is thought that the latter were developed from
ancestors of the former. It will be interesting to trace the
relationship in stem structure. It will first be necessary to learn
something of the structure of the wood strand.
=Wood Strand in Monocotyledons and Dicotyledons.=—Each wood strand
(or fibro-vascular bundle) consists of two parts—the bast and the
wood proper. The wood is on the side of the strand toward the
centre of the stem and contains large tubular canals that take the
watery sap upward from the roots. The bast is on the side toward
the bark, and contains fine tubes through which diffuses the dense
sap containing digested food from the leaves. In the root (Fig. 71)
the bast and the wood are separate, so that there are _two kinds of
strands_.
[Illustration: FIG. 72.—PART OF CROSS-SECTION OF ROOTSTOCK OF
ASPARAGUS, showing a few fibro-vascular bundles. An endogenous stem.]
In monocotyledons, as already said, the strands (or bundles) _are
usually scattered in the stem with no definite arrangement_ (Figs.
72, 73). In dicotyledons the strands, or bundles, _are arranged
in a ring._ As the dicotyledonous seed germinates, five bundles
are usually formed in its hypocotyl (Fig. 74); soon five more are
interposed between them, and the multiplication continues, in tough
plants, until the bundles touch (Fig. 74, right). The inner parts
thus form a ring of wood and the outer parts form the inner bark or
bast. A new ring of wood or bast is formed on stems of dicotyledons
each year, and the age of a cut stem is easily determined.
[Illustration: FIG. 73.—THE SCATTERED BUNDLES OR STRANDS, in
monocotyledons at _a_, and the bundles in a circle in dicotyledons at
_b_.]
[Illustration: FIG. 74.—DICOTYLEDONOUS STEM OF ONE YEAR AT LEFT WITH
FIVE BUNDLES, and a two-year stem at right. _o_, the pith; _c_, the
wood part; _b_, the bast part; _a_, one year’s growth.]
[Illustration: FIG. 75.—FIBRO-VASCULAR BUNDLE OF INDIAN CORN, much
magnified.
_A_, annular vessel; _A′_, annular or spiral vessel; _TT′_,
thick-walled vessels; _W_, tracheids or woody tissue; _F_, sheath
of fibrous tissue surrounding the bundle; _FT_, fundamental tissue
or pith; _S_, sieve tissue; _P_, sieve plate; _C_, companion cell;
_I_, intercellular space, formed by tearing down of adjacent cells;
_W′_, wood parenchyma.]
[Illustration: FIG. 76.—THE DICOTYLEDONOUS BUNDLE OR WOOD STRAND.
Upper figure is of moonseed:
_c_, cambium; _d_, ducts; 1, end of first year’s growth; 2, end of
second year’s growth; bast part at left and wood part at right.
Lower figure (from Wettstein) is sunflower: _h_, wood-cells; _g_,
vessels; _c_, cambium; _p_, fundamental tissue or parenchyma; _b_,
bast; _bp_, bast parenchyma; _s_, sieve-tubes.]
When cross-sections of monocotyledonous and dicotyledonous bundles
are examined under the microscope, it is readily seen why
dicotyledonous bundles form rings of wood and monocotyledonous cannot
(Figs. 75 and 76). The dicotyledonous bundle (Fig. 76) has, running
across it, a layer of brick-shaped cells called =cambium=, which
cells are a specialized form of the parenchyma cells and retain the
power of growing and multiplying. The bundles containing cambium
are called _open bundles_. There is no cambium in monocotyledonous
bundles (Fig. 75) and the bundles are called _closed bundles_.
Monocotyledonous stems _soon cease to grow in diameter_. The stem
of a palm tree is almost as large at the top as at the base. As
dicotyledonous plants grow, the _stems become thicker each year_, for
the delicate active cambium layer forms new cells from early spring
until midsummer or autumn, adding to the wood within and to the bark
without. As the growth in spring is very rapid, the first wood-cells
formed are much larger than the last wood-cells formed by the slow
growth of the late season, and the spring wood is less dense and of
a lighter colour than the summer wood; hence the time between two
years’ growth is readily made out (Figs. 77 and 78). Because of the
rapid growth of the cambium in spring and its consequent soft walls
and fluid contents, the bark of trees “peels” readily at that season.
[Illustration: FIG. 77.—WHITE PINE STEM, 5 years old. The outermost
layer is bark.]
[Illustration: FIG. 78.—ARRANGEMENT OF TISSUES IN TWO-YEAR-OLD STEM
OF MOONSEED.
_p_, pith; _f_, parenchyma. The fibro-vascular bundles, or wood
strands, are very prominent, with thin medullary rays between.]
=Medullary Rays.=—The first year’s growth in dicotyledons forms a
woody ring which almost incloses the pith, and this is left as a
small cylinder which does not grow larger, even if the tree should
live a century. It is not quite inclosed, however, for the narrow
layers of soft cells separating the bundles remain between them (Fig.
78), forming radiating lines called =medullary rays= or =pith rays=.
[Illustration: FIG. 79.—MARKINGS IN CELL WALLS OF WOOD FIBRE
_sp_, spiral; _an_, annular; _sc_, scalariform.]
=The Several Plant Cells and their Functions.=—In the =wood= there
are some parenchyma cells that have thin walls still, but have lost
the power of division. They are now _storage cells_. There are also
wood fibres which are thick-walled and rigid (h, Fig. 76), and serve
to _support_ the =sap-canals= or _wood vessels_ (or tracheids)
that are formed by the absorption of the end walls of upright rows
of cells; the canals pass from the roots to the twigs and even to
ribs of the leaves and serve to transport the root water. They are
recognized (Fig. 79) by the peculiar thickening of the wall on
the inner surface of the tubes, occurring in the form of spirals.
Sometimes the whole wall is thickened except in spots called _pits_
(g, Fig. 76). These thin spots (Fig. 80) allow the sap to pass to
other cells or to neighbouring vessels.
[Illustration: FIG. 80.—PITS IN THE CELL WALL.
Longitudinal section of wall at _b_, showing pit borders at _o_,
_o_.]
The =cambium=, as we have seen, consists of cells whose function is
_growth_. These cells are thin-walled and filled with protoplasm.
During the growing season they are continually adding to the wood
within and the bark without; hence the layer moves outward as it
deposits the new woody layer within.
[Illustration: FIG. 81.—SIEVE-TUBES, _s_, _s_;
_p_ shows a top view of a sieve-plate, with a companion cell, _c_,
at the side; _o_ shows sieve-plates in the side of the cell. In
_s_, _s_ the protoplasm is shrunken from the walls by reagents.]
[Illustration: FIG. 82.—THICK-WALLED BAST CELLS.]
=The bark= consists of inner or _fibrous bark_ or new bast (these
fibres in flax become linen), the _green or middle bark_ which
functions somewhat as the leaves, and the _corky or outer bark_. The
common word “bark” is seen, therefore, not to represent a homogeneous
or simple structure, but rather a collection of several kinds of
tissue, all separating from the wood beneath by means of cambium. The
new bast contains (1) the _sieve-tubes_ (Fig. 81) which transport the
sap containing organic substances, as sugar and proteids, from the
leaves to the parts needing it (_s_, Fig. 76). These tubes have been
formed like the wood vessels, but they have sieve-plates to allow
the dense organic-laden sap to pass with sufficient readiness for
purposes of rapid distribution. (2) There are also thick-walled _bast
fibres_ (Fig. 82) in the bast that serve for _support_. (3) There is
also some parenchyma in the new bast; it is now in part a storage
_tissue_. Sometimes the walls of parenchyma cells in the cortex
thicken at the corners and form _brace cells_ (Fig. 83) (collenchyma)
for _support_; sometimes the whole wall is thickened, forming _grit
cells_ or _stone cells_ (Fig. 84; examples in tough parts of pear, or
in stone of fruits). Some parts serve for secretions (milk, rosin,
etc.) and are called _latex tubes_.
[Illustration: FIG. 83.—COLLENCHYMA IN WILD JEWELWEED OR TOUCH-ME-NOT
(IMPATIENS).]
[Illustration: FIG. 84.—GRIT CELLS.]
The =outer bark= of old shoots consists of _corky_ cells that
_protect_ from mechanical injury, and that contain a fatty substance
(suberin) impermeable to water and of service to _keep in moisture_.
There is sometimes a cork cambium (or phellogen) in the bark that
serves to extend the bark and keep it from splitting, thus increasing
its power to protect.
=Transport of the “Sap.”=—We shall soon learn that the common word
“sap” does not represent a single or simple substance. We may roughly
distinguish two kinds of more or less fluid contents: (1) _the root
water_, sometimes called mineral sap, that is taken in by the root,
containing its freight of such inorganic substances as potassium,
calcium, iron, and the rest; this root water rises, we have found,
_in the wood vessels_,—that is, in the young or “sapwood” (p. 96);
(2) the _elaborated_ or _organized materials_ passing back and
forth, especially from the leaves, to build up tissues in all parts
of the plant, some of it going down to the roots and root-hairs;
this organic material is transported, as we have learned, _in the
sieve-tubes of the inner bast_,—that is, in the “inner bark.”
Removing the bark from a trunk in a girdle will not stop the upward
rise of the root water so long as the wood remains alive; but it will
stop the passage of the elaborated or food-stored materials to parts
below and thus starve those parts; and if the girdle does not heal
over by the deposit of new bark, the tree will in time _starve to
death_. It will now be seen that the common practice of placing wires
or hoops about trees to hold them in position or to prevent branches
from falling is irrational, because such wires interpose barriers
over which the fluids cannot pass; in time, as the trunk increases
in diameter, the wire girdles the tree. It is much better to bolt
the parts together by rods extending through the branches (Fig. 85).
These bolts should fit very tight in their holes. Why?
[Illustration: FIG. 85.—THE WRONG WAY TO BRACE A TREE. (See Fig.
118).]
=Wood.=—The main stem or trunk, and sometimes the larger branches,
are the sources of lumber and timber. Different kinds of wood have
value for their special qualities. The business of raising wood, for
all purposes, is known as _forestry_. The forest is to be considered
as a crop, and the crop must be harvested, as much as corn or rice is
harvested. Man is often able to grow a more productive forest than
nature does.
=Resistance to decay= gives value to wood used for shingles
(_cypress_, heart of _yellow pine_) and for fence posts (_mulberry_,
_cedar_, _post oak_, _bois d’arc_, _mesquite_).
=Hardness and strength= are qualities of great value in building.
_Live oak_ is used in ships. _Red oak_, _rock maple_, and _yellow
pine_ are used for floors. The best flooring is sawn with the
straight edges of the annual rings upward; tangential sawn flooring
may splinter. _Chestnut_ is common in some parts of the country,
being used for ceiling and inexpensive finishing and furniture.
_Locust_ and _bois d’arc_ (osage orange) are used for hubs of wheels;
bois d’arc makes a remarkably durable pavement for streets. _Ebony_
is a tropical wood used for flutes, black piano keys, and fancy
articles. _Ash_ is straight and elastic; it is used for handles for
light implements. _Hickory_ is very strong as well as elastic, and is
superior to ash for handles, spokes, and other uses where strength is
wanted. Hickory is never sawn into lumber, but is split or turned.
The “second growth,” which sprouts from stumps, is most useful, as it
splits readily. Fast-growing hickory in rich land is most valuable.
The supply of useful hickory is being rapidly exhausted.
=Softness= _is often important_. _White pine_ and _sweet gum_ because
of their softness and lightness are useful in box-making. “_Georgia_”
or _southern pine_ is harder and stronger than white pine; it is much
used for floors, ceilings, and some kinds of cabinet work. _White
pine_ is used for window-sash, doors, and moulding, and cheaper
grades are used for flooring. _Hemlock_ is the prevailing lumber in
the east for the framework and clapboarding of buildings. _Redwood_
and _Douglas spruce_ are common building materials on the Pacific
coast. _Cypress_ is soft and resists decay and is superior to white
pine for sash, doors, and posts on the outside of houses. _Cedar_
is readily carved and has a unique use in the making of chests for
clothes, as its odour repels moths and other insects. _Willow_ is
useful for baskets and light furniture. _Basswood_ or _linden_ is
used for light ceiling and sometimes for cheap floors. _Whitewood_
(incorrectly called poplar) is employed for wagon bodies and often
for house finishing. It often resembles curly maple.
[Illustration: FIG. 86.—THE MAKING OF ORDINARY BOARDS, AND ONE WAY OF
MAKING “QUARTERED” BOARDS.]
=Beauty of grain and polish= gives wood value for furniture, pianos,
and the like. _Mahogany_ and _white oak_ are most beautiful, although
red oak is also used. Oak logs which are first quartered and then
sawn radially expose the beautiful silver grain (medullary rays).
Fig. 86 shows one _mode of quartering_. The log is quartered on the
lines _a_, _a_, _b_, _b_; then succeeding boards are cut from each
quarter at 1, 2, 3, etc. The nearer the heart the better the “grain”:
why? Ordinary boards are sawn tangentially, as _c_, _c_. _Curly
pine_, _curly walnut_, and _bird’s-eye maple_ are woods that owe
their beauty of grain to wavy lines or buried knots. A mere stump of
curly walnut is worth several hundred dollars. Such wood is sliced
very thin for veneering and glued over other woods in making pianos
and furniture. If the cause of wavy grain could be found out and such
wood grown at will, the discovery would be very useful. _Maple_ is
much used for furniture. _Birch_ may be coloured so as very closely
to represent mahogany, and it is useful for desks.
=Special Products of Trees.=—Cork from the bark of the cork oak in
Spain, latex from the rubber, and sap from the sugar-maple trees,
turpentine from pine, tannin from oak bark, Peruvian bark from
cinchona, are all useful products.
SUGGESTIONS.—_Parts of a root and stem through which liquids rise._
=49.= Pull up a small plant with abundant leaves, cut off the root
so as to leave two inches or more on the plant (or cut a leafy
shoot of squash or other strong-growing coarse plant), and stand
it in a bottle with a little water at the bottom which has been
coloured with red ink (eosine). After three hours examine the root;
make cross sections at several places. Has the water coloured the
axis cylinder? The cortex? What is your conclusion? Stand some cut
flowers or a leafy plant with cut stem in the same solution and
examine as before: conclusion? =50.= Girdle a twig of a rapidly
growing bush (as willow) in early spring when growth begins (_a_)
by very carefully removing only the bark, and (_b_) by cutting away
also the sapwood. Under which condition do the leaves wilt? Why?
=51.= Stand twigs of willow in water; after roots have formed under
the water, girdle the twig (in the two ways) above the roots. What
happens to the roots, and why? =52.= Observe the swellings on trees
that have been girdled or very badly injured by wires or otherwise:
where are these swellings, and why? =53.= _Kinds of wood._ Let each
pupil determine the kind of wood in the desk, the floor, the door
and window casings, the doors themselves, the sash, the shingles,
the fence, and in the small implements and furniture in the room;
also what is the cheapest and the most expensive lumber in the
community. =54.= How many kinds of wood does the pupil know, and
what are their chief uses?
NOTE TO TEACHER.—The work in this chapter is intended to be mainly
descriptive, for the purpose of giving the pupil a rational
conception of the main vital processes associated with the stem, in
such a way that he may translate it into his daily thought. It is
not intended to give advice for the use of the compound microscope.
If the pupil is led to make a careful study of the text, drawings,
and photographs on the preceding and the following pages, he will
obtain some of the benefit of studying microscope sections without
being forced to spend time in mastering microscope technique. If
the school is equipped with compound microscopes, a teacher is
probably chosen who has the necessary skill to manipulate them and
the knowledge of anatomy and physiology that goes naturally with
such work; and it would be useless to give instruction in such
work in a text of this kind. The writer is of the opinion that
the introduction of the compound microscope into first courses
in botany has been productive of harm. Good and vital teaching
demands first that the pupil have a normal, direct, and natural
relation to his subject, as he commonly meets it, that the obvious
and significant features of the plant world be explained to him
and be made a means of training him. The beginning pupil cannot
be expected to know the fundamental physiological processes, nor
is it necessary that these processes should be known in order to
have a point of view and trained intelligence on the things that
one customarily sees. Many a pupil has had a so-called laboratory
course in botany without having arrived at any real conception of
what plants mean, or without having had his mind opened to any real
sympathetic touch with his environment. Even if one’s knowledge be
not deep or extensive, it may still be accurate as far as it goes,
and his outlook on the subject may be rational.
[Illustration: FIG. 87.—THE MANY-STEMMED THICKETS OF MANGROVE OF
SOUTHERNMOST SEACOASTS, many of the trunks being formed of aërial
roots.]
CHAPTER XI
LEAVES—FORM AND POSITION
Leaves may be studied from four points of view,—with reference to (1)
their _kinds_ and _shapes_; (2) their _position_, or _arrangement_ on
the plant; (3) their _anatomy_, or _structure_; (4) their _function_,
or the work they perform. This chapter is concerned with the first
two categories.
[Illustration: FIG. 88.—A SIMPLE NETTED-VEINED LEAF.]
[Illustration: FIG. 89.—A SIMPLE PARALLEL-VEINED LEAF.]
[Illustration: FIG. 90.—COMPOUND OR BRANCHED LEAF OF BRAKE (a common
fern).]
=Kinds.=—Leaves are =simple= or unbranched (Figs. 88, 89), and
=compound= or branched (Fig. 90). The method of compounding or
branching follows the mode of veining. The veining, or =venation=,
is of two general kinds. In some plants the main veins diverge, and
there is a conspicuous network of smaller veins; such leaves are
=netted-veined=. They are characteristic of the dicotyledons. In
other plants the main veins are parallel, or nearly so, and there is
no conspicuous network; these are =parallel-veined= leaves (Figs.
89, 102). These leaves are the rule in monocotyledonous plants.
The venation of netted-veined leaves is =pinnate= or feather-like
when the veins arise from the side of a continuous midrib (Fig.
91); =palmate= or =digitate= (hand-like) when the veins arise from
the apex of the petiole (Figs. 88, 92). If leaves were divided
between the main veins, the former would be pinnately and the latter
digitately compound.
[Illustration: FIG. 91.—COMPLETE LEAVES OF WILLLOW.]
[Illustration: FIG. 92.—DIGITATE-VEINED PELTATE LEAF OF NASTURTIUM.]
[Illustration: FIG. 93.—PINNATELY COMPOUND LEAF OF ASH.]
It is customary to speak of a leaf as compound only when the parts or
branches are completely separate blades, as when the division extends
to the midrib (Figs. 90, 93, 94, 95). The parts or branches are known
as =leaflets=. Sometimes the leaflets themselves are compound, and
the whole leaf is then said to be =bi-compound= or =twice-compound=
(Fig. 90). Some leaves are three-compound, four-compound, or
five-compound. =Decompound= is a general term to express any degree
of compounding beyond twice-compound.
[Illustration: FIG. 94.—DIGITATELY COMPOUND LEAF OF RASPBERRY.]
[Illustration: FIG. 95.—POISON IVY. LEAF AND FRUIT.]
Leaves that are not divided as far as to the midrib are said to be:
[Illustration: FIG. 96.—LOBED LEAF OF SUGAR MAPLE.]
=lobed=, if the openings or sinuses are not more than half the depth
of the blade (Fig. 96);
=cleft=, if the sinuses are deeper than the middle;
[Illustration: FIG. 97.—DIGITATELY PARTED LEAVES OF BEGONIA.]
=parted=, if the sinuses reach two thirds or more to the midrib (Fig.
97);
=divided=, if the sinuses reach nearly or quite to the midrib.
The parts are called =lobes=, =divisions=, or =segments=, rather than
leaflets. The leaf may be pinnately or digitately lobed, parted,
cleft, or divided. A pinnately parted or cleft leaf is sometimes said
to be =pinnatifid=.
Leaves may have one or all of three parts—=blade=, or expanded
part; =petiole=, or stalk; =stipules=, or appendages at the base
of the petiole. A leaf that has all three of these parts is said
to be =complete= (Figs. 91, 106). The stipules are often green and
leaf-like and perform the function of foliage as in the pea and the
Japanese quince (the latter common in yards).
[Illustration: FIG. 98.—OBLONG-OVATE SESSILE LEAVES OF TEA.]
Leaves and leaflets that have no stalks are said to be =sessile=
(Figs. 98, 103), _i.e._ sitting. Find several examples. The same is
said of flowers and fruits. The blade of a sessile leaf may partly or
wholly surround the stem, when it is said to be =clasping=. Examples:
aster (Fig. 99), corn. In some cases the leaf runs down the stem,
forming a wing; such leaves are said to be =decurrent= (Fig. 100).
When opposite sessile leaves are joined by their bases, they are said
to be =connate= (Fig. 101).
[Illustration: FIG. 99.—CLASPING LEAF OF A WILD ASTER.]
Leaflets may have one or all of these three parts, but the stalks
of leaflets are called =petiolules= and the stipules of leaflets
are called =stipels=. The leaf of the garden bean has leaflets,
petiolules, and stipels.
[Illustration: FIG. 100.—DECURRENT LEAVES OF MULLEIN.]
The blade is usually attached to the petiole by _its lower edge_.
In pinnate-veined leaves, the petiole seems to continue through the
leaf as a =midrib= (Fig. 91). In some plants, however, the petiole
joins the blade inside or beyond the margin (Fig. 92). Such leaves
are said to be =peltate= or shield-shaped. This mode of attachment
is particularly common in floating leaves (_e.g._ the water lilies).
Peltate leaves are usually digitate-veined.
[Illustration: FIG. 101.—TWO PAIRS OF CONNATE LEAVES OF HONEYSUCKLE.]
=How to Tell a Leaf.=—It is often difficult to distinguish compound
leaves from leafy branches, and leaflets from leaves. As a rule
leaves can be distinguished by the following tests: (1) Leaves are
_temporary structures_, sooner or later falling. (2) Usually _buds
are borne in their axils_. (3) Leaves are usually _borne at joints or
nodes_. (4) They arise on wood of the _current year’s growth_. (5)
They have a more or less _definite arrangement_. When leaves fall,
the twig that bore them remains; when leaflets fall, the main petiole
or stalk that bore them also falls.
[Illustration: FIG. 102.—LINEAR-ACUMINATE LEAF OF GRASS.]
[Illustration: FIG. 103.—SHORT-OBLONG LEAVES OF BOX.]
=Shapes.=—Leaves and leaflets are infinitely variable in shape. Names
have been given to some of the more definite or regular shapes. These
names are a part of the language of botany. The names represent ideal
or typical shapes; there are no two leaves alike and very few that
perfectly conform to the definitions. The shapes are likened to those
of familiar objects or of geometrical figures. Some of the commoner
shapes are as follows (name original examples in each class):
[Illustration: (Linear leaf)]
=Linear=, several times longer than broad, with the sides nearly or
quite parallel. Spruces and most grasses are examples (Fig. 102). In
linear leaves, the main veins are usually parallel to the midrib.
[Illustration: (Oblong leaf)]
=Oblong=, twice or thrice as long as broad, with the sides parallel
for most of their length. Fig. 103 shows the short-oblong leaves of
the box, a plant that is used for permanent edgings in gardens.
[Illustration: FIG. 104.—ELLIPTIC LEAF OF PURPLE BEECH.]
[Illustration: (Elliptic leaf)]
=Elliptic= differs from the oblong in having the sides gradually
tapering to either end from the middle. The European beech (Fig. 104)
has elliptic leaves. (This tree is often planted in this country.)
[Illustration: (Lanceolate leaf)]
=Lanceolate=, four to six times longer than broad, widest below
the middle, and tapering to either end. Some of the narrow-leaved
willows are examples. Most of the willows and the peach have
oblong-lanceolate leaves.
[Illustration: (Spatulate leaf)]
=Spatulate=, a narrow leaf that is broadest toward the apex. The top
is usually rounded.
[Illustration: FIG. 105.—OVATE SERRATE LEAF OF HIBISCUS.]
[Illustration: FIG. 106.—LEAF OF APPLE, showing blade, petiole, and
small narrow stipules.]
[Illustration: (Ovate leaf)]
=Ovate=, shaped somewhat like the longitudinal section of an egg:
about twice as long as broad, tapering from near the base to the
apex. This is one of the commonest leaf forms (Figs. 105, 106).
[Illustration: (Obovate leaf)]
=Obovate=, ovate inverted,—the wide part towards the apex. Leaves of
mullein and leaflets of horse-chestnut and false indigo are obovate.
This form is commonest in leaflets of digitate leaves: why?
[Illustration: (Reniform leaf)]
=Reniform=, kidney-shaped. This form is sometimes seen in wild
plants, particularly in root-leaves. Leaves of wild ginger are nearly
reniform.
[Illustration: (Orbicular leaf)]
=Orbicular=, circular in general outline. Very few leaves are
perfectly circular, but there are many that are nearer circular than
of any other shape. (Fig. 107).
[Illustration: FIG. 107.—ORBICULAR LOBED LEAVES.]
[Illustration: FIG. 108.—TRUNCATE LEAF OF TULIP TREE.]
The shape of many leaves is described in combinations of these terms:
as =ovate-lanceolate=, =lanceolate-oblong=.
The shape of the base and the apex of the leaf or leaflet is often
characteristic. The base may be =rounded= (Fig. 104), =tapering=
(Fig. 93), =cordate= or heart-shaped (Fig. 105), =truncate= or
squared as if cut off. The apex may be blunt or =obtuse=, =acute=
or sharp, =acuminate= or long-pointed, =truncate= (Fig. 108). Name
examples.
The shape of the margin is also characteristic of each kind of leaf.
The margin is =entire= when it is not indented or cut in any way
(Figs. 99, 103). When not entire, it may be =undulate= or wavy (Fig.
92), =serrate= or saw-toothed (Fig. 105), =dentate= or more coarsely
notched (Fig. 95), =crenate= or round-toothed, =lobed=, and the like.
Give examples.
[Illustration: FIG. 109.—DIFFERENT FORMS OF LEAVES FROM ONE PLANT OF
AMPELOPSIS.]
Leaves on the same plant often differ greatly in form. Observe the
different shapes of leaves on the young growths of mulberries (Fig.
2) and wild grapes; also on vigorous squash and pumpkin vines. In
some cases there may be simple and compound leaves on the same plant.
This is marked in the so-called Boston ivy or ampelopsis (Fig. 109),
a vine that is used to cover brick and stone buildings. Different
degrees of compounding, even in the same leaf, may often be found in
honey locust. Remarkable differences in forms are seen by comparing
seed-leaves with mature leaves of any plant (Fig. 30).
=The Leaf and its Environment.=—The form and shape of the leaf
often have direct relation to the _place in which the leaf grows_.
_Floating leaves are usually expanded and flat_, and the petiole
varies in length with the depth of the water. _Submerged leaves
are usually linear or thread-like_, or are cut into very narrow
divisions: thereby more surface is exposed, and possibly the leaves
are less injured by moving water. Compare the sizes of the leaves on
the ends of branches with those at the base of the branches or in
the interior of the tree top. In dense foliage masses, the petioles
of the lowermost or undermost leaves _tend to elongate_—to push the
leaf to the light.
On the approach of winter the leaf usually ceases to work, and dies.
It may drop, when it is said to be =deciduous=; or it may remain on
the plant, when it is said to be =persistent=. If persistent leaves
remain green during the winter, the plant is said to be =evergreen=.
Give examples in each class. Most leaves fall by breaking off at the
lower end of the petiole with a _distinct joint or articulation_.
There are many leaves, however, that wither and hang on the plant
until torn off by the wind; of such are the leaves of grasses,
sedges, lilies, orchids, and other plants of the monocotyledons. Most
leaves of this character are parallel-veined.
_Leaves also die and fall from lack of light._ Observe the yellow and
weak leaves in a dense tree top or in any thicket. Why do the lower
leaves die on house plants? Note the carpet of needles under the
pines. All evergreens shed their leaves after a time. Counting back
from the tip of a pine or spruce shoot, determine how many years the
leaves persist. In some spruces a few leaves may be found on branches
ten or more years old.
=Arrangement of Leaves.=—Most leaves have a regular _position or
arrangement_ on the stem. _This position or direction is determined
largely by exposure to sunlight._ In temperate climates they usually
hang in such a way that they receive the greatest amount of light.
One leaf shades another to the least possible degree. If the plant
were placed in a new position with reference to light, the leaves
would make an effort to turn their blades.
_When leaves are_ =opposite= _the pairs usually alternate._ That is,
if one pair stands north and south, the next pair stands east and
west. See the box elder shoot, on the left in Fig. 110. _One pair
does not shade the pair beneath._ The leaves are in four vertical
ranks.
[Illustration: FIG. 110.—PHYLLOTAXY OF BOX ELDER, ELM, APPLE.]
_There are several kinds of_ =alternate= _arrangement._ In the elm
shoot, in Fig. 110, the third bud is vertically above the first. This
is true no matter which bud is taken as the starting point. Draw a
thread around the stem until the two buds are joined. Set a pin at
each bud. Observe that two buds are passed (not counting the last)
and that the thread makes one circuit of the stem. Representing the
number of buds by a denominator, and the number of circuits by a
numerator, we have the fraction ½, _which expresses the part of the
circle that lies between any two buds_. That is, the buds are one
half of 360 degrees apart, or 180 degrees. Looking endwise at the
stem, the leaves are seen to be 2-ranked. Note that in the apple
shoot (Fig. 110, right) the thread makes two circuits and five buds
are passed: _two fifths represents the divergence between the buds_.
The leaves are 5-ranked.
[Illustration: FIG. 111.—PHYLLOTAXY OF THE POTATO TUBER. Work it out
on a fresh long tuber.]
_Every plant has its own arrangement of leaves._ For opposite leaves,
see maple, box elder, ash, lilac, honeysuckle, mint, fuchsia. For
2-ranked arrangement, see all grasses, Indian corn, basswood, elm.
For 3-ranked arrangement, see all sedges. For 5-ranked (which is
one of the commonest), see apple, cherry, pear, peach, plum, poplar,
willow. For 8-ranked, see holly, osage orange, some willows. More
complicated arrangements occur in bulbs, house leeks, and other
condensed plants. The buds or “eyes” on a potato tuber, which is
an underground stem (why?), show a spiral arrangement (Fig. 111).
_The arrangement of leaves on the stem is known as_ =phyllotaxy=
(literally, “leaf arrangement”). Make out the phyllotaxy on six
different plants nearest the schoolhouse door.
In some plants, several leaves occur at one level, being arranged in
a circle around the stem. Such leaves are said to be =verticillate=,
or =whorled=. Leaves arranged in this way are usually narrow: why?
Although a definite arrangement of leaves is the rule in most
plants, _it is subject to modification_. On shoots that receive the
light only from one side or that grow in difficult positions, the
arrangement may not be definite. Examine shoots that grow on the
under side of dense tree tops or in other partially lighted positions.
SUGGESTIONS.—=55.= The pupil should match leaves to determine
whether any two are alike. Why? Compare leaves from the same
plant in size, shape, colour, form of margin, length of petiole,
venation, texture (as to thickness or thinness), stage of maturity,
smoothness or hairiness. =56.= Let the pupil take an average leaf
from each of the first ten different kinds of plants that he meets
and compare them as to the above points (in Exercise 55), and
also name the shapes. Determine how the various leaves resemble
and differ. =57.= Describe the stipules of rose, apple, fig,
willow, violet, pea, or others. =58.= In what part of the world
are parallel-veined leaves the more common? =59.= Do you know of
parallel-veined leaves that have lobed or dentate margins? =60.=
What becomes of dead leaves? =61.= Why is there no grass or other
undergrowth under pine and spruce trees? =62.= Name several leaves
that are useful for decorations. Why are they useful? =63.= What
trees in your vicinity are most esteemed as shade trees? What is
the character of their foliage? =64.= Why are the internodes so
long in water-sprouts and suckers? =65.= How do foliage characters
in corn or sorghum differ when the plants are grown in rows or
broadcast? Why? =66.= Why may removal of half the plants increase
the yield of cotton or sugar-beets or lettuce? =67.= How do leaves
curl when they wither? Do different leaves behave differently in
this respect? =68.= What kinds of leaves do you know to be eaten by
insects? By cattle? By horses? What kinds are used for human food?
=69.= How would you describe the shape of leaf of peach? apple?
elm? hackberry? maple? sweet-gum? corn? wheat? cotton? hickory?
cowpea? strawberry? chrysanthemum? rose? carnation? =70.= Are any
of the fore-going leaves compound? How do you describe the shape of
a compound leaf? =71.= How many sizes of leaves do you find on the
bush or tree nearest the schoolroom door? =72.= How many colours or
shades? =73.= How many lengths of petioles? =74.= Bring in all the
shapes of leaves that you can find.
[Illustration: FIG. 112.—COWPEA. Describe the leaves. For what is the
plant used?]
CHAPTER XII
LEAVES—STRUCTURE OR ANATOMY
Besides the =framework=, or system of veins found in blades of all
leaves, there is a soft cellular tissue called =mesophyll=, or =leaf
parenchyma=, and an =epidermis= or skin that covers the entire
outside part.
[Illustration: FIG. 113.—SECTION OF A LEAF, showing the air-spaces.
Breathing-pore or stoma at _a_. The palisade cells which chiefly
contain the chlorophyll are at _b_. Epidermal cells at _c_.]
=Mesophyll=.—The mesophyll is _not all alike or homogeneous_. The
upper layer is composed of elongated cells placed perpendicular to
the surface of the leaf. These are called =palisade cells=. These
cells are usually filled with green bodies called =chlorophyll
grains=. The grain contains a great number of chlorophyll drops
imbedded in the protoplasm. Below the palisade cells is the spongy
=parenchyma=, composed of cells more or less spherical in shape,
irregularly arranged, and provided with many intercellular air
cavities (Fig. 113). In leaves of some plants exposed to strong
light there may be more than one layer of palisade cells, as in the
India-rubber plant and the oleander. Ivy when grown in bright light
will develop two such layers of cells, but in shaded places it may
be found with only one. Such plants as iris and compass plant, which
have both surfaces of the leaf equally exposed to sunlight, usually
have a palisade layer beneath each epidermis.
=Epidermis.=—The outer or epidermal cells of leaves do not bear
chlorophyll, but are usually so transparent that the green mesophyll
can be seen through them. They often become very thick-walled, and
are in most plants devoid of all protoplasm except a thin layer
lining the walls, the cavities being filled with cell sap. This sap
is sometimes coloured, as in the under epidermis of begonia leaves.
It is not common to find more than one layer of epidermal cells
forming each surface of a leaf. The epidermis _serves to retain
moisture_ in the leaf and as a general _protective covering_. In
desert plants the epidermis, as a rule, is very thick and has a dense
cuticle, thereby preventing loss of water.
There are various _outgrowths of the epidermis_. =Hairs= are the
chief of these. They may be (1) =simple=, as on primula, geranium,
nægelia; (2) =once branched=, as on wall-flower; (3) =compound=,
as on verbascum or mullein; (4) =disk-like=, as on shepherdia; (5)
=stellate=, or star-shaped, as in certain crucifers. In some cases
the hairs are =glandular=, as in Chinese primrose of the greenhouses
(_Primula Sinensis_) and certain hairs of pumpkin flowers. The hairs
often protect the breathing-pores, or stomates, from dust and water.
=Stomates= (sometimes called =breathing-pores=) _are small openings
or pores_ in the epidermis of leaves and soft stems that allow the
passage of air and other gases and vapours (_stomate_ or _stoma_,
singular; _stomates_ or _stomata_, plural). They are _placed near the
large intercellular spaces_ of the mesophyll, usually in positions
least affected by direct sunlight. Fig. 114 shows the structure.
There are =two guard-cells= at the mouth of each stomate, which may
in most cases open or close the passage as the conditions of the
atmosphere may require. The guard-cells contain chlorophyll. In Fig.
115 is shown a case in which there are compound guard-cells, that
of ivy. On the margins of certain leaves, as of fuchsia, impatiens,
cabbage, are openings known as =water-pores=.
[Illustration: FIG. 114.—DIAGRAM OF STOMATE OF IRIS (Osterhout).]
[Illustration: FIG. 115.—STOMATE OF IVY, showing compound
guard-cells.]
_Stomates are very numerous_, as will be seen from the numbers
showing the pores to each square inch of leaf surface:
Lower surface Upper surface
Peony 13,790 None
Holly 63,600 None
Lilac 160,000 None
Mistletoe 200 200
Tradescantia 2,000 2,000
Garden Flag (iris) 11,572 11,572
[Illustration: FIG. 116.—STOMATES OF GERANIUM LEAF.]
The arrangement of stomates on the leaf _differs with each kind of
plant_. Fig. 116 shows stomates and also the outlines of contiguous
epidermal cells.
[Illustration: FIG. 117.—LENTICELS ON YOUNG SHOOT OF RED OSIER
(CORNUS).]
The function or work of the stomates is to _regulate the passage of
gases_ into and out of the plant. The directly active organs or parts
are guard-cells, on either side the opening. One method of opening is
as follows: The thicker walls of the guard-cells (Fig. 114) absorb
water from adjacent cells, these thick walls buckle or bend and part
from one another at their middles on either side the opening, causing
the stomate to open, when the air gases may be taken in and the leaf
gases may pass out. When moisture is reduced in the leaf tissue,
the guard-cells part with some of their contents, the thick walls
straighten, and the faces of the two opposite ones come together,
thus closing the stomate and preventing any water vapour from passing
out. _When a leaf is actively at work making new organic compounds,
the stomates are usually open; when unfavourable conditions arise,
they are usually closed._ They also commonly close at night, when
growth (or the utilizing of the new materials) is most likely to be
active. It is sometimes safer to fumigate greenhouses and window
gardens at night, for the noxious vapours are less likely to enter
the leaf. Dust may clog or cover the stomates. Rains benefit plants
by washing the leaves as well as by providing moisture to the roots.
=Lenticels=.—On the young woody twigs of many plants (marked in
osiers, cherry, birch) there are small corky spots or elevations
known as =lenticels= (Fig. 117). They mark the location of some
loose cork cells that function as stomates, for _green shoots_, as
well as leaves, take in and discharge gases; that is, soft green
twigs _function as leaves_. Under some of these twig stomates,
corky material may form and the opening is torn and enlarged: _the
lenticels are successors to the stomates_. The stomates lie in the
epidermis, but as the twig ages the epidermis perishes and the
bark becomes the external layer. _Gases continue to pass in and out
through the lenticels_, until the branch becomes heavily covered
with thick, corky bark. With the growth of the twig, the lenticel
scars enlarge lengthwise or crosswise or assume other shapes, often
becoming characteristic markings.
=Fibro-vascular Bundles.=—We have studied the fibro-vascular
bundles of stems (Chap. X). These stem bundles _continue into the
leaves, ramifying into the veins_, carrying the soil water inwards
and bringing, by diffusion, the elaborated food out through the
sieve-cells. Cut across a petiole and notice the hard spots or areas
in it; strip these parts lengthwise of the petiole. What are they?
=Fall of the Leaf.=—In most common deciduous plants, when the
season’s work for the leaf is ended, the nutritious matter may be
withdrawn, and _a layer of corky cells is completed over the surface
of the stem where the leaf is attached_. _The leaf soon falls._ It
often falls even before it is killed by frost. Deciduous leaves
begin to show the surface line of articulation in the early growing
season. This articulation may be observed at any time during the
summer. The area of the twig once covered by the petioles is called
the =leaf-scar= after the leaf has fallen. In Chap. XV are shown a
number of leaf-scars. In the plane tree (sycamore or buttonwood), the
leaf-scar is in the form of a ring surrounding the bud, for the bud
is covered by the hollowed end of the petiole; the leaf of sumac is
similar. Examine with a hand lens leaf-scars of several woody plants.
Note the number of bundle-scars in each leaf-scar. Sections may be
cut through a leaf-scar and examined with the microscope. Note the
character of cells that cover the leaf-scar surface.
SUGGESTIONS.—_To study epidermal hairs_: =75.= For this study, use
the leaves of any hairy or woolly plant. A good hand lens will
reveal the identity of many of the coarser hairs. A dissecting
microscope will show them still better. For the study of the cell
structure, a compound microscope is necessary. Cross-sections may
be made so as to bring hairs on the edge of the sections; or in
some cases the hairs may be peeled or scraped from the epidermis
and placed in water on a slide. Make sketches of the different
kinds of hairs. =76.= It is good practice for the pupil to describe
leaves in respect to their covering: Are they smooth on both
surfaces? Or hairy? Woolly? Thickly or thinly hairy? Hairs long or
short? Standing straight out or lying close to the surface of the
leaf? Simple or branched? Attached to the veins or to the plane
surface? Colour? Most abundant on young leaves or old? =77.= Place
a hairy or woolly leaf under water. Does the hairy surface appear
silvery? Why? _Other questions_: =78.= Why is it good practice to
wash the leaves of house plants? =79.= Describe the leaf-scars
on six kinds of plants: size, shape, colour, position with
reference to the bud, bundle-scars. =80.= Do you find leaf-scars on
monocotyledonous plants—corn, cereal grains, lilies, canna, banana,
palm, bamboo, green brier? =81.= Note the table on page =88.= Can
you suggest a reason why there are equal numbers of stomates on
both surfaces of leaves of tradescantia and flag, and none on upper
surface of other leaves? Suppose you pick a leaf of lilac (or some
larger leaf), seal the petiole with wax and then rub the under
surface with vaseline; on another leaf apply the vaseline to the
upper surface; which leaf withers first, and why? Make a similar
experiment with iris or blue flag. =82.= Why do leaves and shoots
of house plants turn towards the light? What happens when the
plants are turned around? =83.= Note position of leaves of beans,
clover, oxalis, alfalfa, locust, at night.
CHAPTER XIII
LEAVES—FUNCTION OR WORK
We have discussed (in Chap. VIII) the work or function of roots and
also (in Chap. X) the function of stems. We are now ready to complete
the view of the main vital activities of plants by considering the
function of the green parts (leaves and young shoots).
=Sources of Food.=—The ordinary green plant has but _two sources
from which to secure food,—the air and the soil_. When a plant is
thoroughly dried in an oven, the water passes off; _this water came
from the soil_. The remaining part is called the =dry substance=
or =dry matter=. If the dry matter is burned in an ordinary fire,
only the =ash= remains; _this ash came from the soil_. The part that
passed off as gas in the burning _contained the elements that came
from the air_; it also contained some of those that came from the
soil—all those (as nitrogen, hydrogen, chlorine) that are transformed
into gases by the heat of a common fire. The part that comes from
the soil (the ash) is small in amount, being considerably less than
10 per cent and sometimes less than 1 per cent. _Water is the most
abundant single constituent or substance of plants._ In a corn plant
of the roasting-ear stage, about 80 per cent of the substance is
water. A fresh turnip is over 90 per cent water. Fresh wood of the
apple tree contains about 45 per cent of water.
=Carbon=.—_Carbon enters abundantly into the composition of all
plants._ Note what happens when a plant is burned without free
access of air, or smothered, as in a charcoal pit. _A mass of
charcoal remains, almost as large as the body of the plant._ Charcoal
is almost pure _carbon_, the ash present being so small in proportion
to the large amount of carbon that we look on the ash as an impurity.
Nearly half of the dry substance of a tree is carbon. Carbon goes
off as _a gas_ when the plant is _burned in air_. It does not go off
alone, but in combination with oxygen in the form of _carbon dioxide_
gas, CO_{2}.
=The green plant secures its carbon from the air.= In other words,
much of the _solid matter_ of the plant comes from _one of the
gases of the air_. By volume, _carbon dioxide forms only a small
fraction of 1 per cent. of the air_. It would be very disastrous to
animal life, however, if this percentage were much increased, for it
excludes the life-giving oxygen. Carbon dioxide is often called “foul
gas.” It may accumulate in old wells, and an experienced person will
not descend into such wells until they have been tested with a torch.
If the air in the well will not support combustion,—that is, if the
torch is extinguished,—it usually means that carbon dioxide has
drained into the place. The air of a closed schoolroom often contains
far too much of this gas, along with little solid particles of waste
matters. Carbon dioxide is often known as carbonic acid gas.
=Appropriation of the Carbon.=—_The carbon dioxide of the air readily
diffuses itself into the leaves and other green parts of the plant._
The leaf is delicate in texture, and when very young the air can
diffuse directly into the tissues. The stomates, however, are the
special inlets adapted for the admission of gases into the leaves and
other green parts. Through these stomates, or diffusion-pores, the
outside air enters into the air-spaces of the plant, and is finally
absorbed by the little cells containing the living matter.
=Chlorophyll= (“leaf green”) is the agent that secures the energy
by means of which carbon dioxide is utilized. This material is
contained in the leaf cells in the form of grains (p. 86); the
grains themselves are protoplasm, only the colouring matter being
chlorophyll. The _chlorophyll bodies or grains are often most
abundant near the upper surface of the leaf, where they can secure
the greatest amount of light_. Without this green colouring matter,
there would be no reason for the large flat surfaces which the
leaves possess, and no reason for the fact that the leaves are borne
most abundantly at the ends of branches, where the light is most
available. Plants with coloured leaves as coleus, have chlorophyll,
but it is masked by other colouring matter. This other colouring
matter is usually soluble in hot water: boil a coleus leaf and notice
that it becomes green and the water becomes coloured.
_Plants grown in darkness are yellow and slender, and do not reach
maturity._ Compare the potato sprouts that have grown from a tuber
lying in a dark cellar with those that have grown normally in
the bright light. The shoots have become slender, and are devoid
of chlorophyll; and when the food that is stored in the tuber is
exhausted these shoots will have lived useless lives. A plant that
has been grown in darkness from the seed will soon die, although for
a time the little seedling will grow very tall and slender. Why?
_Light favours the production of chlorophyll_, and the chlorophyll is
the agent in the making of _the organic carbon compounds_. Sometimes
chlorophyll is found in buds and seeds, but in most cases these
places are not perfectly dark. Notice how potato tubers develop
chlorophyll, or become green, when exposed to light.
=Photosynthesis.=—_Carbon dioxide diffuses into the leaf; during
sunlight it is used, and oxygen is given off._ How the carbon
dioxide which is thus absorbed may be used in making an organic food
is a complex question, and need not be studied here; but it may be
stated that carbon dioxide and water are the constituents. Complex
compounds are built up out of simpler ones.
_Chlorophyll absorbs certain light rays, and the energy thus directly
or indirectly obtained is used by the living matter in uniting the
carbon dioxide absorbed from the air with some of the water brought
up from the roots. The ultimate result usually is starch._ The
process is obscure, but sugar is generally one step; and our first
definite knowledge of the product begins when starch is deposited
in the leaves. The process of using the carbon dioxide of the air
has been known as carbon assimilation, but the term now most used is
=photosynthesis= (from two Greek words meaning _light_ and _placing
together_.)
=Starch and Sugar.=—_All starch is composed of carbon, hydrogen,
and oxygen_ (C_{6}H_{10}O_{5})_{_n_}. The sugars and the substance
of cell walls are very similar to it in composition. All these
substances are called =carbohydrates=. In making fruit sugar from the
carbon and oxygen of carbon dioxide and from the hydrogen and oxygen
of the water, _there is a surplus of oxygen_ (6 parts CO_{2} + 6
parts H_{2}O = C_{6}H_{12}O_{6} + 6 O_{2}). It is this oxygen that is
given off into the air during sunlight.
=Digestion.=—_Starch is in the form of insoluble granules. When such
food material is carried from one part of the plant to another for
purposes of growth or storage, it is made soluble before it can be
transported._ When this starchy material is transferred from place to
place, it is usually changed into sugar by the action of a diastase.
_This is a process of_ =digestion=. It is much like the change of
starchy foodstuffs to sugary foods effected by the saliva.
[Illustration: FIG. 118.—TRUNK GIRDLED BY A WIRE. See Fig. 85.]
=Distribution of the Digested Food.=—After being changed to the
soluble form, _this material is ready to be used in growth_, either
in the leaf, in the stem, or in the roots. With other more complex
products it is then _distributed throughout all the growing parts of
the plant_; and when passing down to the root, it seems to pass more
readily through the _inner bark_, in plants which have a definite
bark. This gradual downward diffusion through the inner bark of
materials suitable for growth is the process referred to when the
“descent of sap” is mentioned. Starch and other products are often
_stored in one growing season to be used in the next season_. If a
tree is constricted or strangled by a wire around its trunk (Fig.
118), the digested food cannot readily pass down and it is stored
above the girdle, causing an enlargement.
=Assimilation.=—_The food from the air and that from the soil unite
in the living tissues._ The “sap” that passes upwards from the roots
in the growing season is made up largely of the soil water and the
salts which have been absorbed in the diluted solutions (p. 67). This
upward-moving water is conducted largely through certain tubular
canals of the _young wood_. These cells are never continuous tubes
from root to leaf; but the water passes readily from one cell or
canal to another in its upward course.
The upward-moving water gradually passes to the growing parts,
and everywhere in the living tissues, it is, of course, in the
most intimate contact with the soluble carbohydrates and products
of photosynthesis. In the building up or reconstructive and other
processes it is therefore available. We may properly conceive of
certain of the simpler organic molecules as passing through a series
of changes, gradually increasing in complexity. There will be formed
substances containing nitrogen in addition to carbon, hydrogen, and
oxygen. Others will contain also sulphur and phosphorus, and the
various processes may be thought of as culminating in =protoplasm=.
_Protoplasm is the living matter in plants._ It is in the cells,
and is usually semi-fluid. Starch is not living matter. The complex
process of building up the protoplasm is called =assimilation=.
=Respiration.=—_Plants need oxygen for respiration, as animals
do._ We have seen that plants need the carbon dioxide of the air.
To most plants the nitrogen of the air is inert, and serves only
to dilute the other elements; but the _oxygen is necessary for all
life_. We know that all animals need this oxygen in order to breathe
or respire. In fact, they have become accustomed to it in just the
proportions found in the air; and this is now best for them. When
animals breathe the air once, they make it foul, because they use
some of the oxygen and give off carbon dioxide. Likewise, _all living
parts of the plant must have a constant supply of oxygen_. Roots
also need it, for they respire. Air goes in and out of the soil by
diffusion, and as the soil is heated and cooled, causing the air to
expand and contract.
The oxygen passes into the air-spaces and is absorbed by the moist
cell membranes. In the living cells it makes possible the formation
of simpler compounds by which energy is released. This energy enables
the plant to work and grow, and the final products of this action
are _carbon dioxide and water_. As a result of the use of this oxygen
by night and by day, plants give off carbon dioxide. _Plants respire;
but since they are stationary, and more or less inactive, they do not
need so much oxygen as animals do, and they do not give off so much
carbon dioxide_. A few plants in a sleeping room need not disturb
one more than a family of mice. It should be noted, however, that
germinating seeds respire vigorously, hence they consume much oxygen;
and opening buds and flowers are likewise active.
=Transpiration.=—Much more water is absorbed by the roots than is
used in growth, _and this surplus water passes from the leaves into
the atmosphere by an evaporation process known as_ =transpiration=.
Transpiration takes place more abundantly from the under surfaces
of leaves, and through the pores or stomates. A sunflower plant of
the height of a man, during an active period of growth, gives off a
quart of water per day. A large oak tree may transpire 150 gallons
per day during the summer. For every ounce of dry matter produced, it
is estimated that 15 to 25 pounds of water usually passes through the
plant.
_When the roots fail to supply to the plant sufficient water
to equalize that transpired by the leaves_, =the plant wilts=.
Transpiration from the leaves and delicate shoots is increased
by all the conditions which increase evaporation, such as higher
temperature, dry air, or wind. The stomata open and close, tending to
regulate transpiration as the varying conditions of the atmosphere
affect the moisture content of the plant. However, in periods of
drought or of very hot weather, and especially during a hot wind, the
closing of these stomates cannot sufficiently prevent evaporation.
The roots may be very active and yet fail to absorb sufficient
moisture to equalize that given off by the leaves. The plant shows
the effect (how?). On a hot dry day, note how the leaves of corn
“roll” towards afternoon. Note how fresh and vigorous the same leaves
appear early the following morning. Any injury to the roots, such as
a bruise, or exposure to heat, drought, or cold may cause the plant
to wilt.
Water is forced up by =root pressure= or =sap pressure=. (Exercise
99.) Some of the dew on the grass in the morning may be the water
forced up by the roots; some of it is the condensed vapour of the air.
_The wilting of a plant is due to the loss of water from the cells._
The cell walls are soft, and collapse. A toy balloon will not stand
alone until it is inflated with air or liquid. In the woody parts of
the plant the cell walls may be stiff enough to support themselves,
even though the cell is empty. Measure the contraction due to wilting
and drying by tracing a fresh leaf on page of notebook, and then
tracing the same leaf after it has been dried between papers. The
softer the leaf, the greater will be the contraction.
=Storage.=—We have said that starch may be stored in twigs to be used
the following year. The very early flowers on fruit trees, especially
those that come before the leaves, and those that come from bulbs,
as crocuses and tulips, are supported by the starch or other food
that was organized the year before. Some plants have very special
storage reservoirs, as the potato, in this case being a thickened
stem although growing underground. (Why a thickened stem? p. 84.) It
is well to make the starch test on winter twigs and on all kinds of
thickened parts, as tubers and bulbs.
[Illustration: FIG. 119.—THE COMMON PITCHER PLANT (_Sarracenia
purpurea_) showing the tubular leaves and the odd, long-stalked
flowers.]
=Carnivorous Plants.=—Certain plants capture insects and other very
small animals and utilize them to some extent as food. Such are the
sundew, which has on the leaves sticky hairs that close over the
insect; the Venus’s fly-trap of the Southern States, in which the
halves of the leaves close over the prey like the jaws of a steel
trap; and the various kinds of pitcher plants that collect insects
and other organic matter in deep, water-filled, flask-like leaf
pouches (Fig. 119).
The sundew and the Venus’s fly-trap are sensitive to contact. Other
plants are _sensitive to the touch_ without being insectivorous. The
common cultivated sensitive plant is an example. This is readily
grown from seeds (sold by seedsmen) in a warm place. Related wild
plants in the south are sensitive. The utility of this sensitiveness
is not understood.
=Parts that Simulate Leaves.=—We have learned that leaves are
endlessly modified to suit the conditions in which the plant is
placed. The most marked modifications are in adaptation to light.
On the other hand, _other organs often perform the functions of
leaves_. Green shoots function as leaves. These shoots may look like
leaves, in which case they are called _cladophylla_. The foliage of
common asparagus is made up of fine branches: the real morphological
leaves are the minute dry functionless scales at the bases of these
branchlets. (What reason is there for calling them leaves?) The broad
“leaves” of the florist’s smilax are cladophylla. Where are the
leaves on this plant? In most of the cacti, the entire plant body
performs the functions of leaves until the parts become cork-bound.
=Leaves are sometimes modified to perform other functions than the
vital processes=: they may be tendrils, as the terminal leaflets of
pea and sweet pea; or spines, as in barberry. Not all spines and
thorns, however, represent modified leaves: some of them (as of
hawthorns, osage orange, honey locust) are branches.
[Illustration: FIG. 120.—EXCLUDING LIGHT AND CO_{2} FROM PART OF A
LEAF.]
[Illustration: FIG. 121.—THE RESULT.]
SUGGESTIONS.—_To test for chlorophyll._ =84.= Purchase about a
gill of wood alcohol. Secure a leaf of geranium, clover, or other
plant that has been exposed to sunlight for a few hours, and,
after dipping it for a minute in boiling water, put it in a white
cup with sufficient alcohol to cover. Place the cup in a shallow
pan of hot water on the stove where it is not hot enough for the
alcohol to take fire. After a time the chlorophyll is dissolved
by the alcohol which has become an intense green. Save this leaf
for the starch experiment (Exercise 85). Without chlorophyll, the
plant cannot appropriate the carbon dioxide of the air. _Starch
and photosynthesis._ =85.= Starch is present in the green leaves
which have been exposed to sunlight; but in the dark no starch can
be formed from carbon dioxide. Apply iodine to the leaf from which
the chlorophyll was dissolved in the previous experiment. Note that
the leaf is coloured purplish-brown throughout. The leaf contains
starch. =86.= Secure a leaf from a plant which has been in the
dark for about two days. Dissolve the chlorophyll as before, and
attempt to stain this leaf with iodine. No purplish-brown colour is
produced. This shows that the starch manufactured in the leaf may
be entirely removed during darkness. =87.= Secure a plant which has
been kept in darkness for twenty-four hours or more. Split a small
cork and pin the two halves on opposite sides of one of the leaves,
as shown in Fig. 120. Place the plant in the sunlight again. After
a morning of bright sunshine dissolve the chlorophyll in this leaf
with alcohol; then stain the leaf with the iodine. Notice that the
leaf is stained deeply except where the cork was; there sunlight
and carbon dioxide were excluded, Fig. 121. There is no starch
in the covered area. =88.= Plants or parts of plants that have
developed no chlorophyll can form no starch. Secure a variegated
leaf of coleus, ribbon grass, geranium, or of any plant showing
both white and green areas. On a day of bright sunshine, test one
of these leaves by the alcohol and iodine method for the presence
of starch. Observe that the parts devoid of green colour have
formed no starch. However, after starch has once been formed in the
leaves, it may be changed into soluble substances and removed, to
be again converted into starch in certain other parts of the living
tissues. _To test the giving off of oxygen by day._ =89.= Make the
experiment illustrated in Fig. 122. Under a funnel in a deep glass
jar containing fresh spring or stream water place fresh pieces
of the common waterweed elodea (or anacharis). Have the funnel
considerably smaller than the vessel, and support the funnel well
up from the bottom so that the plant can more readily get all the
carbon dioxide available in the water. Why would boiled water be
undesirable in this experiment? For a home-made glass funnel, crack
the bottom off a narrow-necked bottle by pressing a red-hot poker
or iron rod against it and leading the crack around the bottle.
Invert a test-tube over the stem of the funnel. In sunlight bubbles
of oxygen will arise and collect in the test-tube. If a sufficient
quantity of oxygen has collected, a lighted taper inserted in the
tube will glow with a brighter flame, showing the presence of
oxygen in greater quantity than in the air. Shade the vessel. Are
bubbles given off? For many reasons it is impracticable to continue
this experiment longer than a few hours. =90.= A simpler experiment
may be made if one of the waterweeds Cabomba (water-lily family) is
available. Tie a number of branches together so that the basal ends
shall make a small bundle. Place these in a large vessel of spring
water, and insert a test-tube of water as before over the bundle.
The bubbles will arise from the cut surfaces. Observe the bubbles
on pond scum and waterweeds on a bright day. _To illustrate the
results of respiration_ (CO_{2}).
[Illustration: FIG. 122.—TO SHOW THE ESCAPE OF OXYGEN.]
[Illustration: FIG. 123.—TO ILLUSTRATE A PRODUCT OF RESPIRATION.]
[Illustration: FIG. 124.—RESPIRATION OF THICK ROOTS.]
[Illustration: FIG. 125.—TO ILLUSTRATE TRANSPIRATION.]
=91.= In a jar of germinating seeds (Fig. 123) place carefully a
small dish of limewater and cover tightly. Put a similar dish in
another jar of about the same air space. After a few hours compare
the cloudiness or precipitate in the two vessels of limewater.
=92.= Or, place a growing plant in a deep covered jar away from
the light, and after a few hours insert a lighted candle or
splinter. =93.= Or, perform a similar experiment with fresh roots
of beets or turnips (Fig. 124) from which the leaves are mostly
removed. In this case, the jar need not be kept dark; why? _To
test transpiration._ =94.= Cut a succulent shoot of any plant,
thrust the end of it through a hole in a cork, and stand it in a
small bottle of water. Invert over this a fruit jar, and observe
that a mist soon accumulates on the inside of the glass. In time
drops of water form. =95.= The experiment may be varied as shown in
Fig. 125. =96.= Or, invert the fruit jar over an entire plant, as
shown in Fig. 126, taking care to cover the soil with oiled paper
or rubber cloth to prevent evaporation from the soil. =97.= The
test may also be made by placing the pot, properly protected, on
balances, and the loss of weight will be noticed (Fig. 127).
[Illustration: FIG. 126.—TO ILLUSTRATE TRANSPIRATION.]
[Illustration: FIG. 127.—LOSS OF WATER.]
[Illustration: FIG. 128.—TO SHOW SAP PRESSURE.]
=98.= Cut a winter twig, seal the severed end with wax, and allow
the twig to lie several days. It shrivels. There must be some
upward movement of water even in winter, else plants would shrivel
and die. =99.= _To illustrate sap pressure._ The upward movement
of sap water often takes place under considerable force. The cause
of this force, known as _root pressure_, is not well understood.
The pressure varies with different plants and under different
conditions. To illustrate: cut off a strong-growing small plant
near the ground. By means of a bit of rubber tube attach a glass
tube with a bore of approximately the diameter of the stem. Pour in
a little water. Observe the rise of the water due to the pressure
from below (Fig 128). Some plants yield a large amount of water
under a pressure sufficient to raise a column several feet; others
force out little, but under considerable pressure (less easily
demonstrated). _The vital processes_ (_i.e._, the life processes).
=100.= The pupil having studied roots, stems, and leaves, should
now be able to describe the main vital functions of plants: what
is the root function? stem function? leaf function? =101.= What is
meant by the “sap”? =102.= Where and how does the plant secure its
water? oxygen? carbon? hydrogen? nitrogen? sulphur? potassium?
calcium? iron? phosphorus? =103.= Where is all the starch in the
world made? What does a starch-factory establishment do? Where are
the real starch factories? =104.= In what part of the twenty-four
hours do plants grow most rapidly in length? When is food formed
and stored most rapidly? =105.= Why does corn or cotton turn yellow
in a long rainy spell? =106.= If stubble, corn stalks, or cotton
stalks are burned in the field, is as much plant-food returned to
the soil as when they are ploughed under? =107.= What process of
plants is roughly analogous to perspiration of animals? =108.= What
part of the organic world uses raw mineral for food? =109.= Why
is earth banked over celery to blanch it? =110.= Is the amount of
water transpired equal to the amount absorbed? =111.= Give some
reasons why plants very close to a house may not thrive or may even
die. =112.= Why are fruit trees pruned or thinned out as in Fig.
129? _Proper balance between top and root._ =113.= We have learned
that the leaf parts and the root parts work together. They may
be said to balance each other in activities, the root supplying
the top and the top supplying the root (how?). If half the roots
were cut from a tree, we should expect to reduce the top also,
particularly if the tree is being transplanted. How would you
prune a tree or bush that is being transplanted? Fig. 130 may be
suggestive.
[Illustration: FIG. 130.—AN APPLE TREE, with suggestions as to
pruning when it is set in the orchard. At _a_ is shown a pruned top.]
[Illustration: FIG. 129.—BEFORE AND AFTER PRUNING.]
CHAPTER XIV
DEPENDENT PLANTS
Thus far we have spoken of plants that have roots and foliage and
that depend on themselves. They collect the raw materials and make
them over into assimilable food. They are =independent=. Plants
without green foliage cannot make food; they must have it made for
them or they die. They are =dependent=. A sprout from a potato tuber
in a dark cellar cannot collect and elaborate carbon dioxide. It
lives on the food stored in the tuber.
[Illustration: FIG. 131.—A MUSHROOM, example of a saprophytic plant.
This is the edible cultivated mushroom.]
_All plants with naturally white or blanched parts are dependent._
Their leaves do not develop. They live on organic matter—that which
has been made by a plant or elaborated by an animal. The dodder,
Indian pipe, beech drop, coral root among flower-bearing plants, also
mushrooms and other fungi (Figs. 131, 132) are examples. The dodder
is common in swales, being conspicuous late in the season from its
thread-like yellow or orange stems spreading over the herbage of
other plants. One kind attacks alfalfa and is a bad pest. The seeds
germinate in the spring, but as soon as the twining stem attaches
itself to another plant, the dodder dies away at the base and becomes
wholly dependent. It produces flowers in clusters and seeds itself
freely (Fig. 133).
[Illustration: FIG. 132.—A PARASITIC FUNGUS, magnified. The mycelium,
or vegetative part, is shown by the dotted-shaded parts ramifying in
the leaf tissue. The rounded haustoria projecting into the cells are
also shown. The long fruiting parts of the fungus hang from the under
surface of the leaf.]
=Parasites and Saprophytes.=—A plant that is dependent on a living
plant or animal is a =parasite=, and the plant or animal on which it
lives is the =host=. The dodder is a true parasite; so are the rusts,
mildews, and other fungi that attack leaves and shoots and injure
them.
[Illustration: FIG. 133.—DODDER IN FRUIT.]
The threads of a parasitic fungus usually creep through the
intercellular spaces in the leaf or the stem and send suckers (or
haustoria) into the cells (Fig. 132). The threads (or the hyphæ) clog
the air-spaces of the leaf and often plug the stomates, and they also
appropriate and disorganize the cell fluids; _thus they injure or
kill their host_. The mass of hyphæ of a fungus is called =mycelium=.
Some of the hyphæ finally grow out of the leaf and produce spores or
reproductive cells that answer the purpose of seeds in distributing
the plant (_b_, Fig. 132).
A plant that lives on dead or decaying matter is a =saprophyte=.
Mushrooms (Fig. 131) are examples; they live on the decaying matter
in the soil. Mould on bread and cheese is an example. Lay a piece
of moist bread on a plate and invert a tumbler over it. In a few
days it will be mouldy. The spores were in the air, or perhaps they
had already fallen on the bread but had not had opportunity to grow.
Most green plants are unable to make any direct use of the humus
or vegetable mould in the soil, for they are not saprophytic. The
shelf fungi (Fig. 134) are saprophytes. They are common on logs and
trees. Some of them are perhaps partially parasitic, extending the
mycelium into the wood of the living tree and causing it to become
black-hearted (Fig. 134).
[Illustration: FIG. 134.—TINDER FUNGUS (_Polyporus igniarius_) on
beech log. The external part of the fungus is shown below; the
heart-rot injury above.]
Some parasites spring from the ground, as other plants do, but they
are _parasitic on the roots of their hosts_. Some parasites may be
_partially parasitic_ and _partially saprophytic_. Many (perhaps
most) of these ground saprophytes are aided in securing their food by
soil fungi, which spread their delicate threads over the root-like
branches of the plant and act as intermediaries between the food and
the saprophyte. These fungus-covered roots are known as =mycorrhizas=
(meaning “fungus root”). Mycorrhizas are not peculiar to saprophytes.
They are found on many wholly independent plants, as, for example,
the heaths, oaks, apples, and pines. It is probable that the fungous
threads perform some of the offices of root-hairs to the host. On the
other hand, the fungus obtains some nourishment from the host. The
association seems to be mutual.
[Illustration: FIG. 135.—BACTERIA OF SEVERAL FORMS, much magnified.]
Saprophytes break down or decompose organic substances. Chief of
these saprophytes are many microscopic organisms known as bacteria
(Fig. 135). These innumerable organisms are immersed in water or in
dead animals and plants, and in all manner of moist organic products.
By breaking down organic combinations, _they produce decay_. Largely
through their agency, and that of many true but microscopic fungi,
_all things pass into soil and gas_. Thus are the bodies of plants
and animals removed and the continuing round of life is maintained.
[Illustration: FIG. 136.—AMERICAN MISTLETOE GROWING ON A WALNUT
BRANCH.]
_Some parasites are green-leaved._ Such is the mistletoe (Fig. 136).
They anchor themselves on the host and absorb its juices, but they
also appropriate and use the carbon dioxide of the air. In some
small groups of bacteria a process of organic synthesis has been
shown to take place.
=Epiphytes.=—To be distinguished from the dependent plants are those
that grow on other plants without taking food from them. These are
green-leaved plants whose roots burrow in the bark of the host plant
and perhaps derive some food from it, but which subsist chiefly on
materials that they secure from air dust, rain water, and the air.
These plants are =epiphytes= (meaning “upon plants”) or air-plants.
Epiphytes abound in the tropics. Certain orchids are among the best
known examples (Fig. 37). The Spanish moss or tillandsia of the South
is another. Mosses and lichens that grow on trees and fences may
also be called epiphytes. In the struggle for existence, the plants
probably _have been driven to these special places_ in which to find
opportunity to grow. Plants grow where they must, not where they will.
SUGGESTIONS.—=114.= Is a puffball a plant? Why do you think so?
=115.= Are mushrooms ever cultivated, and where and how? =116.= In
what locations are mushrooms and toadstools usually found? (There
is really no distinction between mushrooms and toadstools. They are
all mushrooms.) =117.= What kinds of mildew, blight, and rust do
you know? =118.= How do farmers overcome potato blight? Apple scab?
Or any other fungous “plant disease”? =119.= How do these things
injure plants? =120.= What is a plant disease? =121.= The pupil
should know that every spot or injury on a leaf or stem is caused
by something,—as an insect, a fungus, wind, hail, drought, or other
agency. How many uninjured or perfect leaves are there on the plant
growing nearest the schoolhouse steps? =122.= Give formula for
Bordeaux mixture and tell how and for what it is used.
CHAPTER XV.
WINTER AND DORMANT BUDS
=A bud is a growing point=, terminating an axis either long or short,
or being the starting point of an axis. _All branches spring from
buds._ In the growing season the bud is active; later in the season
it ceases to increase the axis in length, and as winter approaches
the growing point becomes more or less thickened and covered by
protecting scales, in preparation for the long resting season. This
resting, dormant, or winter body is what is commonly spoken of as a
“bud.” A winter bud may be defined as an _inactive covered growing
point_, waiting for spring.
[Illustration: FIG. 137.—BUD OF APRICOT, showing the miniature
leaves.]
[Illustration: FIG. 138.—BUD OF PEAR, showing both leaves and
flowers. The latter are the little knobs in the centre.]
_Structurally, a dormant bud is a_ =shortened axis= _or_ =branch=,
_bearing miniature leaves or flowers or both, and protected by a
covering_. Cut in two, lengthwise, a bud of the horse-chestnut or
other plant that has large buds. With a pin separate the tiny leaves.
Count them. Examine the big bud of the rhubarb as it lies under
the ground in late winter or early spring; or the crown buds of
asparagus, hepatica, or other early spring plants. Dissect large buds
of the apricot and pear (Figs. 137, 138).
_The bud is protected by firm and dry scales._ These scales are
modified leaves. The scales fit close. Often the bud is protected
by varnish (see horse-chestnut and the balsam poplars). Most winter
buds are more or less woolly. Examine some of them under a lens. As
we might expect, bud coverings are most prominent in cold and dry
climates. Sprinkle water on velvet or flannel, and note the result
and give a reason.
=All winter buds give rise to branches=, _not to leaves alone_; that
is, the leaves are borne on the lengthening axis. Sometimes the axis,
or branch, remains very short,—so short that it may not be noticed.
Sometimes it grows several feet long.
[Illustration: FIG. 139.—LEAF-SCARS.—Ailanthus.]
_Whether the branch grows large or not depends on the chance it
has_,—position on the plant, soil, rainfall, and many other factors.
The new shoot is the unfolding and enlarging of the tiny axis and
leaves that we saw in the bud. If the conditions are congenial,
the shoot may form more leaves than were tucked away in the bud.
The length of the shoot usually depends more on the length of the
internodes than on the number of leaves.
=Where Buds are.=—_Buds are borne in the_ =axils= _of the leaves_,—in
the acute angle that the leaf makes with the stem. When the leaf
is growing in the summer, a bud is forming above it. When the leaf
falls, the bud remains, and a scar marks the place of the leaf. Fig.
139 shows the large leaf-scars of ailanthus. Observe those on the
horse-chestnut, maple, apple, pear, basswood, or any other tree or
bush.
[Illustration: FIG. 140.—TERMINAL BUD BETWEEN TWO OTHER
BUDS.—Currant.]
Sometimes two or more buds are borne in one axil; the extra ones are
=accessory= or =supernumerary= buds. Observe them in the Tartarian
honeysuckle (common in yards), walnut, butternut, red maple, honey
locust, and sometimes in the apricot and peach.
If the bud is at the end of a shoot, however short the shoot, it is
called a =terminal bud=. _It continues the growth of the axis in a
direct line._ Very often three or more buds are clustered at the tip
(Fig. 140); and in this case there may be more buds than leaf-scars.
Only one of them, however, is strictly terminal.
A bud in the axil of a leaf is an =axillary= or =lateral= bud. Note
that there is normally at least one bud in the axil of every leaf on
a tree or shrub in late summer and fall. The axillary buds, if they
grow, are the _starting points of new shoots the following season_.
If a leaf is pulled off early in summer, what will become of the
young bud in its axil? Try this.
[Illustration: FIG. 141.—A GIGANTIC BUD.—Cabbage.]
_Bulbs and cabbage heads may be likened to buds_; that is, they are
condensed stems, with scales or modified leaves densely overlapping
and forming a rounded body (Fig. 141). They differ from true buds,
however, in the fact that they are condensations of whole main stems
rather than embryo stems borne in the axils of leaves. But bulblets
(as of tiger lily) may be scarcely distinguishable from buds on the
one hand and from bulbs on the other. Cut a cabbage head in two,
lengthwise, and see what it is like.
[Illustration: FIG. 142.—FRUIT-BUD OF PEAR.]
[Illustration: FIG. 143.—THE OPENING OF THE PEAR FRUIT-BUD.]
The buds that appear on roots are unusual or abnormal,—they occur
only occasionally and in no definite order. Buds appearing in unusual
places on any part of the plant are called =adventitious buds=. Such
usually are the buds that arise when a large limb is cut off, and
from which suckers or water-sprouts arise.
[Illustration: FIG. 144.—OPENING PEAR LEAF-BUD.]
=How Buds Open.=—_When the bud swells, the scales are pushed apart,
the little axis elongates and pushes out._ In most plants the outside
scales fall very soon, _leaving a little ring of scars_. With
terminal buds, this ring marks the end of the year’s growth. How?
Notice peach, apple, plum, willow, and other plants. In some others,
all the scales grow for a time, as in the pear (Figs. 142, 143,
144). In other plants the inner bud scales become green and almost
leaf-like. See the maple and hickory.
[Illustration: FIG. 145.—OPENING OF THE PEAR-BUD.]
=Sometimes Flowers come out of the Buds.=—Leaves may or may not
accompany the flowers. We saw the embryo flowers in Fig. 138. The bud
is shown again in Fig. 142. In Fig. 143 it is opening. In Fig. 145
it is more advanced, and the woolly unformed flowers are appearing.
In Fig. 146 the growth is more advanced.
[Illustration: FIG. 146.—A SINGLE FLOWER IN THE PEAR CLUSTER, as seen
at 7 A.M. on the day of its opening. At 10 o’clock it will be fully
expanded.]
[Illustration: FIG. 147.—THE OPENING OF THE FLOWER-BUD OF APRICOT.]
[Illustration: FIG. 148.—APRICOT FLOWER-BUD, enlarged.]
[Illustration: FIG. 149.—FRUIT-BUDS AND LEAF-BUDS OF PEAR.]
Buds that contain or produce only leaves are =leaf-buds=. Those which
contain only flowers are =flower-buds= or =fruit-buds=. The latter
occur on peach, almond, apricot, and many very early spring-flowering
plants. The single flower is emerging from the apricot bud in Fig.
147. A longitudinal section of this bud, enlarged, is shown in Fig.
148. Those that contain both leaves and flowers are =mixed buds=, as
in pear, apple, and most late spring-flowering plants.
_Fruit buds are usually thicker or stouter than leaf-buds. They are
borne in different positions on different plants._ In some plants
(apple, pear) they are on the ends of short branches or spurs; in
others (peach, red maple) they are along the sides of the last year’s
growths. In Fig. 149 are shown three fruit-buds and one leaf-bud on
_E_, and leaf-buds on _A_. See also Figs. 150, 151, 152, 153, and
explain.
[Illustration: FIG. 150.—FRUIT-BUDS OF APPLE ON SPURS: a dormant bud
at the top.]
[Illustration: FIG. 151.—CLUSTER OF FRUIT-BUDS OF SWEET CHERRY, with
one pointed leaf-bud in centre.]
[Illustration: FIG. 152.—TWO FRUIT-BUDS OF PEACH with a leaf-bud
between.]
[Illustration: FIG. 153.—OPENING OF LEAF-BUDS AND FLOWER-BUDS OF
APPLE.]
“_The burst of spring_” means in large part the opening of the buds.
_Everything was made ready the fall before. The embryo shoots and
flowers were tucked away, and the food was stored._ The warm rain
falls, and the shutters open and the sleepers wake.
=Arrangement of Buds.=—We have found that leaves are usually arranged
in a definite order; buds are borne in the axils of leaves: therefore
_buds must exhibit phyllotaxy_. Moreover, branches grow from buds:
branches, therefore, should show a definite arrangement. Usually,
however, they do not show this arrangement because _not all the buds
grow and not all the branches live_. (See Chaps. II and III.) It is
apparent, however, that the mode of arrangement of buds determines to
some extent the form of the tree. Compare bud arrangement in pine or
fir with that in maple or apple.
[Illustration: FIG. 154.—OAK SPRAY. How are the leaves borne with
reference to the annual growths?]
The uppermost buds on any twig, if they are well matured, are
usually the larger and stronger and they are the most likely to grow
the next spring; therefore, branches tend to be arranged in tiers
(particularly well marked in spruces and firs). See Fig. 154 and
explain it.
=Winter Buds show what has been the Effect of Sunlight.=—Buds are
borne in the axils of the leaves, and _the size or the vigour of
the leaf determines to a large extent the size of the bud_. Notice
that, in most instances, _the largest buds are nearest the tip_ (Fig.
157). If the largest buds are not near the tip, there is some special
reason for it. Can you state it? Examine the shoots on trees and
bushes.
[Illustration: FIG. 155.—AN APPLE TWIG.]
[Illustration: FIG. 156.—Same twig before leaves fell.]
SUGGESTIONS.—Some of the best of all observation lessons are those
made on dormant twigs. There are many things to be learned, the
eyes are trained, and the specimens are everywhere accessible.
=123.= At whatever time of year the pupil takes up the study of
branches, he should look for three things: the ages of the various
parts, the relative positions of the buds and the leaves, the
different sizes of similar or comparable buds. If it is late in
spring or early in summer, he should watch the development of the
buds in the axils, and he should determine whether the strength or
size of the bud is in any way related to the size and the vigour
of the subtending (or supporting) leaf. The sizes of buds should
also be noted on leafless twigs, and the sizes of the former leaves
may be inferred from the size of the leaf-scar below the bud. The
pupil should keep in mind the fact of the struggle for food and
light, and its effects on the developing buds. =124.= _The bud
and the branch._ A twig cut from an apple tree in early spring is
shown in Fig. 155. The most hasty observation shows that it has
various parts, or members. It seems to be divided at the point
_f_ into two parts. It is evident that the part from _f_ to _h_
grew last year, and that the part below _f_ grew two years ago.
The buds on the two parts are very unlike, and these differences
challenge investigation.—In order to understand this seemingly
lifeless twig, it will be necessary to see it as it looked late
last summer (and this condition is shown in Fig. 156). The part
from _f_ to _h_,—which has just completed its growth,—is seen to
have its leaves growing singly. In every axil (or angle which the
leaf makes when it joins the shoot) is a bud. The leaf starts
first, and as the season advances the bud forms in its axil. When
the leaves have fallen, at the approach of winter, the buds remain,
as seen in Fig. 155. Every bud on the last year’s growth of a
winter twig, therefore, marks the position occupied by a leaf when
the shoot was growing.—The part below _f_, in Fig. 156, shows a
wholly different arrangement. The leaves are two or more together
(_aaaa_), and there are buds without leaves (_bbbb_). A year ago
this part looked like the present shoot from _f_ to _h_,—that is,
the leaves were single, with a bud in the axil of each. It is now
seen that some of these bud-like parts are longer than others,
and that the longest ones are those which have leaves. It must be
because of the leaves that they have increased in length. The body
_c_ has lost its leaves through some accident, and its growth has
ceased. In other words, the parts at _aaaa_ are like the shoot
_fh_, except that they are shorter, and they are of the same age.
One grew from the end or terminal bud of the main branch, and the
others from the side or lateral buds. Parts or bodies that bear
leaves are, therefore, branches.—The buds at _bbbb_ have no leaves,
and they remain the same size that they were a year ago. They are
dormant. The only way for a mature bud to grow is by making leaves
for itself, for a leaf will never stand below it again. The twig,
therefore, has buds of two ages,—those at _bbbb_ are two seasons
old, and those on the tips, of all the branches (_aaaa_, _h_),
and in the axil of every leaf, are one season old. It is only the
terminal buds that are not axillary. When the bud begins to grow
and to put forth leaves, it gives rise to a branch, which, in its
turn, bears buds.—It will now be interesting to determine why
certain buds gave rise to branches and why others remained dormant.
The strongest shoot or branch of the year is the terminal one
(_fh_). The next in strength is the uppermost lateral one, and the
weakest shoot is at the base of the twig. The dormant buds are on
the under side (for the twig grew in a horizontal position). All
this suggests that those buds grew which had the best chance,—the
most sunlight and room. There were too many buds for the space, and
in the struggle for existence those that had the best opportunities
made the largest growth. This struggle for existence began a year
ago, however, when the buds on the shoot below _f_ were forming in
the axils of the leaves, for the buds near the tip of the shoot
grew larger and stronger than those near its base. The growth of
one year, therefore, is very largely determined by the conditions
under which the buds were formed the previous year. _Other bud
characters._ =125.= It is easy to see the swelling of the bud in a
room in winter. Secure branches of trees and shrubs, two to three
feet long, and stand them in vases or jars, as you would flowers.
Renew the water frequently and cut off the lower ends of the shoots
occasionally. In a week or two the buds will begin to swell. Of
red maple, peach, apricot, and other very early-flowering things,
flowers may be obtained in ten to twenty days. =126.= The shape,
size, and colour of the winter buds are different in every kind of
plant. By the buds alone botanists are often able to distinguish
the kinds of plants. Even such similar plants as the different
kinds of willows have good bud characters. =127.= Distinguish and
draw fruit-buds of apple, pear, peach, plum, and other trees. If
different kinds of maples grow in the vicinity, secure twigs of the
red or swamp maple, and the soft or silver maple, and compare the
buds with those of the sugar maple and the Norway maple. What do
you learn?
[Illustration: FIG. 157.—BUDS OF THE HICKORY.]
CHAPTER XVI
BUD PROPAGATION
We have learned (in Chap. VI) that plants propagate by means of
seeds. _They also propagate by means of bud parts,—as rootstocks
(rhizomes), roots, runners, layers, bulbs._ The pupil should
determine how any plant in which he is interested naturally
propagates itself (or spreads its kind). Determine this for
raspberry, blackberry, strawberry, June-grass or other grass,
nut-grass, water-lily, May apple or mandrake, burdock, Irish potato,
sweet potato, buckwheat, cotton, pea, corn, sugar-cane, wheat, rice.
Plants may be _artificially propagated_ by similar means, as by
_layers_, _cuttings_, and _grafts_. The last two we may discuss here.
=Cuttings in General.=—_A bit of a plant stuck into the ground stands
a chance of growing; and this bit is a_ =cutting=. Plants have
preferences, however, as to the kind of bit which shall be used,
but _there is no way of telling what this preference is except by
trying_. In some instances this preference has not been discovered,
and we say that the plant cannot be propagated by cuttings.
Most plants prefer that the cutting be made of the =soft= or =growing
parts= (called “wood” by gardeners), of which the “slips” of geranium
and coleus are examples. Others grow equally well from cuttings
of the =hard= or =mature parts= or =wood=, as currant and grape;
and in some instances this mature wood may be of roots, as in the
blackberry. In some cases cuttings are made of tubers, as in the
Irish potato (Fig. 60). Pupils should make cuttings now and then. If
they can do nothing more, they can make cuttings of potato, as the
farmer does; and they can plant them in a box in the window.
[Illustration: FIG. 158.—GERANIUM CUTTING.]
[Illustration: FIG. 159.—ROSE CUTTING.]
=The Softwood Cutting.=—The softwood cutting is made from tissue that
is still growing, or at least from that which is not dormant. _It
comprises one or two joints, with a leaf attached_ (Figs. 158, 159).
It must not be allowed to wilt. Therefore, it must be _protected from
direct sunlight and dry air until it is well established; and if it
has many leaves, some of them should be removed, or at least cut in
two, in order to reduce the evaporating surface._ The soil should be
uniformly moist. The pictures show the depth to which the cuttings
are planted.
For most plants, the proper age or maturity of wood for the making
of cuttings may be determined by giving the _twig a quick bend: if
it snaps and hangs by the bark, it is in proper condition; if it
bends without breaking, it is too young and soft or too old; if it
splinters, it is too old and woody_. The tips of strong upright
shoots usually make the best cuttings. Preferably, each cutting
should have a joint or node near its base; and if the internodes are
very short it may comprise two or three joints.
[Illustration: FIG. 160.—CUTTING-BOX.]
_The stem of the cutting is inserted one third or more of its length
in clean sand or gravel, and the earth is pressed firmly about it._
A newspaper may be laid over the bed to exclude the light—if the sun
strikes it—and to prevent too rapid evaporation. The soil should be
moist clear through, not on top only.
[Illustration: FIG. 161.—VERBENA CUTTING READY FOR TRANSPLANTING.]
_Loose sandy or gravelly soil is used._ Sand used by masons is good
material in which to start most cuttings; or fine gravel—sifted
of most of its earthy matter—may be used. Soils are avoided which
contain much decaying organic matter, for these soils are breeding
places of fungi, which attack the soft cutting and cause it to “damp
off,” or to die at or near the surface of the ground. If the cuttings
are to be grown in a window, put three or four inches of the earth
in a shallow box or a pan. A soap box cut in two lengthwise, so that
it makes a box four or five inches deep—as a gardener’s flat—is
excellent (Fig. 160). Cuttings of common plants, as geranium, coleus,
fuchsia, carnation, are kept at a living-room temperature. As long as
the cuttings look bright and green, they are in good condition. It
may be a month before roots form. When roots have formed, the plants
begin to make new leaves at the tip. Then they may be transplanted
into other boxes or into pots. The verbena in Fig. 161 is just ready
for transplanting.
[Illustration: FIG. 162.—OLD GERANIUM PLANT CUT BACK TO MAKE IT THROW
OUT SHOOTS FROM WHICH CUTTINGS CAN BE MADE.]
It is not always easy to find growing shoots from which to make the
cuttings. The best practice, in that case, is _to cut back an old
plant, then keep it warm and well watered, and thereby force it to
throw out new shoots_. The old geranium plant from the window garden,
or the one taken up from the lawn bed, may be treated this way (see
Fig. 162). The best plants of geranium and coleus and most window
plants are those which are not more than one year old. _The geranium
and fuchsia cuttings which are made in January, February, or March
will give compact blooming plants for the next winter; and thereafter
new ones should take their places_ (Fig. 163).
[Illustration: FIG. 163.—EARLY WINTER GERANIUM, from a spring
cutting.]
=The Hardwood Cutting.=—_Best results with cuttings of mature wood
are secured when the cuttings are made in the fall and then buried
until spring in sand in the cellar._ These cuttings are usually six
to ten inches long. They are not idle while they rest. The lower end
calluses or heals, and the roots form more readily when the cutting
is planted in the spring. But if the proper season has passed, take
cuttings at any time in winter, plant them in a deep box in the
window, and watch. They will need no shading or special care. Grape,
currant, gooseberry, willow, and poplar readily take root from the
hardwood. Fig. 164 shows a currant cutting. It has only one bud above
the ground.
[Illustration: FIG. 164.—CURRANT CUTTING.]
=The Graft.=—_When the cutting is inserted in a plant rather than in
the soil, it is a graft_; and the graft may grow. In this case the
cutting grows fast to the other plant, and the two become one. When
the cutting is inserted in a plant, it is no longer called a cutting
but a =scion=; and the plant in which it is inserted is called the
=stock=. Fruit trees are grafted _in order that a certain variety or
kind may be perpetuated_, as a Baldwin or Ben Davis variety of apple,
Seckel or Bartlett pear, Navel or St. Michael orange.
_Plants have preferences as to the stocks on which they will
grow; but we can find out what their choice is only by making the
experiment._ The pear grows well on the quince, but the quince does
not thrive on the pear. The pear grows on some of the hawthorns, but
it is an unwilling subject on the apple. Tomato plants will grow on
potato plants and potato plants on tomato plants. When the potato
is the root, both tomatoes and potatoes may be produced, although
the crop will be very small; when the tomato is the root, neither
potatoes nor tomatoes will be produced. Chestnut will grow on some
kinds of oak. In general, one species or kind is grafted on the same
species, as apple on apple, pear on pear, orange on orange.
_The forming, growing tissue of the stem_ (on the plants we have
been discussing) _is the_ =cambium= (Chap. X), _lying on the outside
of the woody cylinder beneath the bark_. In order that union may
take place, _the cambium of the scion and of the stock must come
together_. Therefore the scion is set in the side of the stock. There
are many ways of shaping the scion and of preparing the stock to
receive it. These ways are dictated largely by the relative sizes
of scion and stock, although many of them are matters of personal
preference. The underlying principles are two: securing _close
contact_ between the cambiums of scion and stock; _covering the
wounded surfaces_ to prevent evaporation and to protect the parts
from disease.
On large stocks the commonest form of grafting is the =cleft-graft=.
The stock is cut off and split; and in one or both sides a
wedge-shaped scion is firmly inserted. Fig. 165 shows the scion; Fig.
166, the scions set in the stock; Fig. 167, the stock waxed. It will
be seen that the lower bud—that lying in the wedge—is covered by the
wax; but being nearest the food supply and least exposed to weather,
it is the most likely to grow: it will push through the wax.
=Cleft-grafting= _is practised in spring, as growth begins. The
scions are cut previously, when perfectly dormant, and from the
tree which it is desired to propagate_. The scions are kept
in sand or moss in the cellar. Limbs of various sizes may be
cleft-grafted,—from a half inch up to four inches in diameter; but a
diameter of one to one and a half inches is the most convenient size.
All the leading or main branches of a tree top may be grafted. If the
remaining parts of the top are gradually cut away and the scions grow
well, the entire top will be changed over to the new variety.
[Illustration: FIG. 165.—SCION OF APPLE.]
[Illustration: FIG. 166.—THE SCION INSERTED.]
[Illustration: FIG. 167.—THE PARTS WAXED.]
Another form of grafting is known as =budding=. In this case a single
bud is used, and it is slipped underneath the bark of the stock and
securely tied (not waxed) with soft material, as bass bark, corn
shuck, yarn, or raffia (the last a commercial palm fibre). Budding is
performed _when the bark of the stock will slip or peel_ (so that the
bud can be inserted), and _when the bud is mature enough to grow_.
Usually budding is performed in late summer or early fall, when the
winter buds are well formed; or it may be practised in spring with
buds cut in winter. In ordinary summer budding (which is the usual
mode) the “bud” or scion forms a union with the stock, and then lies
dormant till the following spring, as if it were still on its own
twig. Budding is mostly restricted to young trees in the nursery.
In the spring following the budding, the stock is cut off just above
the bud, so that only the shoot from the bud grows to make the future
tree. This prevailing form of budding (shield-budding) is shown in
Fig. 168.
SUGGESTIONS.—=128.= Name the plants that the gardener propagates
by means of cuttings. =129.= By means of grafts. =130.= The
cutting-box may be set in the window. If the box does not receive
direct sunlight, it may be covered with a pane of glass to prevent
evaporation. Take care that the air is not kept too close, else
the damping-off fungi may attack the cuttings, and they will rot
at the surface of the ground. See that the pane is raised a little
at one end to afford ventilation; and if the water collects in
drops on the under side of the glass, remove the pane for a time.
=131.= Grafting wax is made of beeswax, resin, and tallow. A good
recipe is one part (as one pound) of rendered tallow, two parts
of beeswax, four parts of resin; melt together in a kettle; pour
the liquid into a pail or tub of water to solidify it; work with
the hands until it has the colour and “grain” of taffy candy, the
hands being greased when necessary. The wax will keep any length
of time. For the little grafting that any pupil would do, it is
better to buy the wax of a seedsman. =132.= Grafting is hardly to
be recommended as a general school diversion, as the making of
cuttings is; and the account of it in this chapter is inserted
chiefly to satisfy the general curiosity on the subject. =133.= In
Chap. V we had a definition of a plant generation: what is “one
generation” of a grafted fruit tree, as Le Conte pear, Baldwin, or
Ben Davis apple? =134.= The Elberta peach originated about 1880:
what is meant by “originated”? =135.= How is the grape propagated
so as to come true to name (explain what is meant by “coming
true”)? currant? strawberry? raspberry? blackberry? peach? pear?
orange? fig? plum? cherry? apple? chestnut? pecan?
[Illustration: FIG. 168.—BUDDING. The “bud”; the opening to receive
it; the bud tied.]
CHAPTER XVII
HOW PLANTS CLIMB
We have found that plants struggle or contend for a place in which
to live. Some of them become adapted to grow in the forest shade,
others to grow on other plants, as epiphytes, others to _climb to the
light_. Observe how woods grapes, and other forest climbers, spread
their foliage on the very top of the forest tree, while their long
flexile trunks may be bare.
There are several ways by which plants climb, but most climbers
may be classified into four groups: (1) =scramblers=, (2) =root
climbers=, (3) =tendril climbers=, (4) =twiners=.
=Scramblers.=—Some plants rise to light and air _by resting their
long and weak stems on the tops of bushes and quick-growing herbs_.
Their stems may be elevated in part by the growing twigs of the
plants on which they recline. Such plants are scramblers. Usually
they are provided with prickles or bristles. In most weedy swamp
thickets, scrambling plants may be found. Briers, some roses,
bed-straw or galium, bittersweet (_Solanum Dulcamara_, not the
_Celastrus_) the tear-thumb polygonums, and other plants are familiar
examples of scramblers.
=Root Climbers.=—Some plants climb by means of _true roots_. These
roots seek the dark places and therefore enter the chinks in walls
and bark. The trumpet creeper is a familiar example (Fig. 36).
The true or English ivy, which is often grown to cover buildings,
is another instance. Still another is the poison ivy. Roots are
distinguished from stem tendrils by their _irregular or indefinite
position_ as well as by their mode of growth.
[Illustration: FIG. 169.—TENDRIL, to show where the coil is changed.]
=Tendril climbers.=—A slender coiling part that serves to hold a
climbing plant to a support is known as a =tendril=. The free end
swings or curves until it strikes some object, when it attaches
itself and then coils and _draws the plant close to the support_.
The spring of the coil also allows the plant _to move in the wind_,
thereby enabling the plant to maintain its hold. Slowly pull a
well-matured tendril from its support, and note how strongly it holds
on. Watch the tendrils in a wind-storm. Usually the tendril attaches
to the support by _coiling about it_, but the Virginia creeper and
the Boston ivy (Fig. 170) attach to walls by means of _disks_ on the
ends of the tendrils.
[Illustration: FIG. 170.—TENDRIL OF BOSTON IVY.]
Since both ends of the tendril are fixed, when it finds a support,
the coiling would tend to twist it in two. It will be found, however,
that the tendril _coils in different directions_ in different parts
of its length. In Fig. 169, showing an old and stretched-out tendril,
the change of direction in the coil occurred at a. In long tendrils
of cucumbers and melons there may be several changes of direction.
Tendrils may represent either _branches_ or _leaves_. In the
Virginia creeper and the grape they are branches; they stand opposite
the leaves in the position of fruit clusters, and sometimes one
branch of a fruit cluster is a tendril. These tendrils are therefore
homologous with fruit clusters, and fruit clusters are branches.
[Illustration: FIG. 171.—LEAVES OF PEA,—very large stipules, opposite
leaflets, and leaflets represented by tendrils.]
In some plants tendrils are _leaflets_ (Chap. XI). Examples are the
sweet pea and the common garden pea. In Fig. 171, observe the leaf
with its two great stipules, petiole, six normal leaflets, and two
or three pairs of leaflet tendrils and a terminal leaflet tendril.
The cobaea, a common garden climber, has a similar arrangement. In
some cases tendrils are _stipules_, as probably in the green briers
(smilax).
The _petiole_ or _midrib may act as a tendril_, as in various kinds
of clematis. In Fig. 172, the common wild clematis or “old man vine,”
this mode is seen.
=Twiners.=—The entire plant or shoot may wind about a support.
Such a plant is a =twiner=. Examples are bean, hop, morning-glory,
moon-flower, false bittersweet or waxwork (_Celastrus_), some
honeysuckles, wistaria, Dutchman’s pipe, dodder. The free tip of the
twining branch _sweeps about in curves_, much as the tendril does,
until it finds support or becomes old and rigid.
Each kind of plant usually coils _in only one direction_. Most plants
coil against the sun, or from the observer’s left across his front to
his right as he faces the plant. Examples are bean, morning-glory.
The hop twines from the observer’s right to his left, or with the sun.
[Illustration: FIG. 172.—CLEMATIS CLIMBING BY LEAF-TENDRIL.]
SUGGESTIONS.—=136.= Set the pupil to watch the behaviour of any
plant that has tendrils at different stages of maturity. A vigorous
cucumber plant is one of the best. Just beyond the point of a young
straight tendril set a stake to compare the position of it. Note
whether the tendril changes position from hour to hour or day to
day. =137.= Is the tip of the tendril perfectly straight? Why? Set
a small stake at the end of a strong straight tendril, so that the
tendril will just reach it. Watch and make drawing. =138.= If a
tendril does not find a support what does it do? =139.= To test the
movement of a free tendril draw an ink line lengthwise of it, and
note whether the line remains always on the concave side or the
convex side. =140.= Name the tendril-bearing plants that you know.
=141.= Make similar observations and experiments on the tips of
twining stems. =142.= What twining plants do you know, and which
way do they twine? =143.= How does any plant that you know shoot
up? =144.= Does the stem of a climbing plant contain more or less
substance (weight) than an erect self-supporting stem of the same
height? Explain.
CHAPTER XVIII
THE FLOWER—ITS PARTS AND FORMS
The function of the flower is to _produce seed_. It is probable that
all its varied forms and colours contribute to this supreme end.
These forms and colours please the human fancy and add to the joy of
living, but the flower exists for the good of the plant, not for the
good of man. The parts of the flower are of two general kinds—those
that are directly concerned in the _production of seeds_, and those
that act as _covering and protecting organs_. The former parts are
known as the =essential organs=; the latter as the =floral envelopes=.
[Illustration: FIG. 173.—FLOWER OF A BUTTERCUP IN SECTION.]
=Envelopes.=—The floral envelopes usually bear a close resemblance to
leaves. These envelopes are very commonly of two series or kinds—the
_outer_ and the _inner_. The outer series, known as the =calyx=, is
usually smaller and green. It usually comprises the outer cover of
the flower-bud. The calyx is the lowest whorl in Fig. 173.
The inner series, known as the =corolla=, is usually coloured and
more special or irregular in shape than the calyx. It is the showy
part of the flower, as a rule. The corolla is the second or large
whorl in Fig. 173.
The _calyx_ may be composed of several leaves. Each leaf is a
=sepal=. If it is of one piece, it may be lobed or divided, in which
case the divisions are called =calyx-lobes=. In like manner, the
corolla may be composed of =petals=, or it may be of one piece and
variously lobed. A calyx of one piece, no matter how deeply lobed, is
=gamosepalous=. A corolla of one piece is =gamopetalous=. When these
series are of separate pieces, as in Fig. 173, the flower is said to
be =polysepalous= and =polypetalous=. Sometimes both series are of
separate parts, and sometimes only one of them is so formed.
[Illustration: FIG. 174.—FLOWER OF FUCHSIA IN SECTION.]
_The floral envelopes are homologous with leaves._ Sepals and petals,
at least when more than three or five, are in more than one whorl,
and one whorl stands below another so that the parts overlap. They
are borne on the expanded or thickened end of the flower stalk; this
end is the =torus=. In Fig. 173 all the parts are seen as attached to
the torus. This part is sometimes called the _receptacle_, but this
word is a common-language term of several meanings, whereas torus has
no other meaning. Sometimes one part is attached to another part,
as in the fuchsia (Fig. 174), in which the petals are borne on the
calyx-tube.
=Subtending Parts.=—Sometimes there are _leaf-like parts just below
the calyx_, looking like a second calyx. Such parts accompany
the carnation flower. These parts are =bracts= (bracts are small
specialized leaves); and they form an =involucre=. We must be careful
that we do not mistake them for true flower-parts. Sometimes the
bracts are large and petal-like, as in the great white blooms of
the flowering dogwood: here the real flowers are several, small and
greenish, forming a small cluster in the centre.
=Essential Organs=.—The essential organs are of two series. The outer
series is composed of the =stamens=. The inner series is composed of
the =pistils=.
_Stamens_ bear the =pollen=, which is made up of grains or spores,
each spore usually being a single plant cell. The stamen is of two
parts, as is readily seen in Figs. 173, 174,—the enlarged terminal
part or =anther=, and the stalk or =filament=. The filament is often
so short as to seem to be absent, and the anther is then said to be
_sessile_. The anther bears the pollen spores. It is made up of two
or four parts (known as sporangia or spore-cases), which burst and
discharge the pollen. _When the pollen is shed, the stamen dies._
[Illustration: FIG. 175.—THE STRUCTURE OF A PLUM BLOSSOM.
_se_, sepals; _p_, petals; _sta_, stamens; _o_, ovary; _s_, style;
_st_, stigma. The pistil consists of the ovary, the style and the
stigma. It contains the seed part. The stamens are tipped with
anthers, in which the pollen is borne. The ovary, _o_, ripens into
the fruit.]
The _pistil has three parts_: the lowest, or seed-bearing part,
which is the =ovary=; the =stigma= at the upper extremity, which is
a flattened or expanded surface, and usually roughened or sticky;
the stalk-like part or =style=, connecting the ovary and the stigma.
Sometimes the style is apparently wanting, and the stigma is said to
be sessile on the ovary. These parts are shown in the fuchsia (Fig.
174). The ovary or seed vessel is at _a_. A long style, bearing a
large stigma, projects from the flower. See also Figs. 175 and 176.
Stamens and pistils probably are homologous with leaves. A pistil is
sometimes conceived to represent anciently a leaf as if rolled into
a tube; and an anther, a leaf of which the edges may have been turned
in on the midrib.
[Illustration: FIG. 176.—SIMPLE PISTILS OF BUTTERCUP, one in
longitudinal section.]
[Illustration: FIG. 177.—PISTIL OF GARDEN PEA, the stamens being
pulled down in order to disclose it; also a section showing the
single compartment (compare Fig. 188).]
The pistil may be of _one part or compartment, or of many parts_.
The different units or parts of which it is composed are =carpels=.
Each carpel is homologous with a leaf. Each carpel bears one or more
seeds. A pistil of one carpel is =simple=; of two or more carpels,
=compound=. Usually the structure of the pistil may be determined by
cutting horizontally across the lower or seed-bearing part, as Figs.
177, 178 explain. A flower may contain a simple pistil (one carpel),
as the pea (Fig. 177); _several simple pistils_ (several separate
carpels), as the buttercup (Fig. 176); or a _compound pistil_ with
carpels united, as the Saint John’s wort (Fig. 178) and apple. How
many carpels in an apple? A peach? An okra pod? A bean pod? The seed
cavity in each carpel is called a =locule= (Latin _locus_, a place).
In these locules _the seeds are borne_.
[Illustration: FIG. 178.—COMPOUND PISTIL OF A ST. JOHN’S WORT. It has
5 carpels.]
=Conformation of the Flower=.—A flower that has calyx, corolla,
stamens, and pistils is said to be =complete= (Fig. 173); all others
are =incomplete=. In some flowers both the floral envelopes are
wanting: such are =naked=. When one of the floral envelope series is
wanting, the remaining series is said to be calyx, and the flower is
therefore =apetalous= (without petals). The knot-weed (Fig. 179),
smartweed, buckwheat, elm are examples.
[Illustration: FIG. 179.—KNOTWEED, a very common but inconspicuous
plant along hard walks and roads. Two flowers, enlarged, are shown at
the right. These flowers are very small and borne in the axils of the
leaves.]
Some flowers lack the pistils: these are =staminate=, whether the
envelopes are missing or not. Others lack the stamens: these are
=pistillate=. Others have neither stamens nor pistils: these are
=sterile= (snowball and hydrangea). Those that have both stamens
and pistils are =perfect=, whether or not the envelopes are
missing. Those that lack either stamens or pistils are =imperfect=
or =diclinous=. Staminate and pistillate flowers are imperfect or
diclinous.
[Illustration: FIG. 180.—STAMINATE CATKINS OF OAK. The pistillate
flowers are in the leaf axils, and not shown in this picture.]
[Illustration: FIG. 181.—BEGONIA FLOWERS. Staminate at _A_:
pistillate below, with the winged ovary at _B_.]
When staminate and pistillate flowers are borne on the same plant,
_e.g._ oak (Fig. 180), corn, beech, chestnut, hazel, walnut, hickory,
pine, begonia (Fig. 181), watermelon, gourd, pumpkin, the plant is
=monœcious= (“in one house”). When they are on different plants,
_e.g._ poplar, cottonwood, bois d’arc, willow (Fig. 182), the plant
is =diœcious= (“in two houses”). Some varieties of strawberry, grape,
and mulberry are partly diœcious. Is the rose either monœcious or
diœcious?
[Illustration: FIG. 182.—CATKINS OF A WILLOW. A staminate flower
is shown at _s_, and a pistillate flower at _p_. The staminate and
pistillate are on different plants.]
Flowers in which the parts of each series are alike are said to be
=regular= (as in Figs. 173, 174, 175). Those in which some parts
are unlike other parts of the same series are =irregular=. Their
regularity may be in calyx, as in nasturtium (Fig. 183); in corolla
(Figs. 184, 185); in the stamens (compare nasturtium, catnip, Fig.
185, sage); in the pistils. Irregularity is most frequent in the
corolla.
[Illustration: FIG. 183.—FLOWER OF GARDEN NASTURTIUM. Separate petal
at _a_. The calyx is produced into a spur.]
[Illustration: FIG. 184.—THE FIVE PETALS OF THE PANSY, detached to
show the form.]
[Illustration: FIG. 185.—FLOWER OF CATNIP.]
=Various Forms of Corolla.=—The corolla often assumes very definite
or distinct forms, especially when gamopetalous. It may have a long
tube with a wide-flaring limb, when it is said to be =funnelform=,
as in morning-glory and pumpkin. If the tube is very narrow and the
limb stands at right angles to it, the corolla is =salverform=, as
in phlox. If the tube is very short and the limb wide-spreading
and nearly circular in outline, the corolla is =rotate= or
=wheel-shaped=, as in potato.
A gamopetalous corolla or gamosepalous calyx is often cleft in
such way as to make two prominent parts. Such parts are said to be
=lipped= or =labiate=. Each of the lips or lobes may be notched or
toothed. In 5-membered flowers, the lower lip is usually 3-lobed and
the upper one 2-lobed. Labiate flowers are characteristic of the mint
family (Fig. 185), and the family therefore is called the Labiatæ.
(Literally, labiate means merely “lipped,” without specifying the
number of lips or lobes; but it is commonly used to designate
2-lipped flowers.) Strongly 2-parted polypetalous flowers may be
said to be labiate; but the term is oftenest used for gamopetalous
corollas.
[Illustration: FIG. 186.—PERSONATE FLOWER OF TOADFLAX.]
Labiate gamopetalous flowers that are closed in the throat (or
entrance to the tube) are said to be grinning or =personate=
(personate means _masked_). Snapdragon is a typical example; also
toadflax or butter-and-eggs (Fig. 186), and many related plants.
Personate flowers usually have definite relations to insect
pollination. Observe how an insect forces his head into the closed
throat of the toadflax.
The peculiar flowers of the pea tribes are explained in Figs. 187,
188.
[Illustration: FIG. 187.—FLOWERS OF THE COMMON BEAN, with one flower
opened (_a_) to show the structure.]
=Spathe Flowers.=—In many plants, very simple (often naked) flowers
are borne in dense, more or less fleshy spikes, and the spike is
inclosed in or attended by a leaf, sometimes corolla-like, known as
a =spathe=. The spike of flowers is technically known as a =spadix=.
This type of flower is characteristic of the great arum family,
which is chiefly tropical. The commonest wild representatives are
Jack-in-the-pulpit, or Indian turnip, and skunk cabbage. In the
former the flowers are all diclinous and naked. In the skunk cabbage
all the flowers are perfect and have four sepals. The common calla is
a good example of this type of inflorescence.
[Illustration: FIG. 188.—DIAGRAM OF ALFALFA FLOWER IN SECTION:
_C_, calyx; _D_, standard; _W_, wing; _K_, keel; _T_, stamen tube;
_F_, filament of tenth stamen; _X_, stigma; _Y_, style; _O_, ovary;
the dotted lines at _E_ show position of stamen tube, when pushed
upward by insects. Enlarged. ]
=Composite Flowers.=—The head (anthodium) or so-called “flower”
of sunflower (Fig. 189), thistle, aster, dandelion, daisy,
chrysanthemum, goldenrod, _is composed of several or many little
flowers, or_ =florets=. These florets are inclosed in a more or less
dense and usually green _involucre_. In the thistle (Fig. 190) this
involucre is prickly. A longitudinal section discloses the florets,
all attached at bottom to a common torus, and densely packed in the
involucre. The pink tips of these florets constitute the showy part
of the head.
[Illustration: FIG. 189.—HEAD OF SUNFLOWER.]
[Illustration: FIG. 190.—LONGITUDINAL SECTION OF THISTLE HEAD; also a
FLORET OF THISTLE.]
Each floret of the thistle (Fig. 190) is a complete flower. At _a_
is the ovary. At _b_ is a much-divided plumy calyx, known as the
=pappus=. The corolla is long-tubed, rising above the pappus, and
is enlarged and 5-lobed at the top, _c_. The style projects at
_e_. The five anthers are united about the style in a ring at _d_.
Such anthers are said to be =syngenesious=. These are the various
parts of the florets of the Compositæ. In some cases the pappus
is in the form of barbs, bristles, or scales, and sometimes it is
wanting. The pappus, as we shall see later, assists in distributing
the seed. Often the florets are not all alike. The corolla of those
in the outer circles may be developed into a _long, straplike, or
tubular part_, and the head then has the appearance of being one
flower with a border of petals. Of such is the sunflower (Fig. 189),
aster, bachelor’s button or cornflower, and field daisy (Fig. 211).
These long corolla-limbs are called =rays=. In some cultivated
composites, all the florets may develop rays, as in the dahlia and
the chrysanthemum. In some species, as dandelion, all the florets
naturally have rays. Syngenesious arrangement of anthers is the most
characteristic single feature of the composites.
[Illustration: FIG. 191.—PETALS ARISING FROM THE STAMINAL COLUMN OF
HOLLYHOCK, and accessory petals in the corolla-whorl. ]
=Double Flowers.=—Under the stimulus of cultivation and increased
food supply, flowers tend to become double. True doubling arises
in two ways, morphologically: (1) _stamens or pistils may produce
petals_ (Fig. 191); (2) _adventitious or accessory petals may arise
in the circle of petals_. Both these categories may be present in the
same flower. In the full double hollyhock the petals derived from
the staminal column are shorter and make a rosette in the centre of
the flower. In Fig. 192 is shown the doubling of a daffodil by the
modification of stamens. Other modifications of flowers are sometimes
known as doubling. For example, double dahlias, chrysanthemums, and
sunflowers are forms in which the disk flowers have developed rays.
The snowball is another case. In the wild snowball the external
flowers of the cluster are large and sterile. In the cultivated
plant all the flowers have become large and sterile. Hydrangea is a
similar case.
[Illustration: FIG. 192.—NARCISSUS OR DAFFODIL. Single flower at the
right.]
SUGGESTIONS.—=145.= If the pupil has been skilfully conducted
through this chapter _by means of careful study of specimens_
rather than as a mere memorizing process, he will be in mood
to challenge any flower that he sees and to make an effort to
understand it. Flowers are endlessly modified in form; but they can
be understood if the pupil looks first for the anthers and ovaries.
How may anthers and ovaries always be distinguished? =146.= It is
excellent practice to find the flowers in plants that are commonly
known by name, and to determine the main points in their structure.
What are the flowers in Indian corn? pumpkin or squash? celery?
cabbage? potato? pea? tomato? okra? cotton? rhubarb? chestnut?
wheat? oats? =147.= Do all forest trees have flowers? Explain.
=148.= Name all the monœcious plants you know. Diœcious. =149.=
What plants do you know that bloom before the leaves appear? Do
any bloom after the leaves fall? =150.= Explain the flowers of
marigold, hyacinth, lettuce, clover, asparagus, garden calla,
aster, locust, onion, burdock, lily-of-the-valley, crocus, Golden
Glow, rudbeckia, cowpea. =151.= Define a _flower_.
* * * * *
NOTE TO THE TEACHER.—It cannot be urged too often that _the
specimens themselves_ be studied. If this chapter becomes a mere
recitation on names and definitions, the exercise will be worse
than useless. Properly taught by means of the flowers themselves,
the names become merely incidental and a part of the pupil’s
language, and the subject has living interest.
CHAPTER XIX
THE FLOWER—FERTILIZATION AND POLLINATION
[Illustration: FIG. 193.—_B_, POLLEN escaping from anther; _A_,
pollen germinating on a stigma. Enlarged. ]
=Fertilization.=—_Seeds result from the union of two elements or
parts._ One of these elements is a cell-nucleus of the pollen-grain.
The other element is the cell-nucleus of an egg-cell, borne in the
ovary. The pollen-grain falls on the stigma (Fig. 193). It absorbs
the juices exuded by the stigma, and grows by sending out a tube
(Fig. 194). This tube grows downward through the style, absorbing
food as it goes, and finally reaches the egg-cell in the interior of
an ovule in the ovary (Fig. 195), and =fertilization=, or union of a
nucleus of the pollen and the nucleus of the egg-cell in the ovule,
takes place. _The ovule and embryo within then develops into a seed._
The growth of the pollen-tube is often spoken of as germination of
the pollen, but it is not germination in the sense in which the word
is used when speaking of seeds.
[Illustration: FIG. 194.—A POLLEN-GRAIN AND THE GROWING TUBE.]
Better seeds—that is, those that produce stronger and more
fruitful plants—often result when the _pollen comes from another
flower_. Fertilization effected between different flowers is
cross-fertilization; that resulting from the application of
pollen to pistils in the same flower is =close-fertilization= or
=self-fertilization=. It will be seen that the cross-fertilization
relationship may be of many degrees—between two flowers in the same
cluster, between those in different clusters on the same branch,
between those on different plants. Usually fertilization takes place
only between plants of the same species or kind.
In many cases there is, in effect, _an apparent selection of pollen_
when pollen from two or more sources is applied to the stigma.
Sometimes the foreign pollen, if from the same kind of plant, grows,
and fertilization results, while pollen from the same flower is less
promptly effective. If, however, no foreign pollen is present, the
pollen from the same flower may finally serve the same purpose.
[Illustration: FIG. 195.—DIAGRAM TO REPRESENT FERTILIZATION.
_s_, stigma; _st_, style; _ov_, ovary; _o_, ovule; _p_,
pollen-grain; _pt_, pollen-tube; _e_, egg-cell; _m_, micropyle.]
In order that the pollen may grow, _the stigma must be ripe_. At
this stage the stigma is usually moist and sometimes sticky. A ripe
stigma is said to be =receptive=. The stigma may remain receptive
for several hours or even days, depending on the kind of plant, the
weather, and how soon pollen is received. Watch a certain flower
every day to see the anther locules open and the stigma ripen. When
fertilization takes place, the stigma dies. Observe, also, how soon
the petals wither after the stigma has received pollen.
=Pollination.=—The transfer of the pollen from anther to stigma is
known as =pollination=. The pollen may fall of its own weight on
the adjacent stigma, or it may be carried from flower to flower by
wind, insects, or other agents. There may be =self-pollination=
or =cross-pollination=, and of course it must always precede
fertilization.
[Illustration: FIG. 196.—ANTHER OF AZALEA, opening by terminal pores.]
Usually the pollen is discharged by the bursting of the anthers. The
commonest method of discharge is through a _slit_ on either side
of the anther (Fig. 193). Sometimes it discharges through a _pore_
at the apex, as in azalea (Fig. 196), rhododendron, huckleberry,
wintergreen. In some plants a part of the anther wall raises or falls
as a _lid_, as in barberry (Fig. 197), blue cohosh, May apple. The
opening of an anther (as also of a seed-pod) is known as =dehiscence=
(_de_, from; _hisco_, to gape). When an anther or seed-pod opens, it
is said to _dehisce_.
[Illustration: FIG. 197.—BARBERRY STAMEN, with anther opening by
lids.]
_Most flowers are so constructed as to increase the chances of
cross-pollination._ We have seen that the stigma may have the power
of choosing foreign pollen. The commonest means of necessitating
cross-pollination is the _different times of maturing of stamens
and pistils in the same flower_. In most cases the stamens mature
first: the flower is then =proterandrous=. When the pistils mature
first, the flower is =proterogynous=. (_Aner_, _andr_, is a Greek
root often used, in combinations, for stamen, and _gyne_ for pistil.)
The difference in time of ripening may be an hour or two, or it may
be a day. The ripening of the stamens and the pistils at different
times is known as =dichogamy=, and flowers of such character are
said to be dichogamous. There is little chance for dichogamous
flowers to pollinate themselves. Many flowers are _imperfectly
dichogamous_—some of the anthers mature simultaneously with the
pistils, so that there is chance for self-pollination in case foreign
pollen does not arrive. Even when the stigma receives pollen from
its own flower, cross-fertilization may result. The hollyhock is
proterandrous. Fig. 198 shows a flower recently expanded. The centre
is occupied by the column of stamens. In Fig. 199, showing an older
flower, the long styles are conspicuous.
[Illustration: FIG. 198.—FLOWER OF HOLLYHOCK; proterandrous.]
[Illustration: FIG. 199.—OLDER FLOWER OF HOLLYHOCK.]
_Some flowers are so constructed as to prohibit self-pollination._
Very irregular flowers are usually of this kind. With some of them,
the petals form a sac to inclose the anthers and the pollen cannot be
shed on the stigma but is retained until a bee forces the sac open;
the pollen is rubbed on the hairs of the bee and transported. Regular
flowers usually depend mostly on dichogamy and the selective power
of the pistil to insure crossing. _Flowers that are very irregular
and provided with nectar and strong perfume are usually pollinated by
insects._ Gaudy colours probably attract insects in many cases, but
perfume appears to be a greater attraction.
[Illustration: FIG. 200.—FLOWER OF LARKSPUR.]
[Illustration: FIG. 201.—ENVELOPES OF A LARKSPUR. There are five wide
sepals, the upper one being spurred. There are four small petals. ]
The insect _visits the flower for the nectar_ (for the making of
honey) _and may unknowingly carry the pollen_. Spurs and sacs in the
flower are =nectaries= (Fig. 200), but in spurless flowers the nectar
is usually secreted _in the bottom of the flower cup_. This compels
the insect to pass by the anther and rub against the pollen before
it reaches the nectar. Sometimes the anther is a long lever poised
on the middle point and the insect bumps against one end and lifts
it, thus bringing the other end of the lever with the pollen sacs
down on its back. Flowers that are pollinated by insects are said to
be =entomophilous= (“insect loving”). Fig. 200 shows a larkspur. The
envelopes are separated in Fig. 201. The long spur at once suggests
insect pollination. The spur is a sepal. Two hollow petals project
into this spur, apparently serving to guide the bee’s tongue. The two
smaller petals, in front, are peculiarly coloured and perhaps serve
the bee in locating the nectary. The stamens ensheath the pistils
(Fig. 202). As the insect stands on the flower and thrusts its head
into the centre, the envelopes are pushed downward and outward and
the pistil and stamens come in contact with its abdomen. Since the
flower is proterandrous, the pollen that the pistils receive from the
bee’s abdomen must come from another flower. Note a somewhat similar
arrangement in the toadflax or butter-and-eggs.
[Illustration: FIG. 202.—STAMENS OF LARKSPUR, surrounding the
pistils. ]
In some cases (Fig. 203) the stamens are longer than the pistil in
one flower and shorter in another. If the insect visits such flowers,
it gets pollen on its head from the long-stamen flower, and deposits
this pollen on the stigma in the long-pistil flower. Such flowers are
=dimorphous= (of two forms). If pollen from its own flower and from
another flower both fall on the stigma, the probabilities are that
the stigma will choose the foreign pollen.
[Illustration: FIG. 203.—DIMORPHIC FLOWERS OF PRIMROSE.]
_Many flowers are pollinated by the wind._ They are said to be
=anemophilous= (“wind loving”). Such flowers produce great
quantities of pollen, for much of it is wasted. They usually have
broad stigmas, which expose large surfaces to the wind. They are
usually lacking in gaudy colours and in perfume. Grasses and pine
trees are typical examples of anemophilous plants.
[Illustration: FIG. 204.—FLOWERS OF BLACK WALNUT: Two Pistillate
flowers at _A_, and staminate catkins at _B_.]
In many cases cross-pollination is assured because the _stamens
and the pistils are in different flowers_ (diclinous). Monœcious
and diœcious plants may be pollinated by wind or insects, or other
agents (Fig. 204). They are usually wind-pollinated, although willows
are often, if not mostly, insect-pollinated. The Indian corn is a
monœcious plant. The staminate flowers are in a terminal panicle
(tassel). The pistillate flowers are in a dense spike (ear), inclosed
in a sheath or husk. Each “silk” is a style. Each pistillate flower
produces a kernel of corn. Sometimes a few pistillate flowers are
borne in the tassel and a few staminate flowers on the tip of the
ear. Is self-fertilization possible with the corn? Why does a
“volunteer” stalk standing alone in a garden have only a few grains
on the ear? What is the direction of the prevailing wind in summer?
If only two or three rows of corn are planted in a garden where
prevailing winds occur, in which direction had they better run?
[Illustration: FIG. 205.—COMMON BLUE VIOLET. The familiar flowers
are shown, natural size. The corolla is spurred. Late in the season,
cleistogamous flowers are often borne on the surface of the ground. A
small one is shown at _a_. A nearly mature pod is shown at _b_. Both
_a_ and _b_ are one third natural size.]
Although most flowers are of such character as to insure or increase
the chances of cross-pollination, there are some _that absolutely
forbid crossing_. These flowers are usually borne beneath or on the
ground, and they lack showy colours and perfumes. They are known
as =cleistogamous= flowers (meaning self-fertilizing flowers). The
plant has normal showy flowers that may be insect-pollinated, and
in addition is provided with these simplified flowers. Only a few
plants bear cleistogamous flowers. Hog-peanut, common blue violet,
fringed wintergreen, and dalibarda are the best subjects in this
country. Fig. 205 shows a cleistogamous flower of the blue violet at
_a_. Above the true roots, slender stems bear these flowers, that
are provided with a calyx, and a curving corolla which does not
open. Inside are the stamens and the pistils. Late in the season
the cleistogamous flowers may be found just underneath the mould.
They never rise above ground. The following summer one may find a
seedling plant, in some kinds of plants, with the remains of the
old cleistogamous flower still adhering to the root. Cleistogamous
flowers usually appear after the showy flowers have passed. They
seem to insure a crop of seed by a method that expends little of the
plant’s energy. The pupil will be interested to work out the fruiting
of the peanut (Fig. 206). Unbaked fresh peanuts grow readily and can
easily be raised in Canada, in a warm sandy garden.
[Illustration: FIG. 206.—PODS OF PEANUTS RIPENING UNDERGROUND.]
[Illustration: FIG. 207.—STRUGGLE FOR EXISTENCE AMONG THE APPLE
FLOWERS.]
SUGGESTIONS.—=152.= _Not all the flowers produce seeds._ Note that
an apple tree may bloom very full, but that only relatively few
apples may result (Fig. 207). _More pollen is produced than is
needed to fertilize the flowers_; this increases the chances that
sufficient stigmas will receive acceptable pollen to enable the
plant to perpetuate its kind. At any time in summer, or even in
fall, examine the apple trees carefully to determine whether any
dead flowers or flower stalks still remain about the apple; or,
examine any full-blooming plant to see whether any of the flowers
fail. =153.= Keep watch on any plant to see whether insects visit
it. What kind? When? What for? =154.= Determine whether the calyx
serves any purpose in protecting the flower. Very carefully remove
the calyx from a bud that is normally exposed to heat and sun and
rain, and see whether the flower then fares as well as others.
=155.= Cover a single flower on its plant with a tiny paper or
muslin bag so tightly that no insect can get in. If the flower sets
fruit, what do you conclude? =156.= Remove carefully the corolla
from a flower nearly ready to open, preferably one that has no
other flowers very close to it. Watch for insects. =157.= Find the
nectar in any flower that you study. =158.= Remove the stigma. What
happens? =159.= Which of the following plants have perfect flowers:
pea, bean, pumpkin, cotton, clover, buckwheat, potato, Indian
corn, peach, chestnut, hickory, watermelon, sunflower, cabbage,
rose, begonia, geranium, cucumber, calla, willow, cottonwood,
cantaloupe? What have the others? =160.= On wind-pollinated plants,
are either anthers or stigmas more numerous? =161.= Are very small
coloured flowers usually borne singly or in clusters? =162.= Why
do rains at blooming time often lessen the fruit crop? =163.= Of
what value are bees in orchards? =164.= _The crossing of plants to
improve varieties or to obtain new varieties._—It may be desired to
perform the operation of pollination by hand. In order to insure
the most definite results, every effort should be made rightly to
apply the pollen which it is desired shall be used, and rigidly
to exclude all other pollen. (_a_) The first requisite is to
remove the anthers from the flower which it is proposed to cross,
and they _must be removed before the pollen has been shed_. The
flower-bud is therefore opened and the anthers taken out. Cut off
the floral envelopes with small, sharp-pointed scissors, then cut
out or pull out the anthers, leaving only the pistil untouched; or
merely open the corolla at the end and pull out the anthers with
a hook or tweezers; and this method is often the best one. It is
best to delay the operation as long as possible and yet not allow
the bud to open (and thereby expose the flower to foreign pollen)
nor the anthers to discharge the pollen. (_b_) _The flower must
next be covered with a paper bag to prevent the access of pollen_
(Figs. 208, 209). If the stigma is not receptive at the time (as
it usually is not), the desired pollen is not applied at once. The
bag may be removed from time to time to allow of examination of
the pistil, and when the stigma is mature, which is told by its
glutinous or roughened appearance, the time for pollination has
come. If the bag is slightly moistened, it can be puckered more
tightly about the stem of the plant. The time required for the
stigma to mature varies from several hours to a few days. (_c_)
When the stigma is ready, an unopened anther from the desired
flower is crushed on the finger nail or a knife blade, and the
pollen is rubbed on the stigma by means of a tiny brush, the
point of a knife blade, or a sliver of wood. The flower is _again
covered with the bag_, which is allowed to remain for several days
until all danger of other pollination is past. Care must be taken
completely to cover the stigmatic surface with pollen, if possible.
The seeds produced by a crossed flower produce _hybrids_, or plants
having parents belonging to different varieties or species. =165.=
One of the means of securing new forms of plants is by making
hybrids. Why?
[Illustration: FIG. 208.—A PAPER BAG, with string inserted.]
[Illustration: FIG. 209.—THE BAG TIED OVER A FLOWER.]
[Illustration: FIG. 210.—The fig is a hollow torus with flowers borne
on the inside, and pollinated by insects that enter at the apex.]
CHAPTER XX
FLOWER-CLUSTERS
=Origin of the Flower-cluster.=—We have seen that branches arise from
the axils of leaves. Sometimes the leaves may be reduced to bracts
and yet branches are borne in their axils. Some of the branches grow
into long limbs; others become short spurs; _others bear flowers_. In
fact, a flower is itself a specialized branch.
[Illustration: FIG. 211.—TERMINAL FLOWERS OF THE WHITEWEED (in some
places called ox-eye daisy).]
Flowers are usually borne near the top of the plant. Often they
are produced in great numbers. It results, therefore, that flower
branches usually stand close together, forming a =cluster=. The shape
and the arrangement of the flower-cluster _differ with the kind of
plant_, since each plant has its own mode of branching.
Certain definite or well-marked types of flower-clusters have
received names. Some of these names we shall discuss, but the
flower-clusters that perfectly match the definitions are the
exception rather than the rule. The determining of the kinds of
flower-clusters is one of the most perplexing subjects in descriptive
botany. We may classify the subject around three ideas: =solitary
flowers=, =centrifugal= or =determinate clusters=, =centripetal= or
=indeterminate clusters=.
[Illustration: FIG. 212.—LATERAL FLOWER OF AN ABUTILON. A greenhouse
plant.]
=Solitary Flowers.=—In many cases flowers are borne singly; they are
separated from other flowers by leaves. They are then said to be
=solitary=. The solitary flower may be either at the end of the main
shoot or axis (Fig. 211), when it is said to be =terminal=; or from
the side of the shoot (Fig. 212), when it is said to be =lateral= or
=axillary=.
=Centripetal Clusters.=—If the flower-bearing axils were rather
close together, an open or leafy flower-cluster might result. If the
plant continues to grow from the tip, the older flowers are left
farther and farther behind. If the cluster were so short as to be
flat or convex on top, the outermost flowers would be the older. A
flower-cluster in which the lower or outer flowers open first is
said to be a =centripetal cluster=. It is sometimes said to be an
=indeterminate= cluster, since it is the result of a type of growth
which may go on more or less continuously from the apex.
The simplest form of a definite centripetal cluster is a =raceme=,
which is an open elongated cluster in which the _flowers are borne
singly on very short branches_ and open from below (that is, from
the older part of the shoot) upwards (Fig. 213). The raceme may be
_terminal_ to the main branch; or it may be _lateral_ to it, as in
Fig. 214. Racemes often bear the flowers on one side of the stem,
thus forming a single row.
[Illustration: FIG. 213.—RACEME OF CURRANT. Terminal or lateral?]
When a centripetal flower-cluster is long and dense and the flowers
are sessile or nearly so, it is called a =spike= (Fig. 215). Common
examples of spikes are plantain, mignonette, mullein.
[Illustration: FIG. 214.—LATERAL RACEMES (in fruit) of BARBERRY.]
[Illustration: FIG. 215.—SPIKE OF PLANTAIN.]
A very _short and dense spike_ is a =head=. Clover (Fig. 216) is
a good example. The sunflower and related plants bear many small
flowers in a very dense and often flat head. Note that in the
sunflower (Fig. 189) the outside or exterior flowers open first.
Another special form of spike is the =catkin=, which usually has
scaly bracts, the whole cluster being deciduous after flowering or
fruiting, and the flowers (in typical cases) having only stamens
or pistils. Examples are the “pussies” of willows (Fig. 182) and
flower-clusters of oak (Fig. 180), walnuts (Fig. 204), poplars.
[Illustration: FIG. 216.—HEAD OF CLOVER BLOSSOMS.]
[Illustration: FIG. 217.—CORYMB OF CANDY-TUFT.]
When a loose, elongated centripetal flower-cluster has some primary
branches simple, and others irregularly branched, it is called a
=panicle=. It is a branching raceme. Because of the earlier growth of
the lower branches, the panicle is usually broadest at the base or
conical in outline. True panicles are not very common.
When an indeterminate flower-cluster is short, so that the _top is
convex or flat_, it is a =corymb= (Fig. 217). The outermost flowers
open first. Centripetal flower-clusters are sometimes said to be
corymbose in mode.
When the branches of an indeterminate cluster _arise from a common
point_, like the frame of an umbrella, the cluster is an =umbel=
(Fig. 218). Typical umbels occur in carrot, parsnip, caraway, and
other plants of the parsley family: the family is known as the
Umbelliferæ, or umbel-bearing family. In the carrot and many other
Umbelliferæ, there are small or secondary umbels, called =umbellets=,
at the end of each of the main branches. (In the centre of the wild
carrot umbel one often finds a single, blackish, often aborted
flower, comprising a 1-flowered umbellet.)
[Illustration: FIG. 218.—REMAINS OF A LAST YEAR’S UMBEL OF WILD
CARROT.]
=Centrifugal or Determinate Clusters.=—When the terminal or central
flower opens first, the cluster is said to be =centrifugal=. The
growth of the shoot or cluster is =determinate=, since the length is
definitely determined or stopped by the terminal flower. Fig. 219
shows a determinate or centrifugal mode of flower bearing.
[Illustration: FIG. 219.—DETERMINATE OR CYMOSE ARRANGEMENT.—Wild
geranium. ]
Dense centrifugal clusters are usually flattish on top because of
the cessation of growth in the main or central axis. These compact
flower-clusters are known as =cymes=. Centrifugal clusters are
sometimes said to be cymose in mode. Apples, pears (Fig. 220), and
elders bear flowers in cymes. Some cyme-forms are like umbels in
general appearance. A head-like cymose cluster is a =glomerule=; it
blooms from the top downwards rather than from the base upwards.
=Mixed Clusters.=—Often the cluster is mixed, being determinate in
one part and indeterminate in another part of the same cluster. The
main cluster may be indeterminate, but the branches determinate. The
cluster has the appearance of a panicle, and is usually so called,
but it is really a =thyrse=. Lilac is a familiar example of a thyrse.
In some cases the main cluster is determinate and the branches are
indeterminate, as in hydrangea and elder.
[Illustration: FIG. 220.—CYME OF PEAR. Often imperfect.]
=Inflorescence.=—The _mode_ or _method_ of flower arrangement is
known as the =inflorescence=. That is, the inflorescence is cymose,
corymbose, paniculate, spicate, solitary, determinate, indeterminate.
By custom, however, the word “inflorescence” has come to be used
in works on descriptive botany for the _flower-cluster itself_. Thus
a cyme or a panicle may be called an inflorescence. It will be seen
that even solitary flowers follow either indeterminate or determinate
methods of branching.
[Illustration: FIG. 221.—FORMS OF CENTRIPETAL FLOWER-CLUSTERS.
1, raceme; 2, spike; 3, umbel; 4, head or anthodium; 5, corymb.]
[Illustration: FIG. 222.—CENTRIPETAL INFLORESCENCE, _continued_.
6, spadix; 7, compound umbel; 8, catkin.]
[Illustration: FIG. 223.—CENTRIFUGAL INFLORESCENCE.
1, cyme; 2, scirpioid raceme (or half cyme).]
=The flower-stem.=—The stem of a solitary flower is known as a
=peduncle=; also the general stem of a _flower-cluster_. The stem of
the individual flower in a cluster is a =pedicel=. In the so-called
stemless plants the peduncle may arise directly from the ground, or
crown of the plant, as in dandelion, hyacinth, garden daisy; this
kind of peduncle is called a =scape=. A scape may bear one or many
flowers. It has no foliage leaves, but it may have bracts.
SUGGESTIONS.—=166.= Name six columns in your notebook as follows:
spike, raceme, corymb, umbel, cyme, solitary. Write each of the
following in its appropriate column: larkspur, grape, rose,
wistaria, onion, bridal wreath, banana, hydrangea, phlox, China
berry, lily-of-the-valley, Spanish dagger (or yucca), sorghum,
tuberose, hyacinth, mustard, goldenrod, peach, hollyhock, mullein,
crêpe myrtle, locust, narcissus, snapdragon, peppergrass,
shepherd’s purse, coxcomb, wheat, hawthorn, geranium, carrot,
elder, millet, dogwood, castor bean; substitute others for
plants that do not grow in your region. =167.= In the study
of flower-clusters, it is well to choose first those that are
fairly typical of the various classes discussed in the preceding
paragraphs. As soon as the main types are well fixed in the
mind, random clusters should be examined, for the pupil must
never receive the impression that all flower-clusters follow the
definitions in books. Clusters of some of the commonest plants are
very puzzling, but the pupil should at least be able to discover
whether the inflorescence is determinate or indeterminate. Figures
221 to 223 illustrate the theoretical modes of inflorescence. The
numerals indicate the order of opening.
CHAPTER XXI
FRUITS
The ripened ovary, with its attachments, is known as the fruit. _It
contains the seeds._ If the pistil is simple, or of one carpel, the
fruit also will have one compartment. If the pistil is compound,
or of more than one carpel, the fruit usually has an equal number
of compartments. The compartments in pistil and fruit are known as
=locules= (from Latin _locus_, meaning “a place”).
[Illustration: FIG. 224.—DENTARIA, OR TOOTH-WORT, in fruit.]
The simplest kind of fruit is a _ripened 1-loculed ovary_. The first
stage in complexity is a ripened _2- or many-loculed ovary_. Very
complex forms may arise by the _attachment of other parts to the
ovary_. Sometimes the style persists and becomes a beak (mustard
pods, dentaria, Fig. 224), or a tail as in clematis; or the calyx
may be attached to the ovary; or the ovary may be embedded in the
receptacle, and ovary and receptacle together constitute the fruit:
or an involucre may become a part of the fruit, as possibly in the
walnut and the hickory (Fig. 225), and the cup of the acorn (Fig.
226). The chestnut and the beech bear a prickly involucre, but the
nuts, or true fruits, are not grown fast to it, and the involucre
can scarcely be called a part of the fruit. A ripened ovary is a
=pericarp=. A pericarp to which other parts adhere has been called an
accessory or reënforced fruit. (Page 169.)
[Illustration: FIG. 225.—HICKORY-NUT. The nut is the fruit, contained
in a husk. ]
[Illustration: FIG. 226.—LIVE-OAK ACORN. The fruit is the “seed”
part; the involucre is the “cup.” ]
Some fruits are =dehiscent=, or split open at maturity and liberate
the seeds; others are =indehiscent=, or do not open. A dehiscent
pericarp is called a =pod=. The parts into which such a pod breaks
or splits are known as =valves=. In indehiscent fruits the seed is
liberated by the decay of the envelope, or by the rupturing of the
envelope by the germinating seed. Indehiscent winged pericarps are
known as =samaras= or =key fruits=. Maple (Fig. 227), elm (Fig. 228),
and ash (Fig. 93) are examples.
[Illustration: FIG. 227.—KEY OF SUGAR MAPLE.]
[Illustration: FIG. 228.—KEY OF COMMON AMERICAN ELM.]
=Pericarps.=—The simplest pericarp is a dry, one-seeded, indehiscent
body. It is known as an =akene=. A head of akenes is shown in Fig.
229, and the structure is explained in Fig. 230. Akenes may be seen
in buttercup, hepatica, anemone, smartweed, buckwheat.
[Illustration: FIG. 229.—AKENES OF BUTTERCUP.]
[Illustration: FIG. 230.—AKENES OF BUTTERCUP, one in longitudinal
section. ]
A 1-loculed pericarp which dehisces along the front edge (that is,
the inner edge, next the centre of the flower) is a =follicle=. The
fruit of the larkspur (Fig. 231) is a follicle. There are usually
five of these fruits (sometimes three or four) in each larkspur
flower, each pistil ripening into a follicle. If these pistils were
united, a single compound pistil would be formed. Columbine, peony,
ninebark, milkweed, also have follicles.
[Illustration: FIG. 231.—FOLLICLE OF LARKSPUR.]
[Illustration: FIG. 232.—A BEAN POD.]
A 1-loculed pericarp that dehisces on both edges is a =legume=. Peas
and beans are typical examples (Fig. 232); in fact, this character
gives name to the pea family,—Leguminosæ. Often the valves of the
legume twist forcibly and expel the seeds, throwing them some
distance. The word “pod” is sometimes restricted to legumes, but it
is better to use it generically for all dehiscent pericarps.
[Illustration: FIG. 233.—CAPSULE OF CASTOR-OIL BEAN AFTER DEHISCENCE. ]
A compound pod—dehiscing pericarp of two or more carpels—is a
=capsule= (Figs. 233, 234, 236, 237). Some capsules are of one
locule, but they may have been compound when young (in the ovary
stage) and the partitions may have vanished. Sometimes one or more
of the carpels are uniformly crowded out by the exclusive growth of
other carpels (Fig. 235). The seeds or parts which are crowded out
are said to be _aborted_.
[Illustration: FIG. 234.—CAPSULE OF MORNING GLORY.]
[Illustration: FIG. 235.—THREE-CARPELED FRUIT OF HORSE-CHESTNUT. Two
locules are closing by abortion of the ovules. ]
There are several ways in which capsules dehisce or open. When
they break _along the partitions_ (or septa), the mode is known as
=septicidal dehiscence= (Fig. 236); In septicidal dehiscence the
fruit separates into parts representing the original carpels. These
carpels may still be entire, and they then dehisce individually,
usually along the inner edge as if they were follicles. When the
compartments _split in the middle, between the partitions_, the mode
is =loculicidal dehiscence= (Fig. 237). In some cases the dehiscence
is _at the top_, when it is said to be =apical= (although several
modes of dehiscence are here included). When the _whole top comes
off_, as in purslane and garden portulaca (Fig. 238), the pod is
known as a =pyxis=. In some cases apical dehiscence is by means of a
hole or clefts.
[Illustration: FIG. 236.—ST. JOHN’S WORT. Septicidal.]
[Illustration: FIG. 237.—LOCULICIDAL POD OF DAY-LILY.]
The peculiar capsule of the mustard family, or Cruciferæ, is known
as a =silique= when it is distinctly longer than broad (Fig. 224),
and a =silicle= when its breadth nearly equals or exceeds its length.
A cruciferous capsule is 2-carpeled, with a thin partition, each
locule containing seeds in two rows. The two valves detach from
below upwards. Cabbage, turnip, mustard, water-cress, radish, rape,
shepherd’s purse, sweet alyssum, wall-flower, honesty, are examples.
[Illustration: FIG. 238.—PYXIS OF PORTULACA OR ROSE-MOSS.]
[Illustration: FIG. 239.—BERRIES OF GOOSEBERRY. Remains of calyx at
_c_.]
[Illustration: FIG. 240.—BERRY OF THE GROUND CHERRY OR HUSK TOMATO,
contained in the inflated calyx. ]
[Illustration: FIG. 241.—ORANGE; example of a berry.]
The pericarp may be _fleshy and indehiscent_. A pulpy pericarp
with several or many seeds is a berry (Figs. 239, 240, 241).
To the horticulturist a berry is a small, soft, edible fruit,
without particular reference to its structure. The botanical and
horticultural conceptions of a berry are, therefore, unlike. In
the botanical sense, gooseberries, currants, grapes, tomatoes,
potato-balls, and even eggplant fruits and oranges (Fig. 241) are
berries; strawberries, raspberries, blackberries are not.
[Illustration: FIG. 242.—PLUM; example of a drupe.]
A fleshy pericarp containing one relatively large seed or stone is a
=drupe=. Examples are plum (Fig. 242), peach, cherry, apricot, olive.
The walls of the pit in the plum, peach, and cherry are formed from
the inner coats of the ovary, and the flesh from the outer coats.
Drupes are also known as _stone-fruits_.
[Illustration: FIG. 243.—FRUIT OF RASPBERRY.]
Fruits that are formed by the subsequent union of separate pistils
are =aggregate fruits=. The carpels in aggregate fruits are usually
more or less fleshy. In the raspberry and the blackberry flower,
the pistils are essentially distinct, but as the pistils ripen they
cohere and form one body (Figs. 243, 244).
[Illustration: FIG. 244.—AGGREGATE FRUIT OF MULBERRY; and a separate
fruit.]
Each of the carpels or pistils in the raspberry and the blackberry
is a little drupe or =drupelet=. In the raspberry the entire fruit
separates from the torus, leaving the torus on the plant. In the
blackberry and the dewberry the fruit adheres to the torus, and the
two are removed together when the fruit is picked.
=Accessory Fruits.=—When the pericarp and some other part grow
together, the fruit is said to be =accessory= or =reënforced=. An
example is the strawberry (Fig. 245). The edible part is a greatly
_enlarged torus_, and the pericarps are akenes embedded in it. These
akenes are commonly called seeds.
[Illustration: FIG. 245.—STRAWBERRY; fleshy torus in which akenes are
embedded.]
Various kinds of reënforced fruits have received special names. One
of these is the =hip=, characteristic of roses. In this case, the
torus is deep and hollow, like an urn, and the separate akenes are
borne inside it. The mouth of the receptacle may close, and the walls
sometimes become fleshy; the fruit may then be mistaken for a berry.
The fruit of the pear, apple, and quince is known as a =pome=. In
this case the five united carpels are completely buried in the hollow
torus, and the torus makes most of the edible part of the ripe fruit,
while the pistils are represented by the core (Fig. 246). Observe the
sepals on the top of the torus (apex of the fruit) in Fig. 246. Note
the outlines of the embedded pericarp in Fig. 247.
[Illustration: FIG. 246.—SECTION OF AN APPLE.]
[Illustration: FIG. 247.—CROSS-SECTION OF AN APPLE.]
=Gymnospermous Fruits.=—In pine, spruces, and their kin, there is no
fruit in the sense in which the word is used in the preceding pages,
because _there is no ovary_. The ovules are naked or uncovered, in
the axils of the scales of the young cone, and they have neither
style nor stigma. The pollen falls directly on the mouth of the
ovule. The ovule ripens into a seed, which is usually winged. Because
the ovule is not borne in a sac or ovary, these plants are called
=gymnosperms= (Greek for “naked seeds”). All the true cone-bearing
plants are of this class; also certain other plants, as red cedar,
juniper, yew. The plants are monœcious or sometimes diœcious. The
staminate flowers are mere naked stamens borne beneath scales,
in small yellow catkins which soon fall. The pistillate flowers
are naked ovules beneath scales on cones that persist (Fig. 29).
Gymnospermous seeds may have several cotyledons.
SUGGESTIONS.—=168.= Study the following fruits, or any five fruits
chosen by the teacher, and answer the questions for each: Apple,
peach, bean, tomato, pumpkin. What is its form? Locate the scar
left by the stem. By what kind of stem was it attached? Are there
any remains of the blossom at the blossom end? Describe texture and
colour of surface. Divide the fruit into the seed vessel and the
surrounding part. Has the fruit any pulp or flesh? Is it within or
without the seed vessel? Is the seed vessel simple or sub-divided?
What is the number of seeds? Are the seeds free, attached to
the wall of the vessel, or to a support in the centre? Are they
arranged in any order? What kind of wall has the seed vessel? What
is the difference between a peach stone and a peach seed? =169.=
The nut fruits are always available for study. Note the points
suggested above. Determine what the meat or edible part represents,
whether cotyledons or not. Figure 248 is suggestive. =170.= Mention
all the fleshy fruits you know, tell where they come from, and
refer them to their proper groups. =171.= What kinds of fruit
can you buy in the market, and to what groups or classes do they
belong? Of which fruits are the seeds only, and not the pericarps,
eaten? =172.= An ear of corn is always available for study. What is
it—a fruit or a collection of fruits? How are the grains arranged
on the cob? How many rows do you count on each of several ears?
Are all the rows on an ear equally close together? Do you find
an ear with an odd number of rows? How do the parts of the husk
overlap? Does the husk serve as protection from rain? Can birds
pick out the grains? How do insect enemies enter the ear? How and
when do weevils lay eggs on corn? =173.= _Study a grain of corn._
Is it a seed? Describe the shape of a grain. Colour. Size. Does its
surface show any projections or depressions? Is the seed-coat thin
or thick? Transparent or opaque? Locate the hilum. Where is the
silk scar? What is the silk? Sketch the grain from the two points
of view that show it best. Where is the embryo? Does the grain have
endosperm? What is dent corn? Flint corn? How many kinds of corn do
you know? For what are they used?
[Illustration: FIG. 248.—PECAN FRUIT.]
NOTE TO TEACHER.—There are few more interesting subjects to
beginning pupils than fruits,—the pods of many kinds, forms, and
colours, the berries, and nuts. This interest may well be utilized
to make the teaching alive. All common edible fruits of orchard and
vegetable garden should be brought into this discussion. Of dry
fruits, as pods, burs, nuts, collections may be made for the school
museum. Fully mature fruits are best for study, particularly if it
is desired to see dehiscence. For comparison, pistils and partially
grown fruits should be had at the same time. If the fruits are not
ripe enough to dehisce, they may be placed in the sun to dry. In
the school it is well to have a collection of fruits for study. The
specimens may be kept in glass jars. _Always note exterior of fruit
and its parts; interior of fruit with arrangement and attachment of
contents._
CHAPTER XXII
DISPERSAL OF SEEDS
It is to the plant’s advantage to have its seeds distributed as
widely as possible. _It has a better chance of surviving in the
struggle for existence._ It gets away from competition. Many seeds
and fruits are of such character as to increase their chances of
wide dispersal. The commonest means of dissemination may be classed
under four heads: _explosive fruits_; _transportation by wind_;
_transportation by birds_; _burs_.
[Illustration: FIG. 249.—EXPLOSION OF THE BALSAM POD.]
[Illustration: FIG. 250.—EXPLOSIVE FRUITS OF OXALIS.
An exploding pod is shown at _c_. The dehiscence is shown at _b_.
The structure of the pod is seen at _a_.]
=Explosive Fruits.=—_Some pods open with explosive force and
discharge the seeds._ Even beans and everlasting peas do this.
More marked examples are the locust, witch hazel, garden balsam
(Fig. 249), wild jewel-weed or impatiens (touch-me-not), violet,
crane’s-bill or wild geranium, bull nettle, morning-glory, and the
oxalis (Fig. 250). The oxalis is common in several species in the
wild and in cultivation. One of them is known as wood sorrel. Figure
250 shows the common yellow oxalis. The pod opens loculicidally. The
elastic tissue suddenly contracts when dehiscence takes place, and
the seeds are thrown violently. The squirting cucumber is easily
grown in a garden (procure seeds of seedsmen), and the fruits
discharge the seeds with great force, throwing them many feet.
=Wind Travelers.=—Wind-transported seeds are of two general kinds:
those that are _provided with wings_, as the flat seeds of catalpa
(Fig. 251) and cone-bearing trees and the samaras of ash, elm, tulip
tree, ailanthus, and maple; and those which have _feathery buoys_ or
_parachutes_ to enable them to float in the air. Of the latter kind
are the fruits of many composites, in which the pappus is copious and
soft. Dandelion and thistle are examples. The silk of the milkweed
and probably the hairs on the cotton seed have a similar office, and
also the wool of the cat-tail. Recall the cottony seeds of the willow
and the poplar.
[Illustration: FIG. 251.—WINGED SEEDS OF CATALPA.]
=Dispersal by Birds.=—_Seeds of berries and of other small fleshy
fruits are carried far and wide by birds._ The pulp is digested,
but the seeds are not injured. Note how the cherries, raspberries,
blackberries, June-berries, and others spring up in the fence rows,
where the birds rest. Some berries and drupes persist far into
winter, when they supply food to cedar birds, robins, and the winter
birds. Red cedar is distributed by birds. Many of these pulpy fruits
are agreeable as human food, and some of them have been greatly
enlarged or “improved” by the arts of the cultivator. The seeds are
usually indigestible.
=Burs.=—Many seeds and fruits bear spines, hooks, and hairs, which
_adhere to the coats of animals and to clothing_. The burdock has
an involucre with hooked scales, containing the fruits inside. The
clotbur is also an involucre. Both are composite plants, allied to
thistles, but the whole head, rather than the separate fruits, is
transported. In some composite fruits the pappus takes the form of
hooks and spines, as in the “Spanish bayonets” and “pitch-forks.”
Fruits of various kinds are known as “stick tights,” as of the
agrimony and hound’s-tongue. Those who walk in the woods in late
summer and fall are aware that plants have means of disseminating
themselves (Fig. 252). If it is impossible to identify the burs which
one finds on clothing, the seeds may be planted and specimens of the
plant may then be grown.
[Illustration: FIG. 252.—STEALING A RIDE.]
SUGGESTIONS.—=174.= What advantage is it to the plant to have its
seeds widely dispersed? =175.= What are the leading ways in which
fruits and seeds are dispersed? =176.= Name some explosive fruits.
=177.= Describe wind travelers. =178.= What seeds are carried by
birds? =179.= Describe some bur with which you are familiar. =180.=
Are adhesive fruits usually dehiscent or indehiscent? =181.= Do
samaras grow on low or tall plants, as a rule? =182.= Are the
cotton fibres on the seed or on the fruit? =183.= Name the ways in
which the common weeds of your region are disseminated. =184.= This
lesson will suggest other ways in which seeds are transported.
Nuts are buried by squirrels for food; but if they are not eaten,
they may grow. The seeds of many plants are blown on the snow. The
old stalks of weeds, standing through the winter, may serve to
disseminate the plant. Seeds are carried by water down the streams
and along shores. About woollen mills strange plants often spring
up from seed brought in the fleeces. Sometimes the entire plant is
rolled for miles before the winds. Such plants are “tumbleweeds.”
Examples are Russian thistle, hair grass or tumblegrass (_Panicum
capillare_), cyclone plant (_Cycloloma platyphyllum_), and white
amaranth (_Amarantus albus_). About seaports strange plants are
often found, having been introduced in the earth that is used in
ships for ballast. These plants are usually known as “ballast
plants.” Most of them do not persist long. =185.= Plants are able
to spread themselves by means of the great numbers of seeds that
they produce. How many seeds may a given elm tree or apple tree or
raspberry bush produce?
[Illustration: FIG. 253.—THE FRUITS OF THE CAT-TAIL ARE LOOSENED BY
WIND AND WEATHER. ]
CHAPTER XXIII
PHENOGAMS AND CRYPTOGAMS
[Illustration: FIG. 254.—CHRISTMAS FERN.—Dryopteris acrostichoides;
known also as Aspidium. ]
The plants thus far studied produce flowers; and the flowers produce
seeds by means of which the plant is propagated. There are other
plants, however, that produce no seeds, and these plants (including
bacteria) are probably more numerous than the seed-bearing plants.
These plants propagate by means of =spores=, _which are generative
cells, usually simple, containing no embryo_. These spores are very
small, and sometimes are not visible to the naked eye.
[Illustration: FIG. 255.—FRUITING FROND OF CHRISTMAS FERN.
Sori at _a_. One sorus with its indusium at _b_.]
Prominent among the spore-propagated plants are _ferns_. The common
_Christmas fern_ (so called because it remains green during winter)
is shown in Fig. 254. The plant has no trunk. The leaves spring
directly from the ground. The leaves of ferns are called =fronds=.
They vary in shape, as other leaves do. Some of the fronds in Fig.
254 are seen to be narrower at the top. If these are examined more
closely (Fig. 255), it will be seen that the leaflets are contracted
and are densely covered beneath with brown bodies. These bodies are
collections of =sporangia= or =spore-cases=.
[Illustration: FIG. 256.—COMMON POLYPODE FERN. _Polypodium vulgare_.]
[Illustration: FIG. 257.—SORI AND SPORANGIUM OF POLYPODE. A chain
of cells lies along the top of the sporangium, which springs back
elastically on drying, thus disseminating the spores. ]
[Illustration: FIG. 258.—THE BRAKE FRUITS UNDERNEATH THE REVOLUTE
EDGES OF THE LEAF.]
[Illustration: FIG. 259.—FRUITING PINNULES OF MAIDENHAIR FERN.]
The sporangia are collected into little groups, known as =sori=
(singular, sorus) or =fruit-dots=. Each sorus is covered with a thin
scale or shield, known as an =indusium=. This indusium separates from
the frond at its edges, and the sporangia are exposed. Not all ferns
have indusia. The polypode (Figs. 256, 257) does not; the sori are
naked. In the brake (Fig. 258) and maidenhair (Fig. 259) the edge
of the frond turns over and forms an indusium. The nephrolepis or
sword fern of greenhouses is allied to the polypode. The sori are
in a single row on either side the midrib (Fig. 260). The indusium
is circular or kidney-shaped and open at one edge or finally all
around. The Boston fern, Washington fern, Pierson fern, and others,
are horticultural forms of the common sword fern. In some ferns (Fig.
261) an entire frond becomes contracted to cover the sporangia.
[Illustration: FIG. 260.—PART OF FROND OF SWORD FERN. To the pupil:
Is this illustration right side up? ]
The sporangium or spore-case of a fern is a more or less globular
body and usually with a stalk (Fig. 257). _It contains the spores.
When ripe it bursts and the spores are set free._
[Illustration: FIG. 261.—FERTILE AND STERILE FRONDS OF THE SENSITIVE
FERN.]
In a moist, warm place _the spores germinate_. They produce a small,
flat, thin, green, more or less heart-shaped membrane (Fig. 262).
This is the =prothallus=. Sometimes the prothallus is an inch or
more across, but oftener it is less than a ten cent piece in size.
Although easily seen, it is commonly unknown except to botanists.
Prothalli may often be found in greenhouses where ferns are grown.
Look on the moist stone or brick walls, or on the firm soil of
undisturbed pots and beds; or spores may be sown in a damp, warm
place.
[Illustration: FIG. 262.—PROTHALLUS OF A FERN. Enlarged:
Archegonia at _a_; antheridia at _b_.]
On the under side of the prothallus two kinds of organs are
borne. These are the =archegonium= (containing egg-cells) and the
=antheridium= (containing sperm-cells). These organs are minute
specialized parts of the prothallus. Their positions on a particular
prothallus are shown at _a_ and _b_ in Fig. 262, but in some ferns
they are on separate prothalli (plant diœcious). _The sperm-cells
escape from the antheridium and in the water that collects on the
prothallus are carried to the archegonium, where fertilization
of the egg takes place._ From the fertilized egg-cell a plant
grows, becoming a “fern.” In most cases the prothallus soon dies.
The prothallus is the =gametophyte= (from Greek, signifying the
fertilized plant).
The fern plant, arising from the fertilized egg in the archegonium,
becomes a perennial plant, each year producing spores from its fronds
(called the =sporophyte=); but these spores—which are merely detached
special kinds of cells—produce the prothallic phase of the fern
plant, from which new individuals arise. _A fern is fertilized but
once in its lifetime._ The “fern” bears the spore, the spore gives
rise to the prothallus, and the egg-cell of the prothallus (when
fertilized) gives rise to the fern.
A similar =alternation of generations= runs all through the vegetable
kingdom, although there are some groups of plants in which it is very
obscure or apparently wanting. It is very marked in ferns and mosses.
In algæ (including the seaweeds) the gametophyte is the “plant,”
as the non-botanist knows it, and the sporophyte is inconspicuous.
_There is a general tendency, in the evolution of the vegetable
kingdom, for the gametophyte to lose its relative importance and for
the sporophyte to become larger and more highly developed._ In the
seed-bearing plants the sporophyte generation is the only one seen by
the non-botanist. The gametophyte stage is of short duration and the
parts are small; it is confined to the time of fertilization.
The sporophyte of seed-plants, or the “plant” as we know it, produces
two kinds of spores—one kind becoming =pollen-grains= and the other
kind =embryo-sacs=. The pollen-spores are borne in sporangia,
which are united into what are called =anthers=. The embryo-sac,
which contains the egg-cell, is borne in a sporangium known as an
=ovule=. _A gametophytic stage is present in both pollen and embryo
sac: fertilization takes place, and a sporophyte arises. Soon this
sporophyte becomes dormant, and is then known as an_ =embryo=. The
embryo is packed away within tight-fitting coats, and the entire body
is the =seed=. When the conditions are right the seed grows, and
the sporophyte grows into herb, bush, or tree. The utility of the
alternation of generations is not understood.
The spores of ferns are borne on leaves; the spores of seed-bearing
plants are also borne amongst a mass of specially developed
conspicuous leaves known as =flowers=; therefore these plants
have been known as the =flowering plants=. Some of the leaves are
developed as envelopes (calyx, corolla), and others as spore-bearing
parts, or =sporophylls= (stamens, pistils). But the spores of the
lower plants, as of ferns and mosses, may also be borne in specially
developed foliage, so that the line of demarcation between flowering
plants and flowerless plants is not so definite as was once supposed.
The one definite distinction between these two classes of plants is
the fact that _one class produces seeds and the other does not_. The
seed-plants are now often called =spermaphytes=, but there is no
single coordinate term to set off those which do not bear seeds. It
is quite as well, for popular purposes, to use the terms =phenogams=
for the seed-bearing plants and =cryptogams= for the others. These
terms have been objected to in recent years because their etymology
does not express literal facts (_phenogam_ signifying “showy
flowers,” and _cryptogam_ “hidden flowers”), but the terms represent
distinct ideas in classification. The cryptogams include three great
series of plants—the =Thallophytes= or algæ, lichens, and fungi; the
=Bryophytes= or moss-like plants; the =Pteridophytes= or fernlike
plants.
[Illustration: FIG. 263.—DIAGRAM TO EXPLAIN THE TERMINOLOGY OF THE
FROND. ]
SUGGESTIONS.—=186.= _The parts of a fern leaf._ The primary
complete divisions of a frond are called pinnæ, no matter whether
the frond is pinnate or not. In ferns the word “pinna” is used
in essentially the same way that leaflet is in the once-compound
leaves of other plants. The secondary leaflets are called pinnules,
and in thrice, or more, compound fronds, the last complete parts
or leaflets are ultimate pinnules. The diagram (Fig. 263) will aid
in making the subject clear. If the frond were not divided to the
midrib, it would be simple, but this diagram represents a compound
frond. The general outline of the frond, as bounded by the dotted
line, is ovate. The stipe is very short. The midrib of a compound
frond is known as the rachis. In a decompound frond, this main
rachis is called the primary rachis. Segments (not divided to the
rachis) are seen at the tip, and down to _h_ on one side and to _m_
on the other. Pinnæ are shown at _i_, _k_, _l_, _o_, _n_. The pinna
_o_ is entire; _n_ is crenate-dentate; _i_ is sinuate or wavy, with
an auricle at the base; _k_ and _l_ are compound. The pinna _k_ has
twelve entire pinnules. (Is there ever an even number of pinnules
on any pinna?) Pinna _l_ has nine compound pinnules, each bearing
several entire ultimate pinnules. _The spores._—=187.= Lay a mature
fruiting frond of any fern on white paper, top side up, and allow
it to remain in a dry, warm place. The spores will discharge on the
paper. =188.= Lay the full-grown (but not dry) cap of a mushroom or
toadstool bottom down on a sheet of clean paper, under a ventilated
box in a warm, dry place. A day later raise the cap.
CHAPTER XXIV
STUDIES IN CRYPTOGAMS
The pupil who has acquired skill in the use of the compound
microscope may desire to make more extended excursions into the
cryptogamous orders. The following plants have been chosen as
examples in various groups. Ferns are sufficiently discussed in the
preceding chapter.
BACTERIA
If an infusion of ordinary hay is made in water and allowed to stand,
it becomes turbid or cloudy after a few days, and a drop under the
microscope will show the presence of minute oblong cells swimming in
the water, perhaps by means of numerous hair-like appendages, that
project through the cell wall from the protoplasm within. At the
surface of the dish containing the infusion the cells are non-motile
and are united in long chains. Each of these cells or organisms is a
_bacterium_ (plural, _bacteria_). (Fig. 135.)
Bacteria are very minute organisms,—the smallest known—consisting
either of separate oblong or spherical cells, or of chains, plates,
or groups of such cells, depending on the kind. They possess a
membrane-like wall which, unlike the cell walls of higher plants,
contains nitrogen. The presence of a nucleus has not been definitely
demonstrated. Multiplication is by the fission of the vegetative
cells; but under certain conditions of drought, cold, or exhaustion
of the nutrient medium, the protoplasm of the ordinary cells may
become invested with a thick wall, thus forming an _endospore_ which
is very resistant to extremes of environment. No sexual reproduction
is known.
Bacteria are very widely distributed as parasites and saprophytes in
almost all conceivable places. _Decay_ is largely caused by bacteria,
accompanied in animal tissue by the liberation of foul-smelling
gases. Certain species grow in the reservoirs and pipes of water
supplies, rendering the water brackish and often undrinkable. Some
kinds of _fermentation_ (the breaking down or decomposing of organic
compounds, usually accompanied by the formation of gas) are due to
these organisms. Other bacteria oxidize alcohol to _acetic acid_, and
produce _lactic acid_ in milk and _butyric acid_ in butter. Bacteria
live in the mouth, the stomach, the intestines, and on the surface
of the skins of animals. Some secrete gelatinous sheaths around
themselves; others secrete sulphur or iron, giving the substratum a
vivid colour.
Were it not for bacteria, man could not live on the earth, for not
only are they agents in the process of decay, but they are concerned
in certain healthful processes of plants and animals. We have learned
in Chapter VIII how bacteria are related to nitrogen-gathering.
Bacteria are of economic importance not alone because of their effect
on materials used by man, but also because of the _disease-producing
power_ of certain species. _Pus_ is caused by a spherical form,
_tetanus_ or _lock-jaw_ by a rod-shaped form, _diphtheria_ by short
oblong chains, _tuberculosis_ or “_consumption_” by more slender
oblong chains, and _typhoid fever_, _cholera_, and other diseases
by other forms. Many _diseases of animals and plants_ are caused by
bacteria. Disease-producing bacteria are said to be _pathogenic_.
The ability to grow in other nutrient substances than the natural
one has greatly facilitated the study of these minute forms of life.
By the use of suitable culture media and proper precautions, _pure
cultures_ of a particular disease-producing bacterium may be obtained
with which further experiments may be conducted.
Milk provides an excellent collecting place for bacteria coming
from the air, from the coat of the cow and from the milker. Disease
germs are sometimes carried in milk. If a drop of milk is spread on
a culture medium (as agar), and provided with proper temperature,
the bacteria will multiply, each one forming a colony visible to the
naked eye. In this way, the number of bacteria originally contained
in the milk may be counted.
Bacteria are disseminated in water, as the germ of typhoid fever and
cholera; in milk and other fluids; in the air; and on the bodies of
flies, feet of birds, and otherwise.
Bacteria are thought by many to have descended from algæ by the loss
of chlorophyll and decrease in size due to the more specialized
acquired saprophytic and parasitic habit.
ALGÆ
The algæ comprise most of the green floating “scum” which covers the
surfaces of ponds and other quiet waters. The masses of plants are
often called “frog spittle.” Others are attached to stones, pieces of
wood, and other objects submerged in streams and lakes, and many are
found on moist ground and on dripping rocks. Aside from these, all
the plants commonly known as seaweeds belong to this category; these
latter are inhabitants of salt water.
The simplest forms of algæ consist of a single spherical cell, which
multiplies by repeated division or fission. Many of the forms found
in fresh water are filamentous, _i.e._ the plant body consists of
long threads, either simple or branched. Such a plant body is termed
a _thallus_. This term applies to the vegetative body of all plants
that are _not differentiated into stem and leaves_. Such plants are
known as _thallophytes_ (p. 181). All algæ contain chlorophyll,
and are able to assimilate carbon dioxide from the air. This
distinguishes them from the fungi.
* * * * *
_Nostoc._—On wet rocks and damp soil dark, semitransparent irregular
or spherical gelatinous masses about the size of a pea are often
found. These consist of a colony of contorted filamentous algæ
embedded in the jelly-like mass. The chain of cells in the filament
is necklace-like. Each cell is homogeneous, without apparent nucleus,
and blue-green in colour, except one cell which is larger and
clearer than the rest. The plant therefore belongs to the group of
_blue-green algæ_. The jelly probably serves to maintain a more even
moisture and to provide mechanical protection. Multiplication is
wholly by the breaking up of the threads. Occasionally certain cells
of the filament thicken to become _resting-spores_, but no other
spore formation occurs.
[Illustration: FIG. 264.—FILAMENT OF OSCILLATORIA, showing one dead
cell where the strand will break.]
* * * * *
_Oscillatoria._—The blue-green coatings found on damp soil and
in water frequently show under the microscope the presence of
filamentous algæ composed of many short homogeneous cells (Fig. 264).
If watched closely, some filaments will be seen to wave back and
forth slowly, showing a peculiar power of movement characteristic
of this plant. Multiplication is by the breaking up of the threads.
There is no true spore formation.
* * * * *
[Illustration: FIG. 265.—STRAND OF SPIROGYRA, showing the chlorophyll
bands. There is a nucleus at _a_. How many cells, or parts of cells,
are shown in this figure? ]
_Spirogyra._—One of the most common forms of the green algæ is
spirogyra (Fig. 265). This plant often forms the greater part of the
floating green mass (or “frog spittle”) on ponds. The thread-like
character of the thallus can be seen with the naked eye or with a
hand lens, but to study it carefully a microscope magnifying two
hundred diameters or more must be used. The thread is divided into
long cells by cross walls which, according to the species, are either
straight or curiously folded (Fig. 266). The chlorophyll is arranged
in _beautiful spiral bands_ near the wall of each cell. From the
character of these bands the plant takes its name. Each cell is
provided with a _nucleus_ and other _protoplasm_. The nucleus is
suspended near the centre of the cell (_a_, Fig. 265) by delicate
strands of protoplasm radiating toward the wall and terminating
at certain points in the chlorophyll band. The remainder of the
protoplasm forms a thin layer lining the wall. The interior of the
cell is filled with cell-sap. The protoplasm and nucleus cannot be
easily seen, but if the plant is stained with a dilute alcoholic
solution of eosine they become clear.
[Illustration: FIG. 266.—CONJUGATION OF SPIROGYRA. Ripe zygospores on
the left; _a_, connecting tubes. ]
Spirogyra is propagated vegetatively by the breaking off of parts of
the threads, which continue to grow as new plants. Resting-spores,
which may remain dormant for a time, are formed by a process known
as _conjugation_. Two threads lying side by side send out short
projections, usually from all the cells of a long series (Fig. 266).
The projections or processes from opposite cells grow toward each
other, meet, and fuse, forming a connecting tube between the cells.
The protoplasm, nucleus, and chlorophyll band of one cell now pass
through this tube, and unite with the contents of the other cell. The
entire mass then becomes surrounded by a thick cellulose wall, thus
completing the _resting-spore_, or _zygospore_ (_z_, Fig. 266).
* * * * *
_Zygnema_ is an alga closely related to spirogyra and found in
similar places. Its life history is practically the same, but it
differs from spirogyra in having _two star-shaped chlorophyll bodies_
(Fig. 267) in each cell, instead of a chlorophyll-bearing spiral band.
[Illustration: FIG. 267.—STRAND, OR FILAMENT OF ZYGNEMA, freed from
its gelatinous covering. ]
_Vaucheria_ is another alga common in shallow water and on damp
soil. The thallus is much branched, but the threads are not divided
by cross walls as in spirogyra. The plants are attached by means of
colourless root-like organs which are much like the root-hairs of the
higher plants: these are _rhizoids_. The chlorophyll is in the form
of _grains scattered through the thread_.
Vaucheria has a special mode of asexual reproduction by means of
swimming spores or _swarm-spores_. These are formed singly in a
short enlarged lateral branch known as the _sporangium_. When the
sporangium bursts, the entire contents escape, forming a single large
swarm-spore, which swims about by means of numerous lashes or cilia
on its surface. The swarm-spores are so large that they can be seen
with the naked eye. After swimming about for some time they come to
rest and germinate, producing a new plant.
[Illustration: FIG. 268.—THREAD OF VAUCHERIA WITH OÖGONIA AND
ANTHERIDIA.]
The formation of resting-spores of vaucheria is accomplished by means
of special organs, _oögonia_ (_o_, Fig. 268) and _antheridia_ (_a_,
Fig. 268). These are both specially developed branches from the
thallus. The antheridia are nearly cylindrical, and curved toward
the oögonia. The upper part of an antheridium is cut off by a cross
wall, and within it numerous ciliated _sperm-cells_ are formed. These
escape by the ruptured apex of the antheridium. The oögonia are more
enlarged than the antheridia, and have a beak-like projection turned
a little to one side of the apex. They are separated from the thallus
thread by a cross wall, and contain a single large green cell, the
_egg-cell_. The apex of the oögonium is dissolved, and through the
opening the sperm-cells enter. Fertilization is thus accomplished.
After fertilization the egg-cell becomes invested with a thick wall
and is thus converted into a resting-spore, the _oöspore_.
[Illustration: FIG. 269.—FUCUS. Fruiting branches at _s_, _s_. On the
stem are two air-bladders. ]
* * * * *
_Fucus._—These are rather large specialized algæ belonging to the
group known as brown seaweeds and found attached by a disk to the
rocks of the seashore just below high tide (Fig. 269). They are firm
and strong to resist wave action and are so attached as to avoid
being washed ashore. They are very abundant algæ. In shape the plants
are long, branched, and multicellular, with either flat or terete
branches. They are olive-brown. Propagation is by the breaking off of
the branches. No zoöspores are produced, as in many other seaweeds;
and reproduction is wholly sexual. The _antheridia_, bearing
_sperm-cells_, and the _oögonia_, each bearing eight _egg-cells_,
are sunken in pits or _conceptacles_. These pits are aggregated in
the swollen lighter coloured tips of some of the branches (_s_, _s_,
Fig. 269). The egg-cells and sperm-cells escape from the pits and
fertilization takes place in the water. The matured eggs, or spores,
reproduce the fucus plant directly.
* * * * *
[Illustration: FIG. 270.—NITELLA.]
_Nitella._—This is a large branched and specialized fresh-water alga
found in tufts attached to the bottom in shallow ponds (Fig. 270).
Between the whorls of branches are long _internodes consisting of a
single cylindrical cell_, which is one of the largest cells known in
vegetable tissue. Under the microscope the walls of this cell are
found to be lined with a layer of small stationary chloroplastids,
within which layer the protoplasm, in favourable circumstances,
will be found in motion, moving up one side and down the other (in
rotation). Note the clear streak up the side of the cell and its
relation to the moving current.
FUNGI
Some forms of fungi are familiar to every one. Mushrooms and
toadstools, with their varied forms and colours, are common in
fields, woods, and pastures. In every household the common moulds
are familiar intruders, appearing on old bread, vegetables, and
even within tightly sealed fruit jars, where they form a felt-like
layer dusted over with blue, yellow, or black powder. The strange
occurrence of these plants long mystified people, who thought
they were productions of the dead matter upon which they grew, but
now we know that a mould, as any other plant, cannot originate
spontaneously; it must start from something which is analogous to a
seed. The “seed” in this case is a _spore_. A spore may be produced
by a _vegetative process_ (growing out from the ordinary plant
tissues), or it may be the result of a _fertilization process_.
* * * * *
_Favourable conditions for the growth of fungi._—Place a piece of
bread under a moist bell jar and another in an uncovered place near
by. Sow mould on each. Note the result from day to day. Moisten a
third piece of bread with weak copper sulphate (blue vitriol) or
mercuric chloride solution, sow mould, cover with bell jar, note
results, and explain. Expose pieces of different kinds of food in
a damp atmosphere and observe the variety of organisms appearing.
Fungi are saprophytes or parasites, and must be provided with organic
matter on which to grow. They are usually most abundant in moist
places and wet seasons.
* * * * *
[Illustration: FIG. 271.—MUCOR MUCEDO, showing habit.]
_Mould._—One of these moulds (_Mucor mucedo_), which is very common
on all decaying fruits and vegetables, is shown in Fig. 271, somewhat
magnified. When fruiting, this mould appears as a _dense mass of long
white hairs_, often over an inch high, standing erect from the fruit
or the vegetable on which it is growing.
The life of this mucor begins with a minute rounded spore (_a_,
Fig. 272), which lodges on the decaying material. When the spore
germinates, it sends out a delicate thread that grows rapidly in
length and forms very many branches that soon permeate every part
of the substance on which the plant grows (_b_, Fig. 272). One of
these threads is termed a _hypha_. All the threads together form the
_mycelium_ of the fungus. The mycelium disorganizes the material in
which it grows, and thus the mucor plant (Fig. 271) is nourished.
It corresponds physiologically to the roots and the stems of other
plants.
[Illustration: FIG. 272.—SPORES OF MUCOR, some germinating. ]
When the mycelium is about two days old, it begins to form the
long fruiting stalks which we first noticed. To study them, use a
compound microscope magnifying about two hundred diameters. One
of the stalks, magnified, is shown in _a_, Fig. 274. It consists
of a rounded head, the _sporangium_, _sp_, supported on a long,
delicate stalk, the _sporangiophore_. The stalk is separated from the
sporangium by a wall which is formed at the base of the sporangium.
This wall, however, does not extend straight across the thread, but
it arches up into the sporangium like an inverted pear. It is known
as the _columella_, _c_. When the sporangium is placed in water,
the wall immediately dissolves and allows hundreds of spores, which
were formed in the cavity within the sporangium, to escape, _b_.
All that is left of the fruit is the stalk, with the pear-shaped
columella at its summit, _c_. The spores that have been set free by
the breaking of the sporangium wall are now scattered by the wind and
other agents. Those that lodge in favourable places begin to grow
immediately and reproduce the fungus. The others soon perish.
[Illustration: FIG. 274.—MUCOR.
_a_, sporangium; _b_, sporangium bursting; _c_, columella.]
The mucor may continue to reproduce itself in this way indefinitely,
but these spores are very delicate and usually die if they do not
fall on favourable ground, so that the fungus is provided with
another means of carrying itself over unfavourable seasons, as
winter. This is accomplished by means of curious _thick-walled
resting-spores_ or _zygospores_. The zygospores are formed on the
mycelium buried within the substance on which the plant grows. They
originate in the following way: Two threads that lie near together
send out short branches, which grow toward each other and finally
meet (Fig. 273). The walls at the ends, _a_, then disappear, allowing
the contents to flow together. At the same time, however, two other
walls are formed at points farther back, _b_, _b_, separating the
short section, _c_, from the remainder of the thread. This section
now increases in size and becomes covered with a thick, dark brown
wall ornamented with thickened tubercles. The zygospore is now mature
and, after a period of rest, it germinates, either producing a
sporangium directly or growing out as mycelium.
[Illustration: FIG. 273.—MUCOR, showing formation of zygospore on the
right; germinating zygospore on the left. ]
The zygospores of the mucors form one of the most interesting and
instructive objects among the lower plants. They are, however, very
difficult to obtain. One of the mucors (_Sporodinia grandis_) may be
frequently found in summer growing on toadstools. This plant usually
produces zygospores that are formed on the aërial mycelium. The
zygospores are large enough to be recognized with a hand lens. The
material may be dried and kept for winter study, or the zygospores
may be prepared for permanent microscopic mounts in the ordinary way.
* * * * *
_Yeast._—This is a very much reduced and simple fungus, consisting
normally of isolated spherical or elliptical cells (Fig. 275)
containing abundant protoplasm and probably a nucleus, although the
latter is not easily observed. It propagates rapidly by _budding_,
which consists of the gradual extrusion of a wart-like swelling that
is sooner or later cut off at the base by constriction, thus forming
a separate organism. Although simple in structure, the yeast is found
to be closely related to some of the higher groups of fungi as shown
by the method of spore formation. When grown on special substances
like potato or carrot, the contents of the cell may _form spores
inside of the sac-like mother cell_, thus resembling the sac-fungi to
which blue mould and mildews belong. The yeast plant is remarkable on
account of its power to induce alcoholic fermentation in the media in
which it grows.
[Illustration: FIG. 275.—YEAST PLANTS.]
There are many kinds of yeasts. One of them is found in the common
_yeast cakes_. In the process of manufacture of these cakes, the
yeast cells grow to a certain stage, and the material is then dried
and fashioned into small cakes, each cake containing great numbers of
the yeast cells. When the yeast cake is added to dough, and proper
conditions of warmth and moisture are provided, the yeast grows
rapidly and breaks up the sugar of the dough into carbon dioxide
and alcohol. This is _fermentation_. The gases escape and puff up
the dough, causing the _bread to rise_. In this loosened condition
the dough is baked; if it is not baked quickly enough, _the bread_
“_falls_.” Shake up a bit of yeast cake in slightly sweetened water:
the water soon becomes cloudy from the growing yeasts.
* * * * *
_Parasitic fungi._—Most of the moulds are saprophytes. Many other
fungi are parasitic on living plants and animals (Fig. 285). Some
of them have complicated life histories, undergoing many changes
before the original spore is again produced. The _willow mildew_ and
the common _rust of wheat_ will serve to illustrate the habits of
parasitic fungi.
The _willow mildew_ (_Uncinula salicis_).—This is one of the sac
fungi. It forms white downy patches on the leaves of willows (Fig.
276). These patches consist of numerous interwoven threads that may
be recognized under the microscope as the mycelium of the fungus. The
mycelium in this case lives on the surface of the leaf and nourishes
itself by sending short branches into the cells of the leaf to absorb
food materials from them.
[Illustration: FIG. 276.—COLONIES OF WILLOW MILDEW.]
[Illustration: FIG. 277.—SUMMER-SPORES OF WILLOW MILDEW.]
[Illustration: FIG. 278.—PERITHECIUM OF WILLOW MILDEW.]
[Illustration: FIG. 279.—SECTION THROUGH PERITHECIUM OF WILLOW MILDEW. ]
Numerous _summer-spores_ are formed of short, erect branches all over
the white surface. One of these branches is shown in Fig. 277. When
it has grown to a certain length, the upper part begins to segment or
divide into spores which fall and are scattered by the wind. Those
falling on other willows reproduce the fungus there. This process
continues all summer, but in the later part of the season provision
is made to maintain the mildew through the winter. If some of the
white patches are closely examined in July or August, a number of
little black bodies will be seen among the threads. These little
bodies are called _perithecia_, shown in Fig. 278. To the naked eye
they appear as minute specks, but when seen under a magnification of
200 diameters they present a very interesting appearance. They are
hollow spherical bodies decorated around the outside with a fringe
of crook-like hairs. The _resting-spores_ of the willow mildew are
produced in sacs or _asci_ inclosed within the leathery perithecia.
Figure 279 shows a cross-section of a perithecium with the asci
arising from the bottom. The spores remain securely packed in the
perithecia. They do not ripen in the autumn, but fall to the ground
with the leaf, and there remain securely protected among the dead
foliage. The following spring they mature and are liberated by the
decay of the perithecia. They are then ready to attack the unfolding
leaves of the willow and repeat the work of the summer before.
* * * * *
_The wheat rust._—The development of some of the rusts, as the
common _wheat rust_ (_Puccinia graminis_), is even more interesting
and complicated than that of the mildews. Wheat rust is also a true
parasite, affecting wheat and a few other grasses. The mycelium
here cannot be seen by the unaided eye, for it consists of threads
which are present within the host plant, mostly in the intercellular
spaces. These threads also send short branches, or _haustoria_ (Fig.
132), into the neighbouring cells to absorb nutriment.
[Illustration: FIG. 280.—SORI CONTAINING TELEUTOSPORES OF WHEAT RUST. ]
[Illustration: FIG. 281.—TELEUTOSPORE OF WHEAT RUST. ]
The _resting-spores_ of wheat rust are produced in late summer, when
they may be found in black lines breaking through the epidermis of
the wheat stalk (black-rust stage). They are formed in masses, called
_sori_ (Fig. 280), from the ends of numerous crowded mycelial strands
just beneath the epidermis of the host. The individual spores are
very small and can be well studied only with a microscope of high
power (× about 400). They are brown two-celled bodies with a thick
wall (Fig. 281). Since they are the resting or winter-spores, they
are termed _teleutospores_ (“completed spores”). Usually they do not
fall, but remain in the sori during winter. The following spring each
cell of the teleutospore puts forth a rather stout thread, which
does not grow more than several times the length of the spore and
terminates in a blunt extremity. This germ tube, _promycelium_, now
becomes divided into four cells by cross walls, which are formed from
the top downwards. Each cell gives rise to a short, pointed branch
which, in the course of a few hours, forms at its summit a single
spore called a _sporidium_. This in turn germinates and produces a
mycelium. In Fig. 282 a germinating teleutospore is drawn to show the
promycelium, _p_, divided into four cells, each producing a short
branch with a little _sporidium_, _s_.
[Illustration: FIG. 282.—GERMINATING TELEUTOSPORE OF WHEAT RUST. ]
A most remarkable circumstance in the life history of the wheat rust
is the fact that the mycelium produced by the sporidium _can live
only in barberry leaves_, and it follows that if no barberry bushes
are in the neighbourhood the sporidia finally perish. Those which
happen to lodge on a barberry bush germinate immediately, producing a
mycelium that enters the barberry leaf and grows within its tissues.
Very soon the fungus produces a new kind of spores on the barberry
leaves. These are called _æcidiospores_. They are formed in long
chains in little fringed cups, or _æcidia_, which appear in groups on
the lower side of the leaf (Fig. 283). These orange or yellow æcidia
are termed _cluster-cups_. In Fig. 284 is shown a cross-section
of one of the cups, outlining the long chains of spores, and the
mycelium in the tissues.
[Illustration: FIG. 283.—LEAF OF BARBERRY WITH CLUSTER-CUPS. ]
[Illustration: FIG. 284.—SECTION THROUGH A CLUSTER-CUP ON BARBERRY
LEAF.]
The æcidiospores are formed in the spring, and after they have been
set free, some of them lodge on wheat or other grasses, where they
germinate immediately. The germ-tube enters the leaf through a
stomate, whence it spreads among the cells of the wheat plant. In
summer one-celled reddish _uredospores_ (“blight spores,” red-rust
stage) are produced in a manner similar to the teleutospores. These
are capable of germinating immediately, and serve to disseminate
the fungus during the summer on other wheat plants or grasses. Late
in the season, teleutospores are again produced, completing the life
cycle of the plant.
Many rusts besides _Puccinia graminis_ produce different spore forms
on different plants. The phenomenon is called _heterœcism_, and
was first shown to exist in the wheat rust. Curiously enough, the
peasants of Europe had observed and asserted that barberry bushes
cause wheat to blight long before science explained the relation
between the cluster-cups on barberry and the rust on wheat. The true
relation was actually demonstrated, as has since been done for many
other rusts on their respective hosts, by sowing the æcidiospores on
healthy wheat plants and thus producing the rust. The _cedar apple_
is another rust, producing the curious swellings often found on the
branches of red cedar trees. In the spring the teleutospores ooze out
from the “apple” in brownish yellow masses. It has been found that
these attack various fruit trees, producing æcidia on their leaves.
Fig. 285 explains how a parasitic fungus works.
[Illustration: FIG. 285.—HOW A PARASITIC FUNGUS WORKS. Anthracnose on
a bean pod entering the bean beneath. (Whetzel.)]
[Illustration: FIG. 286.—PART OF GILL OF THE CULTIVATED MUSHROOM.
_tr_, trama tissue; _sh_, hymenium; _b_, basidium; _st_, sterigma;
_sp_, spore. (Atkinson.)]
* * * * *
_Puffballs, mushrooms, toadstools, and shelf fungi._—These represent
what are called the _higher fungi_, because of the size and the
complexity of the plant body as well as from the fact that they seem
to stand at the end of one line of evolution. The mycelial threads
grow together in extensive strands in rotten wood or in the soil,
and send out large complex growths of mycelium in connection with
which the spores are borne. These aërial parts are the only ones
which we ordinarily see, and which constitute the “mushroom” part
(Fig. 131). Only asexual spores (_basidiospores_) are produced, and
on short stalks (_basidia_) (Fig. 286). In the puff-balls the spores
are inclosed and constitute a large part of the “smoke.” In the
mushrooms and toadstools they are borne on _gills_, and in the shelf
fungi (Fig. 134) on the walls of minute pores of the underside. The
mycelium of these shelf fungi frequently lives and grows for a long
time concealed in the substratum before the visible fruit bodies are
sent out. Practically all timber decay is caused by such growth, and
the damage is largely done before the fruiting bodies appear. For
other accounts of mushrooms, see Chapter XIV.
LICHENS
[Illustration: FIG. 287.—LICHEN ON AN OAK TRUNK. (A species of
_Physcia_.) ]
Lichens are so common everywhere that the attention of the student
is sure to be drawn to them. They grow on rocks, trunks of trees
(Fig. 287), old fences, and on the earth. They are thin, usually gray
ragged objects, apparently lifeless. Their study is too difficult for
beginners, but a few words of explanation may be useful.
Lichens were formerly supposed to be a distinct or separate division
of plants. They are now known to be organisms, each species of which
is a constant association of a fungus and an alga. The thallus is
ordinarily made up of fungous mycelium or tissue within which the
imprisoned alga is definitely distributed. The result is a growth
unlike either component. This association of alga and fungus is
usually spoken of as _symbiosis_, or mutually helpful growth, the
alga furnishing some things, the fungus others, and both together
being able to accomplish work that neither could do independently. By
others this union is considered to be a mild form of parasitism, in
which the fungus profits at the expense of the alga. As favourable to
this view, the facts are cited that each component is able to grow
independently, and that under such conditions the algal cells seem to
thrive better than when imprisoned by the fungus.
Lichens propagate by means of _soredia_, which are tiny parts
separated from the body of the thallus, and consisting of one or more
algal cells overgrown with fungus threads. These are readily observed
in many lichens. They also produce spores, usually ascospores, which
are always the product of the fungus element, and which reproduce the
lichen by germinating in the presence of algal cells, to which the
hyphæ immediately cling.
Lichens are found in the most inhospitable places, and, by means of
acids which they secrete, they attack and slowly disintegrate even
the hardest rocks. By making thin sections of the thallus with a
sharp razor and examining under the compound microscope, it is easy
to distinguish the two components in many lichens.
LIVERWORTS
[Illustration: FIG. 288. FIG. 289.
PLANTS OF MARCHANTIA.]
The liverworts are peculiar flat green plants usually found on wet
cliffs and in other moist, shady places. They frequently occur
in greenhouses where the soil is kept constantly wet. One of the
commonest liverworts is _Marchantia polymorpha_, two plants of which
are shown in Figs. 288, 289. The plant consists of a ribbon-like
thallus that creeps along the ground, becoming repeatedly forked as
it grows. The end of each branch is always conspicuously notched.
There is a prominent midrib extending along the centre of each branch
of the thallus. On the under side of the thallus, especially along
the midrib, there are numerous rhizoids which serve the purpose of
roots, absorbing nourishment from the earth and holding the plant in
its place. The upper surface of the thallus is divided into minute
rhombic areas that can be seen with the naked eye. Each of these
areas is perforated by a small breathing pore or stomate that leads
into a cavity just beneath the epidermis. This space is surrounded
by chlorophyll-bearing cells, some of which stand in rows from the
bottom of the cavity (Fig. 290). The delicate assimilating tissue is
thus brought in close communication with the outer air through the
pore in the thick, protecting epidermis.
[Illustration: FIG. 290.—SECTION OF THALLUS OF MARCHANTIA. Stomate at
_a_.]
At various points on the midrib are little cups containing small
green bodies. These bodies are buds or _gemmæ_ which are outgrowths
from the cells at the bottom of the cup. They become loosened and are
then dispersed by the rain to other places, where they take root and
grow into new plants.
The most striking organs on the thallus of marchantia are the
peculiar stalked bodies shown in Figs. 288, 289. These are termed
_archegoniophores_ and _antheridiophores_ or _receptacles_. Their
structure and function are very interesting, but their parts are so
minute that they can be studied only with the aid of a microscope
magnifying from 100 to 400 times. Enlarged drawings will guide the
pupil.
[Illustration: FIG. 291.—SECTION THROUGH ANTHERIDIOPHORE OF
MARCHANTIA, showing antheridia. One antheridium more magnified.]
The _antheridiophores_ are fleshy, lobed disks borne on short stalks
(Fig. 291). The upper surface of the disk shows openings scarcely
visible to the naked eye. However, a section of the disk, such as is
drawn in Fig. 291, shows that the pores lead into oblong cavities
in the receptacle. From the base of each cavity there arises a
thick, club-shaped body, the _antheridium_. Within the antheridium
are formed many sperm-cells which are capable of swimming about in
water by means of long lashes or cilia attached to them. When the
antheridium is mature, it bursts and allows the ciliated sperm cells
to escape.
[Illustration: FIG. 292.—ARCHEGONIUM OF MARCHANTIA.]
The _archegoniophores_ are also elevated on stalks (Fig. 289).
Instead of a simple disk, the receptacle consists of nine or
more finger-like rays. Along the under side of the rays, between
delicately fringed curtains, peculiar flask-like bodies, or
_archegonia_, are situated. The archegonia are not visible to the
naked eye. They can be studied only with the microscope (x about
400). One of them much magnified is represented in Fig. 292. Its
principal parts are the long _neck_, _a_, and the rounded _venter_,
_b_, inclosing a large free cell—the egg-cell.
[Illustration: FIG. 293.—ARCHEGONIOPHORE, WITH SPOROGONIA, OF
MARCHANTIA.]
We have seen that the antheridium at maturity discharges its
sperm-cells. These swim about in the water provided by the dew and
rain. Some of them finally find their way to the archegonia and
egg-cells, the latter being fertilized, as pollen fertilizes the
ovules of higher plants.
After fertilization the egg-cell develops into the spore capsule
or _sporogonium_. The mature spore capsules may be seen in Fig.
293. They consist of an oval spore-case on a short stalk, the base
of which is imbedded in the tissue of the receptacle, from which
it derives the necessary nourishment for the development of the
sporogonium. At maturity the sporogonium is ruptured at the apex,
setting free the spherical spores together with numerous filaments
having spirally thickened walls (Fig. 294). These filaments are
called _elaters_. When drying, they exhibit rapid movements by means
of which the spores are scattered. The spores germinate and again
produce the thallus of marchantia.
[Illustration: FIG. 294.—SPORES AND ELATERS OF MARCHANTIA.]
MOSSES (Bryophyta)
[Illustration: FIG. 295.—POLYTRICHUM COMMUNE.
_f_, _f_, fertile plants, one on the left in fruit;
_m_, antheridial plant.]
If we have followed carefully the development of marchantia, the
study of one of the mosses will be comparatively easy. The mosses
are more familiar plants than the liverworts. They grow on trees,
stones, and on the soil in both wet and dry places. One of the
common larger mosses, known as _Polytrichum commune_, may serve as
an example, Fig. 295. This plant grows on rather dry knolls, mostly
in the borders of open woods, where it forms large beds. In dry
weather these beds have a reddish brown appearance, but when moist
they form beautiful green cushions. This colour is due, in the first
instance, to the colour of the old stems and leaves, and, in the
second instance, to the peculiar action of the green living leaves
under the influence of changing moisture-conditions. The inner or
upper surface of the leaf is covered with thin, longitudinal ridges
of delicate cells which contain chlorophyll. These cells are shown in
cross-section in Fig. 296, as dots or granules. All the other tissue
of the leaf consists of thick-walled, corky cells which do not allow
moisture to penetrate. When the air is moist the green leaves spread
out, exposing the chlorophyll cells to the air, but in dry weather
the margins of the leaves roll inward, and the leaves fold closely
against the stem, thus protecting the delicate assimilating tissue.
[Illustration: FIG. 296.—SECTION OF LEAF OF POLYTRICHUM COMMUNE.]
The _antheridia_ and _archegonia_ of polytrichum are borne in groups
at the ends of the branches on different plants (many mosses bear
both organs on the same branch). They are surrounded by involucres
of characteristic leaves termed _perichætia_ or _perichætal leaves_.
Multicellular hairs known as _paraphyses_ are scattered among the
archegonia and antheridia. The involucres with the organs borne
within them are called _receptacles_, or, less appropriately, “moss
flowers.” As in marchantia, the organs are very minute and must be
highly magnified to be studied.
[Illustration: FIG. 297.—SECTION THROUGH A RECEPTACLE OF POLYTRICHUM
COMMUNE, showing paraphyses and antheridia. ]
The antheridia are borne in broad cup-like receptacles on the
antheridial plants (Fig. 297). They are much like the antheridia of
marchantia, but they stand free among the paraphyses and are not
sunk in cavities. At maturity they burst and allow the sperm-cells
or _spermatozoids_ to escape. In polytrichum, when the receptacles
have fulfilled their function, the stem continues to grow from the
centre of the cup (_m_, Fig. 295). The archegonia are borne in other
receptacles on different plants. They are like the archegonia of
marchantia except that they stand erect on the end of the branch.
The _sporogonium_ which develops from the fertilized egg is shown
in _a_, _b_, Fig. 295. It consists of a long, brown stalk bearing
the spore-case at its summit. The base of the stalk is imbedded
in the end of the moss stem by which it is nourished. The capsule
is entirely inclosed by a hairy cap, the _calyptra_, _b_. The
calyptra is really the remnant of the archegonium, which, for
a time, increases in size to accommodate and protect the young
growing capsule. It is finally torn loose and carried up on the
spore-case. The mouth of the capsule is closed by a circular lid, the
_operculum_, having a conical projection at the centre.
The operculum soon drops, or it may be removed, displaying a fringe
of sixty-four teeth guarding the mouth of the capsule. This ring of
teeth is known as the _peristome_. In most mosses the teeth exhibit
peculiar hygroscopic movements; _i.e._ when moist they bend outwards,
and upon drying curve in toward the mouth of the capsule. This
motion, it will be seen, serves to disperse these spores gradually
over a long period of time.
Not the entire capsule is filled with spores. There are no elaters,
but the centre of the capsule is occupied by a columnar strand of
tissue, the _columella_, which expands at the mouth into a thin,
membranous disk, closing the entire mouth of the capsule except the
narrow annular chink guarded by the teeth. In this moss the points of
the teeth are attached to the margin of the membrane, allowing the
spores to sift out through the spaces between them.
When the spores germinate they form a green, branched thread, the
_protonema_. This gives rise directly to moss plants, which appear
as little buds on the thread. When the moss plants have sent their
little rhizoids into the earth, the protonema dies, for it is no
longer necessary for the support of the little plants, and the moss
plants grow independently.
* * * * *
[Illustration: FIG. 298.—FUNARIA HYGROSCOPICA. ]
_Funaria_ is a moss very common on damp, open soil. It forms green
patches of small fine leaves from which arise long brown stalks
terminated by curved capsules (Fig. 298). The structure is similar to
that of polytrichum, except the absence of plates on the under side
of the leaves, the continuous growth of the stem, the curved capsule,
double peristome, monœcious rather than diœcious receptacles, and
nearly glabrous unsymmetrical calyptra.
EQUISETUMS, OR HORSETAILS (Pteridophyta)
There are about twenty-five species of equisetum, constituting the
only genus of the unique family _Equisetaceæ_. Among these _E.
arvense_ (Fig. 299) is common on clayey and sandy soils.
In this species the work of nutrition and that of spore production
are performed by separate shoots from an underground rhizome. The
fertile branches appear early in spring. The stem, which is 3
to 6 inches high, consists of a number of cylindrical, furrowed
internodes, each sheathed at the base by a circle of scale leaves.
The shoots are of a pale yellow colour. They contain no chlorophyll,
and are nourished by the food stored in the rhizome (Fig. 299).
The spores are formed on specially developed fertile leaves or
_sporophylls_ which are collected into a spike or cone at the end of
the stalk (_a_, Fig. 299). A single sporophyll is shown at _b_. It
consists of a short stalk expanded into a broad, mushroom-like head.
Several large _sporangia_ are borne on its under side. The spores
formed in the sporangia are very interesting and beautiful objects
when examined under the microscope (× about 200). They are spherical,
green bodies, each surrounded by two spiral bands attached to the
spore at their intersection, _s_. These bands exhibit hygroscopic
movements by means of which the spores become entangled, and are held
together. This is of advantage to the plant, as we shall see. All
the spores are alike, but some of the _prothallia_ grow to a greater
size than the others. The large prothallia produce only _archegonia_
while the smaller ones produce _antheridia_. Both these organs are
much like those of the ferns, and fertilization is accomplished in
the same way. Since the prothallia are usually diœcious, the special
advantage of the spiral bands, holding the spores together so that
both kinds of prothallia may be in close proximity, will be easily
understood. As in the fern, the fertilized egg-cell develops into an
equisetum plant.
[Illustration: FIG. 299.—EQUISETUM ARVENSE.
_st_, sterile shoot;
_f_, fertile shoot showing the spike at _a_;
_b_, sporophyll, with sporangia;
_s_, spore.]
The sterile shoots (_st_, Fig. 299) appear much later in the season.
They give rise to repeated whorls of angular or furrowed branches.
The leaves are very much reduced scales, situated at the internodes.
The stems are provided with chlorophyll and act as assimilating
tissue, nourishing the rhizome and the fertile shoots. Nutriment is
also stored in special tubers developed on the rhizome.
Other species of equisetum have only one kind of shoot—a tall, hard,
leafless, green shoot with the spike at its summit. Equisetum stems
are full of silex, and they are sometimes used for scouring floors
and utensils; hence the common name “scouring rush.”
ISOËTES (Pteridophyta)
_Isoëtes_ or quillwort is usually found in water or damp soil on the
edges of ponds and lakes. The general habit of the plant is seen
in Fig. 300, _a_. It consists of a short, perennial stem bearing
numerous erect, quill-like leaves with broad sheathing bases. The
plants are commonly mistaken for young grasses.
[Illustration: FIG. 300.—ISOËTES, showing habit of plant at _a_; _b_,
base of leaf, showing sporangium, velum, and ligule. ]
Isoëtes bears two kinds of spores, large roughened ones, the
_macrospores_, and small ones or _microspores_. Both kinds are formed
in _sporangia_ borne in an excavation in the expanded base of the
leaf. The macrospores are formed on the outer and the microspores on
the inner leaves. A sporangium in the base of a leaf is shown at _b_.
It is partially covered by a thin membrane, the _velum_. The minute
triangular appendage at the upper end of the sporangium is called the
_ligule_.
The spores are liberated by the decay of the sporangia. They form
rudimentary prothallia of two kinds. The microspores produce
prothallia with _antheridia_, while the macrospores produce
prothallia with _archegonia_. Fertilization takes place as in the
mosses or liverworts, and the fertilized egg-cell, by continued
growth, gives rise again to the isoëtes plant.
CLUB-MOSSES (Pteridophyta)
The club-mosses are low trailing plants of moss-like looks and
habit, although more closely allied to ferns than to true mosses.
Except one genus in Florida, all the club-mosses belong to the
genus _Lycopodium_. They grow mostly in woods, having 1-nerved
evergreen leaves arranged in four or more ranks. Some of them make
long strands, as the ground pine, and are much used for Christmas
decorations. The spores are all _of one kind_ or form, borne in
_1-celled sporangia_ that open on the margin into _two valves_. The
sporangia are borne in some species (Fig. 301) as small yellow bodies
in the axils of the ordinary leaves near the tip of the shoot; in
other species (Fig. 302) they are borne in the axils of small scales
that form a catkin-like spike. The spores are very numerous, and
they contain an oil that makes them inflammable. About 100 species
of lycopodium are known. The plants grown by florists under the name
of lycopodium are of the genus _Selaginella_, more closely allied to
isoëtes, bearing two kinds of spores (microspores and macrospores).
[Illustration: FIG. 301.—A LYCOPODIUM WITH SPORANGIA IN THE AXILS OF
THE FOLIAGE LEAVES. (_Lycopodium lucidulum._) ]
[Illustration: FIG. 302.—A CLUB-MOSS (_Lycopodium complanatum_).]
INDEX
Aborted seeds, 166.
Abutilon, 156.
Accessory fruit, 164, 169.
Adaptation to environment, 6.
Adventitious roots, 36;
buds, 114.
Aërial roots, 34.
Aggregate fruit, 168.
Air plants, 35.
Akenes, 165.
Algæ, 179, 183, 195.
Alternation of generation, 179.
Anemophilous, 149.
Annual plant, 17.
Anther, 135, 144, 180.
Antheridium, 178, 186, 198, 200, 202, 203.
Apical dehiscence, 166.
Archegonium, 178, 198, 200, 202, 203.
Arum family, 140.
Ash, 92.
Assimilation, 97.
Axil, 112.
Axis, plant, 15.
Bacteria, 39, 109, 182.
Barberry, 157, 193.
Bark, 54, 66, 67.
Bark-bound trees, 54.
Bast, 61, 66.
Bean, 20, 28, 39, 194.
Berry, 167.
Biennial plant, 17.
Brace cells, 67.
Bracts, 134.
Branch, 111.
Breeding, plant, 7, 8.
Bryophytes, 181.
Budding, 127, 128.
Bud propagation, 181.
Buds, 72, 82, 87, 111;
flower, 115;
fruit, 115.
Burs, 172, 174.
Bushes, 191.
Cabbage, 113.
Callus, 56.
Calyx, 133.
Cambium, 63, 65.
Capsule, 165.
Carbohydrate, 95.
Carbon, 92.
Carbon dioxide, 22, 93, 106.
Carnivorous, 99.
Carpel, 136.
Castor bean, 24.
Catkin, 158.
Caulicle, 20, 22, 25.
Cedar apple, 194.
Cell, 42, 63, 145, 176.
Chlorophyll, 86, 94, 101, 183, 186.
Cladophylla, 100.
Cleft graft, 126.
Cleft leaf, 75.
Cleistogamous, 151.
Climbing plants, 129.
Clover, 39.
Club mosses, 203.
Cluster, flower, 155, 159;
centrifugal, 156, 159;
centripetal, 156;
indeterminate, 156.
Colonies, plant, 11.
Composite flowers, 140.
Conjugation, 185.
Cork, 66, 67.
Corn, 3, 25, 26.
Corolla, 133;
funnel form, 138;
labiate, 138;
personate, 139;
rotate, 138;
salver form, 138.
Cortex, 44.
Corymb, 159.
Cotton plant, 7.
Cotyledon, 20.
Cryptogam, 176, 180, 183-204.
Currant, 157.
Cuttings, 121, 123, 124.
Cyme, 159, 160.
Deciduous, 82.
Decumbent, 50.
Dehiscence, 144, 164.
Deliquescent, 51.
Dependent plants, 106.
Dichogamy, 144.
Dicotyledon, 20.
Dicotyledonous stems, 61.
Digestion, 95.
Digitate, 74.
Dimorphous, 144.
Diœcious, 138, 170.
Dispersal of seeds, 172.
Dissection, 30.
Dodder, 35, 106.
Drupe, 168.
Drupelet, 168.
Ecology, 14.
Elaters, 198.
Embryo, 26, 180.
Embryo sac, 180.
Endodermis, 44.
Endosperm, 21, 24.
Entomophilous, 148.
Environment, 6.
Epicotyl, 23, 25.
Epidermis, of leaf, 86, 87.
Epigeal, 23.
Epiphyte, 35, 110.
Equisetums, 201.
Essential organs, 135.
Excurrent, 51.
Explosive seeds, 172.
Fermentation, 190.
Fern, 176.
Fertilization, 144;
cross, 144, 146;
self, 145, 147, 188.
Fibro-vascular bundles, 61, 90.
Field study, 3, 6, 8, 14, 19, 27, 46, 57, 71, 84, 91, 101, 110,
118, 128, 132, 143, 152, 162, 170, 174, 181.
Filament, 135.
Floral envelopes, 133.
Florets, 140.
Flower, 133, 180;
apetalous, 136;
clusters, 155;
complete, 136;
diclinous, 137;
double, 142;
imperfect, 137;
incomplete, 136;
lateral, 136;
naked, 136;
perfect, 137;
pistillate, 137;
regular, 138;
staminate, 137;
sterile, 137;
solitary, 156;
terminal, 156.
Foliage, 16.
Follicle, 165.
Forestry, 68.
Framework of plant, 15.
Frond, 176, 178, 181.
Fruit, 163.
Fucus, 186.
Funaria, 201.
Fungi, 187.
Fungus, 107, 108, 184, 187, 195.
Gametophyte, 179.
Gamopetalous, 134.
Gamosepalous, 134.
Generation of plants, 16.
Geotropism, 44, 47.
Germination, 22, 23, 27.
Glomerule, 160.
Grafting, 125.
Grit cells, 67.
Guard cells, 88.
Gymnosperm, 26, 170.
Hairs, 87.
Herb, 17.
Hilum, 21, 26.
Hip, 168.
Hollyhock, 147.
Homologous, 134, 135.
Host, 107.
Haustoria, 107.
Hyphæ, 107, 188.
Hypocotyl, 22.
Hypogeal, 23.
Indehiscent, 164.
Indusium, 177.
Inflorescence, 155, 160.
Internode, 52.
Involucre, 34, 141, 163, 164.
Iron, 39.
Isoëtes, 203.
Key fruit, 164.
Laboratory, 3.
Landscape, 13.
Larkspur, 148, 149.
Latex tubes, 67.
Leaf, apex of, 80;
base of, 80;
function of, 92;
margin of, 80;
structure, 86.
Leaf scar, 90.
Leaves, arrangement of, 82;
shapes of, 78, 85.
Legume, 165.
Legume family, 35, 169.
Lenticel, 89.
Lichens, 195.
Ligneous, 17.
Liverworts, 196.
Lobes of leaf, 75.
Locule, 136, 163, 166.
Loculicidal dehiscence, 166.
Lumber, 68.
Lycopodium, 204.
Macrospore, 203, 204.
Marchantia, 196.
Medullary ray, 64.
Mesophyll, 86.
Micropyle, 21, 26.
Microspore, 203.
Midrib, 77.
Mint family, 139.
Mistletoe, 109.
Mould, 188.
Monocotyledons, 20, 25, 63.
Monœcious, 138, 150, 170.
Mosses, 199.
Moss, Spanish, 110.
Mullein, 87.
Muscadine, 36.
Mushroom, 107, 194.
Mycelium, 107, 108, 188.
Mycorrhiza, 108.
Natural selection, 8.
Nectar, 148.
Nitella, 187.
Nitrogen, 39, 40.
Nodes, 20, 52.
Nodules, 39, 40.
Nostoc, 184.
Notebooks, 3.
Nucleus, 144, 185.
Nuts, 164.
Oleander, 86.
Oögonia, 186.
Orchid, 35, 110.
Oscillatoria, 184.
Osmosis, 42, 48.
Ovary, 135, 144, 163, 170.
Overgrowth, 12.
Ovule, 144, 186.
Palisade cells, 86.
Palmate, 74.
Panicle, 158.
Pappus, 141.
Parasites, 107.
Parenchyma, 60, 86.
Pedicel, 162.
Peduncle, 62.
Peltate, 77.
Perennial, 17.
Pericarp, 164, 165, 169.
Petals, 134.
Petiole, 76.
Phenogam, 177, 180.
Photosynthesis, 94, 101.
Phyllotaxy, 84.
Pine cone, 27, 170.
Pinna, 181.
Pinnate, 74.
Pinnatifid, 76.
Pistil, 135.
Plantain, 157.
Plant societies, 9.
Plants, unlikeness of, 9.
Plumule, 20, 23, 25.
Plur-annual, 18.
Pod, 164.
Pollen, 135, 144, 180.
Pollination, 144, 145;
artificial, 153.
Polypetalous, 134.
Polysepalous, 134.
Polytrichum, 199.
Pome, 169.
Primrose, 140.
Propagation by buds, 121.
Prop-roots, 36.
Proterandrous, 146.
Proterogynous, 146.
Prothallus, 178, 202.
Protoplasm, 42, 94, 97, 185.
Pruning, 105.
Pseud-annual, 17.
Pteridophytes, 181, 201, 203.
Puffball, 194.
Pyxis, 166.
Quarter-sawed, 70.
Receptacle, 134, 163.
Respiration, in plants, 97, 103.
Resting spore, 184, 185, 189, 191, 192.
Rhizome, 52, 202.
Root cap, 44.
Root climber, 129.
Root hairs, 41, 42, 46.
Rootlet, 41.
Root pressure, 99, 104.
Roots, and air, 41;
forms of, 32;
function, 38;
structure, 38, 43;
systems, 32.
Rust, 192.
Samara, 164.
Sap, 67.
Saprophyte, 107, 108.
Scape, 161.
Scion, 125.
Scouring rush, 203.
Scramblers, 129.
Seed, 20, 163, 180;
coat, 21.
Selaginella, 204.
Selection, natural, 8;
artificial, 8.
Sepal, 133, 169.
Septicidal capsule, 166.
Sessile, 77.
Shelf fungus, 194.
Shrub, 19.
Sieve tubes, 66.
Silicle, 167.
Silique, 167.
Societies, 9.
Soil, 40, 47.
Soredia, 196.
Sori, 177, 192.
Spadix, 140.
Spathe, 138, 140.
Spermatophytes, 180.
Spike, 157.
Spirogyra, 184.
Sporangium, 177, 186, 188, 201, 203, 204.
Spore, 176, 178, 181, 184.
Sporophyll, 180, 201.
Sporophyte, 177.
Stamen, 135.
Starch, 95, 101.
Stem, 49;
endogenous, 59;
exogenous, 61;
kinds of, 49.
Stigma, 135, 144, 145.
Stipule, 76, 84.
Stock, 125.
Stomate, 87.
Stone fruit, 168.
Storage of food, 99.
Struggle to live, 4, 6.
Style, 135, 163.
Summer-spore, 191.
Sun energy, 95.
Survival of fittest, 7.
Swarm-spores, 186.
Symbiosis, 196.
Syngenesious, 141.
Teleutospores, 192.
Tendril, 101.
Thallophyte, 181, 184.
Thallus, 184, 197.
Thorns, 101.
Thyrse, 160.
Tillandsia, 110.
Timber, decay of, 195.
Tissue, 60, 62.
Toadstool, 194.
Torus, 134, 169.
Tracheid, 65.
Transpiration, 98, 103.
Twiners, 129, 131.
Umbel, 159.
Undergrowth, 12.
Valve, 164.
Variation, 2.
Vaucheria, 186.
Verticellate, 84.
Water-pore, 88.
Wheat rust, 192.
Whiteweed or ox-eye daisy, 155.
Whorled, 84.
Willow mildew, 190.
Wind travelers, 173.
Woody fibre, 17.
Wounds of plants, 56.
Yeast plants, 190.
Zygnema, 185.
Zygospore, 185, 189.
GLOSSARY
_accessory_ or _reinforced fruits_ are those in which the ripened
pericarp is combined with other parts, as with the torus or the
calyx.
_adventitious._ Coming by chance or without order, as the sprouts
that arise where a limb is cut off.
_aggregate fruits_ are those in which several distinct pistils
cohere to form one body, as in blackberry, raspberry, mulberry.
_akene_ or _achene_. A one-seeded indehiscent fruit, usually small
and seed-like.
_analogous._ Like to, in function or use. See _homologous_.
_anemophilous._ Said of flowers that are pollinated by the wind.
_annual._ A plant that naturally does not live more than one year,
as garden bean, pea, Indian corn, buckwheat, cowpea.
_anther._ The knob or enlargement of the stamen, bearing the pollen.
_antheridium._ The male or sperm-cell organ, such as occurs on the
prothallus of ferns and similar plants. See _archegonium_.
_apetalous._ Without petals.
_apical._ At the top.
_archegonium._ The female or egg-cell organ on the prothallus of
ferns and related plants. See _antheridium_.
_assimilation._ The building up of protoplasm from the materials
elaborated in the plant.
_axil._ The angle or place just above the petiole of a leaf (or
pedicel of a flower) where it joins the twig.
_berry._ In botany, a fleshy pericarp containing a number of seeds,
as current, orange, tomato, grape, cranberry, but not strawberry,
blackberry, raspberry, or mulberry.
_biennial._ A plant that lives two years. It usually blooms the
second year.
_blade._ The expanded part of a leaf.
_bract._ A small or much-reduced leaf, often a mere scale.
_calyx._ The outer row or series in the floral envelope. The outer
“leaves” of the flower, usually green.
_cambium._ Growth tissue, lying between the bark-part and wood-part
of the fibro-vascular bundle and giving rise to the cells of both.
_capsule._ A pod consisting of two or more carpels or parts,
usually opening naturally.
_carbohydrate._ The compounds of the starch and sugar class.
_carpel._ One part or member of a compound pistil, or a simple
pistil itself.
_catkin_ or _ament_. A raceme-like or spike-like flower-cluster
that falls away after flowering or fruiting, as of willows and
staminate flower-clusters of walnuts and birches.
_centrifugal._ From the inside out; as a flower-cluster of
which the inside, terminal, or uppermost flowers open first; a
determinate cluster.
_centripetal._ From the outside in; as a flower-cluster of which
the outer flowers open first; an indeterminate cluster.
_chlorophyll._ Leaf green. Chlorophyll is the pigment that gives
the characteristic colour to plants.
_cladophylla._ Stems that look like leaves, and function as leaves,
as in asparagus and the florists’ smilax.
_cleistogamous._ Applied to small flowers, usually hidden beneath
the earth, that are little developed as to floral envelopes, and
are self-fertilized; also to self-fertilization in flowers that do
not open.
_complete flowers_ have all the parts,—calyx, corolla, stamens,
pistil.
_corolla._ The inner row or series of flower-leaves, usually
coloured, and often of irregular shape. It may be all one piece or
of many pieces.
_corymb._ A flattish flower-cluster in which the outermost flowers
open first.
_cotyledon._ A leaf of the embryo; seed-leaf. The embryo may have
one cotyledon (monocotyledon), or two cotyledons (dicotyledon), or
sometimes more than two.
_cross-fertilization_ is fertilization by means of pollen produced
in another flower.
_cryptogam._ One of the group of flowerless or non-seed-bearing
plants, as a fern, fungus, moss, seaweed.
_cutting._ A shoot planted in soil or water for the purpose of
making a new plant.
_cyme._ A flattish or broad flower-cluster in which the innermost
or terminal flowers open first.
_decumbent._ Said of branches or stems that lop or lie over on the
ground.
_decurrent._ Said of a leaf that runs down on the stem, thereby not
having a distinct petiole.
_dehiscence._ The mode of opening, as of a seed-pod or an anther.
_deliquescent._ Said of trees in which the leader or main trunk
disappears at the tree top, forking into several or many main
branches.
_determinate._ See _centrifugal_.
_dichogamy._ The condition when stamens and pistils in the same
flower mature at different times; this prevents or hinders
self-pollination. See _proterandrous_ and _proterogynous_.
_diclinous._ Said of flowers that are imperfect,—lacking either
stamens or pistils.
_dicotyledon._ Having two cotyledons or seed-leaves.
_digestion._ Change in the food materials whereby they may be
transported, or used in assimilation. Starch is changed into sugar
in the plant by a process of digestion.
_dimorphous._ Of two forms; as flowers that bear two kinds of
stamens.
_diœcious._ Said of plants that bear stamens and pistils in flowers
on different plants.
_drupe._ A fleshy pericarp or fruit, containing a relatively large
stone or pit, as peach, cherry, plum.
_drupelet._ A very small drupe, particularly one comprising part of
an aggregate fruit, as a drupelet of raspberry.
_embryo._ The dormant plantlet comprising part of the seed. It is
enclosed within the seed-coats. Its parts are the caulicle (or
stemlet), cotyledons or seed-leaves, and plumule. The food may be
stored in the embryo, or around the embryo (endosperm).
_endogen._ A plant of the monocotyledon class, not enlarging in
diameter by means of outside rings; as palms. All grasses and
lilies and orchids and cereal grains are of this kind. Now used, if
at all, to express a general mode of growth rather than a class of
plants. See _exogen_.
_endosperm._ The food material that is packed around the embryo
(rather than inside it) in the seed.
_entomophilous._ Said of flowers that are pollinated by insects.
_environment._ The surroundings; or the conditions in which a plant
or animal lives. The environment comprises the soil, climate, and
the influence of the other plants and animals with which or among
which the plant or animal grows.
_epicotyl._ The internode or “joint” above the seed-leaves or
cotyledons.
_epidermis._ The outermost layer or part of the cortex; the skin.
_epigeal._ Said of seeds (as common bean) in which the cotyledons
or seed-leaves rise above ground in germination. See _hypogeal_.
_epiphyte._ A plant that grows on another plant, or on other
objects above ground, but which does not derive much or any of its
nourishment from its host; an air-plant.
_excurrent._ Said of trees (as firs and spruces) in which the main
trunk or leader continues through the tree top.
_exogen_ (see _endogen_). Of the dicotyledon class, the stem
enlarging by external layers or rings.
_fertilization_ takes place in the flower when a pollen nucleus and
an egg-cell nucleus unite in a forming ovule.
_fibro-vascular._ Bundles or strands of tissue composed of
sieve-tubes, mechanical fibre and vessels or ducts.
_filament._ The stalk part of a stamen.
_follicle._ A single-cavity fruit or pod opening along its inner
edge.
_frond._ A leaf of a fern and related plants.
_fruit._ In botany, the ripened ovary with the attached parts. All
flowering plants, therefore, produce fruits. The term is also used
for the ripened reproductive bodies of flowerless plants.
_fruit-dot_, _sorus_. A collection or cluster of sporangia, as in
ferns.
_function._ What a plant or an organ does; how it works.
_gamete._ A cell or nucleus that takes part in fertilization.
_gametophyte._ The stage of the plant (as the prothallus) that
bears or produces the sex organs; sexual stage of the plant.
_gamopetalous._ Said of a corolla with the petals united.
_gamosepalous._ Said of a calyx with the sepals united.
_generation._ The entire life period of a plant.
_geotropism._ Turning toward the earth, as the action of the roots.
_glomerule._ A dense globular or oblong flower-cluster in which the
upper or inner flowers open first.
_graft._ A cutting inserted in another plant for the purpose of
having it grow there.
_gymnosperm_ (“naked seed”). A name applied to a group of plants
(pines, spruces, cedars, and the like) in which the seeds are not
contained in an ovary.
_head._ A very dense globular or oblong flower-cluster in which the
outer flowers open first; often applied to any dense flower-cluster.
_herb._ A plant that never becomes woody and that dies to the
ground, or dies entirely, in winter.
_hilum._ The scar or spot where the seed was attached to its stalk.
_hip._ The fruit of the rose, which is a hollowed torus containing
the dry fruits or “seeds.”
_homologous._ Related to, or with, in origin or structure. Thus, a
tendril of grape is homologous with a branch; a tendril of grape
is _analogous_ to a tendril of pea (similar in function), but not
homologous, for one represents a branch (or flower-cluster) and the
other represents a leaf.
_host._ A plant or animal on which another organism grows or feeds.
_hypha_ (plural _hyphæ_). The threads of the mycelium of a fungus.
_hypocotyl._ The stem or internode below the seed-leaves.
_hypogeal._ Said of seeds (as garden pea) in which the cotyledons
remain under ground in germination. See _epigeal_.
_imperfect flowers_ lack either stamens or pistils.
_incomplete flowers_ lack one of the parts or series, as the calyx,
corolla, stamens, or pistil.
_indeterminate._ See _centripetal_.
_indusium._ The scale or lid covering a sorus, in ferns and allied
plants.
_inflorescence._ Properly, the mode of flowering (page 160), but
sometimes used in the sense of a flower-cluster.
_involucre._ A set or whorl of leaves or bracts beneath a flower or
a cluster of flowers; sometimes looks like an outer or extra calyx.
_irregular flowers_ have some members of one or more of the series
unlike their fellows.
_key fruit._ See _samara_.
_labiate._ Lipped; that is, divided into parts, as the lips of a
mouth. Said usually of corollas that are lobed into two parts.
_lateral._ On the side; as a flower or leaf borne on the side of a
shoot rather than at its end. See _terminal_.
_leaflet._ One of the divisions or parts of a compound leaf.
_legume._ Like a follicle, but opening along both edges. In some
cases, as in peanut, the pod does not actually open.
_leguminous plants_ are those that bear legumes or true pods, as
peas, beans, clovers, alfalfa, vetch, sweet pea, peanut, locusts,
red-bud.
_lenticel._ Very small spots or corky elevations on young twigs,
marking the place of former twig stomates.
_locule._ One cavity or “cell” in a pistil or anther.
_loculicidal._ Said of capsules when the carpels or compartments
open between the partitions.
_mesophyll._ The parenchyma in the leaf.
_micropyle._ The place on the seed at which the pollen-tube entered
when the seed was forming (when impregnation took place).
_monocotyledon._ Having one cotyledon or seed-leaf.
_monœcious._ Said of plants that bear the stamens and pistils in
different flowers on the same plant.
_mycelium._ The vegetative part of a fungus, composed of threads or
hyphæ.
_mycorrhiza._ A root covered with or bearing a fungus that aids the
root in securing nourishment from the soil.
_naked._ Said of flowers that lack envelopes (calyx and corolla).
_nectary._ A cup, sac, or place in the flower in which nectar (or
“honey”) is borne.
_osmosis._ The passing or diffusion of liquids or gases through
membranes.
_ovary._ The lower enlarged part of the pistil, containing the
ovules or forming seeds.
_ovule._ The young or forming seed.
_palmate._ Palm-like; said of venation that arises from the base of
the leaf (top of petiole), or of leaflets similarly arranged.
_panicle._ A branching raceme. The lower or outer flowers open
first; but the word is often used loosely.
_pappus._ The hair, plumes, bristles, or scales on the top of a
dry fruit, particularly of a fruit (or “seed”) of the Compositæ or
sunflower family.
_parasite._ A plant or animal that lives on a living host (as
on a plant or an animal), taking its food from the host. See
_saprophyte_.
_parenchyma._ The general underlying tissue, from which other
tissue arises.
_pedicel._ Stem of a single flower in a cluster.
_peduncle._ A flower-stem, supporting a solitary flower or a
cluster of flowers.
_perennial._ A plant that lives more than two years, as most
grasses, docks, alfalfa, asparagus, and all trees and shrubs.
_perfect flowers_ bear both stamens and pistils.
_pericarp._ A ripened ovary, without counting attached parts.
_personate._ Masked; that is, so formed as to suggest a masked
face, in labiate corollas with a large lower lip.
_petal._ One of the parts or leaves of the corolla.
_petiole._ Stem or stalk of a leaf.
_petiolule._ Stem of a leaflet.
_phenogam_ (_phænogam_, _phanerogam_). A seed-bearing plant; that
is, one of the seed-bearing or flowering group of plants.
_phloem._ Bark or soft bast tissue.
_photosynthesis._ The process whereby the carbon dioxide of the air
is appropriated in the formation of material for plant growth.
_phyllotaxy._ Mode of arrangement of leaves or flowers on the plant
or stem.
_pinnate._ Feather-like; said of leaves in which the veins strike
off from a continuing midrib, or in which the leaflets are arranged
in a similar order.
_pistil._ The innermost member in the flower, bearing the forming
seeds.
_pistillate._ Of pistils only; a flower that contains pistils and
no stamens, or a plant that bears only pistils.
_plur-annual._ A plant of a tropical or semi-tropical climate that
is annual in a colder country only because it is killed by frost;
as tomato, castor bean.
_pollen._ The dust or grains contained in the anther, which,
falling on the stigma, grows and fertilizes the forming ovules.
_pollination._ The transfer of pollen from the anther to the
stigma. The transfer may be accomplished by wind, insects, birds,
water (in the case of water plants), or by the natural falling of
the pollen.
_polypetalous_ (“many-petaled”). Said of a corolla with the petals
not united.
_polysepalous_ (“many-sepaled”). Said of a calyx with sepals not
united.
_pome._ An apple-like or pear-like or quince-like fruit, with a
five-carpeled or en-carpeled “core.”
_proterandrous._ Said of a flower when the anthers mature in
advance of the pistils in the same flower.
_proterogynous._ Said of a flower when its pistils mature before
its anthers.
_prothallus_ (“first thallus”). The minute leaf-like body or organ
produced by the germination of a spore, in ferns and allied plants.
It bears the sex organs.
_protoplasm._ The living matter in plants. It is the living part of
the cells, usually in a semi-fluid, translucent state.
_pseud-annual._ A plant that is apparently annual, but which is
carried over winter by a bulb, tuber, or similar body; as potato,
onion.
_pyxis._ A dry fruit or capsule in which the top comes off, like a
cover to a jar.
_raceme._ A simple (unbranched) cluster in which the flowers are on
short pedicels and open from the base upwards.
_raphe._ A ridge or elevation on some seeds caused by the
seed-stalk and seed-coats growing together.
_ray._ The elongated corolla-limb of some members of the Compositæ
family.
_receptive._ Said of a stigma when it is “ripe” or ready to receive
the pollen.
_regular flowers_ are those in which all the members of each series
(as all the sepals, or all the petals, or all the stamens) are like
each other in shape, size, and colour.
_reinforced._ See _accessory fruits_.
_respiration._ Breathing; manifest by oxygen taken in and carbon
dioxide given off.
_rhizome._ A rootstock; an underground root-like stem. It has
joints, usually scales representing leaves, and is often thick and
fleshy.
_samara._ A key fruit, being an indehiscent (not opening) fruit
provided with a wing or wings.
_saprophyte._ A plant that lives on dead or decaying material. See
_parasite_.
_scape._ A flower-stem rising directly from the crown of the plant
at the surface of the ground or near it. A scape may have bracts.
_self-fertilization_ or _close-fertilization_ is fertilization by
means of pollen produced in the same flower.
_sepal._ One of the parts or leaves of the calyx.
_septicidal._ A form of dehiscence or opening along the natural
partitions of the capsule.
_sessile._ Sitting; without a stem, as a leaf without petiole, or a
flower without pedicel or peduncle.
_shrub._ A low woody plant that does not have a distinct trunk.
When a plant normally has a trunk, it is a tree.
_silicle._ A short, more or less circular, capsule of the mustard
family.
_silique._ A long capsule of the mustard family.
_society._ An aggregation or company of plants, comprising a more
or less distinct group.
_sorus_ (plural sori). See _fruit-dot_.
_spadix._ A spike of flowers (each flower usually minute), with a
more or less fleshy axis, and usually accompanied by a spathe.
_spathe._ A corolla-like or involucre-like leaf or bract (or a pair
of them) surrounding or accompanying a spadix. In the calla, the
spathe is the large white flower-like part.
_spermaphyte_ (“seed plant”). A seed-bearing plant; one of the
flowering-plant class.
_spike._ A dense and slender flower-cluster in which the flowers
open from below upwards.
_sporangium_ (plural _sporangia_; _spore-case_). A body or
receptacle holding spores.
_spore._ A reproductive or generative cell; in flowerless plants
answering the purpose of a seed, but containing no embryo. It may
not be the direct product of fertilization.
_sporophyll_ (“spore leaf”). A member or part that bears spores.
_sporophyte._ The stage of the plant arising directly from the
fertilized egg, and which ordinarily produces asexual spores (as
the “plant” or conspicuous part of the fern or of a seed plant).
_stamen._ The pollen-bearing organ of the flower, of which the
essential part is the anther (usually borne on a stem or filament).
_staminate._ Said of a flower that has stamens and no pistils.
_stigma._ The part of the pistil (usually on a stalk or style)
on which the pollen germinates; it is sticky, rough, or hairy at
maturity.
_stipel._ A stipule of a leaflet.
_stipule._ A leafy or scale-like appendage at the base of a
petiole. Stipules are usually two in each case.
_stomate_, _stoma_ (plural _stomates_ or _stomata_). The openings
on leaves and green parts through which gases pass; diffusion-pores
or “breathing-pores.”
_stone-fruit._ A drupe.
_strict._ Said of a stem that grows straight up, without breaking
into branches.
_style._ The stalk between the ovary and the stigma; sometimes not
present.
_syngenesious._ Said of anthers when they cohere in a ring, as in
the Compositæ, the style usually being inclosed.
_tap-root._ A single or leading strong root that runs straight down
into the earth.
_tendril._ A slender coiling member of a plant that enables it to
climb. A tendril may represent a branch, a petiole, a leaflet, a
stipule, an entire leaf.
_terminal._ At the end; as a flower borne on the end of a shoot.
See _lateral_.
_thyrse._ A compound, usually elongated or pyramidal flower-cluster
in which the mode of inflorescence is mixed.
_torus._ The end of the flower-stalk (usually somewhat enlarged) to
which the flower-parts are attached; receptacle.
_transpiration._ Evaporation or loss of water from plants.
_umbel._ A flower-cluster opening from the outside, in which the
branches or stems arise from one place, as the rays of an opened
umbrella.
_umbellet._ A small umbel, comprising part of a larger or compound
umbel.
_valve._ One of the integral parts into which a fruit or an anther
naturally splits, or into which it is divided.
_venation._ The mode or fashion of veining, as in a leaf or petal.
_xylem._ Wood tissue.
TRANSCRIBER’S NOTE
Illustrations have been positioned between paragraphs.
Illustrations without captions have had a description added, this is
denoted with parentheses.
The index was not checked for correct page references.
Some hyphens in words have been silently removed, some added,
when a predominant preference was found in the original book.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained.
Pg. 13 “dstinguishes” replaced with “distinguishes”.
Pg. 29 “with experiment” replaced with “with the experiment”.
Pg. 46 “grow” replaced with “grows”.
Pg. 50 “lop over” replaced with “flop over”.
Pg. 86 “sphercal” replaced with “spherical”.
Pg. 94 “Chlorohyll” replaced with “Chlorophyll”.
Pg. 111 “buds of the apple and” changed to
“buds of the apricot and” to match figure caption.
Pg. 131 “cobea” replaced with “cobaea”
Pg. 135 “tigma” replaced with “stigma”.
Pg. 170 “aranged” replaced with “arranged”.
Pg. 186 “acomplished” replaced with “accomplished”.
Pg. 188 “is this case” replaced with “in this case”.
Pp. 205,208 Corrected location of “Scion” from Pg. 205 to Pg. 208.
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