Botany

ai

Richard M. Holman

-55*-

BOTANY

*Appleton\s Scientific ^Primers
Edited by J.^ynolds Green,Sc.T>.,F.R.S.

BOTANY

BY

J. REYNOLDS GREEN, Sc.D., F.R.S.

Downing College, Cambridge

ILLUSTRATIONS

New Tork

tAppleton and Company

BIOLOGY
LIBfcARY

Apple ton's Scientific Primers

Edited by

J. Reynolds Green, Sc.D.

BIOLOGY. By Prof. HARVEY

GIBSON.
CHEMISTRY. By Prof.

W. A. TILDEN.
BOTANY. By J. REYNOLDS

GREEN, Sc.D:, F.R.S.

N\,

PREFACE

IN writing this little introduction to the study of a plant
I have endeavoured especially to present it to the reader
as a living organism. Botany is now regarded as a
branch of biology, and is not satisfactorily studied by
gathering plants and, after ascertaining their names and
the natural orders to which they belong, drying them
and putting them away in a cabinet. I have tried to
present them as they are engaged in the struggle for
existence, and to call my readers' attention not only to
their form and structure but especially to what they do
in life, and why and how they do it.

I hope that those who study them by the assistance
of this little primer will try to have the living plant
under observation whiler they read it. I have not
written any detailed scheme of laboratory work, but I
hope my readers will be able to construct such a scheme
for themselves as they follow the directions for study
given in the text.

I should like to suggest that students should read the
Chemistry primer first, to gain some acquaintance with
the phenomena underlying the processes of construction
and decomposition going on in the plant. It would be
well to read the Biology primer also before beginning
Botany.

J. REYNOLDS GREEN.

CAMBRIDGE, 1909.

921904

CONTENTS

CHAP. PAGE

I. INTRODUCTORY ...... -7

II. THE EARLY DEVELOPMENT OF A PLANT— THE GERMINA-
TION OF A DICOTYLEDONOUS SEED

III. THE FORMATION OF THE ROOT SYSTEM

IV. THE STRUCTURE OF THE ROOT .

V. THE CHARACTERISTIC FEATURES OF THE SHOOT .
VI. THE CONSTRUCTION OF THE SHOOT SYSTEM
VII. THE STRUCTURE OF THE SHOOT .
VIII. THE MONOCOTYLEDONOUS PLANT
IX. THE FOOD OF PLANTS

X. THE RESPIRATION OF PLANTS .....
XI. THE EVOLUTION OF THE FORMS OF PLANTS — ALG.E

XII. THE DEVELOPMENT OF THE REPRODUCTIVE PROCESSES IN
THE ALG.E ... ....

XIII. THE ORIGIN OF TERRESTRIAL PLANTS — EVOLUTION OF

MOSSES AND FERNS ... . .

XIV. REPRODUCTION OF FLOWERING PLANTS — VEGETATIVE

PROPAGATION .......

XV. THE INFLORESCENCE AND THE FLOWER
XVI. POLLINATION AND ITS MECHANISMS — FERTILISATION

XVII. FORMATION OF THE SEED AND ITS MIGRATION — THE
FRUIT ........

VI

BOTANY

CHAPTER I

INTRODUCTORY

OF all the things we see about us as soon as we escape
from the life and surroundings of the town, none is more
familiar to us than the common green plant. We
tread upon grass and other plants which clothe the
earth's surface, we walk under trees, around bushes, and
by the sides of hedges, or we wander through more
cultivated scenes, enjoying the beauty and fragrance of
the well-cared-for garden. In all this wealth of vegeta-
tion perhaps, however, one fact sometimes escapes our
notice. These plants, trees, shrubs, weeds, or what not
are alive. We do not deny this when we hear it said,
but the idea is hardly a prominent one in the view we
take of things in general. It is based probably on the
fact that we do not see the plants move, except as their
slender twigs and branches or their numerous leaves are
swayed to and fro by the wind, for to our own some-
what narrow experience life is so closely connected with'
restless change of position or locomotion. Yet if we
wish to study plants to learn something more about
them than a casual glance can tell us, we must bear in
mind these two facts on which their whole story turns :
first, they are living creatures ; second, they spend their
lives in the same place in which they commenced them.
This is true of the greater number of plants we see
around us, though there are some exceptions, chiefly

7

8 BOTANY

plants which, living in water, are passively moved about
by the currents of the stream.

The fact that a plant is alive and conducts itself as a
living organism implies certain things. It must receive
suitable and sufficient nourishment; it must possess a
certain power of adjusting itself to its surroundings,
defending itself against possible dangers and over-
coming definite difficulties which these surroundings
occasion, and taking advantage of such benefits as are
met with in them. It must possess, to at any rate a
limited extent, a power of appreciating its relations to
such surroundings, of realising variations in certain of
them, such as light, moisture, and temperature, that it
may adapt itself accordingly.

The second fact, that it cannot alter its position by
moving freely about, makes those requirements more
essential. It also demands that it shall be possessed of
such a safe attachment to its situation as shall secure
it an appropriate position and shall enable it to enjoy
undisturbed such advantages as the surroundings offer.
Further, it calls for a certain power of adjustment of its
various parts to the air above it and the earth in which
it is fastened, as changes in both of them are frequent
and sometimes violent. As the only sources of nourish-
ment possible to it are the air and the soil, together
with the water which both contain in constantly vary-
ing amount, its construction must be such that the same
parts which secure anchorage or support shall be capable
of securing supplies of the various materials which ulti-
mately become the medium of nourishment.

A further requirement of every living organism is the
need of possessing the means of bearing offspring which
shall succeed it in the great scene of nature. To a
stationary organism this introduces difficulties from
which the readily moving animal is free, but these diffi-
culties have been overcome by adaptations to the habit

INTRODUCTORY

of life which are among the most complicated and the
most perfect that nature shows us.

So the life of a plant shows us conflict and struggle
waged against disadvantages of a very formidable
nature; a power of appreciating difficulties and of
struggling against them ; further, it exhibits a capacity
of seizing upon such advantages as present themselves,
not only in the air and in the soil, but in relative
association and competition with each other.

We are familiar with the fact that part of an ordinary
green plant is embedded in the soil. Such a part is
commonly known to us as its
root, and we distinguish it in
several ways from the part
which rises into the air (Fig. i).
In the case of plants which live
in water we find much the same
division of the plant body.
There is in their case also a root
part, which is not green and
which is buried in the soil or
mud at the bottom of the
water ; there is a part which
stretches up into the water, in
some cases extending into the
air above the surface. We
often express this fact by say-
ing that the plant is differenti-
ated into a root and a shoot.

This differentiation is a funda- FIG. i. Diagram showing the
mental one, for the two parts
behave very differently. They
always grow in opposite directions, and as these direc-
tions are generally upwards and downwards they are
spoken of as the ascending and the descending axes of
the plant.

general structure of
dicotyledonous plant.

io BOTANY

We need not at present consider very fully the case
of the water-plant, and will therefore examine the
relations between the root and shoot in general and
the surroundings in which each finds itself.

The anchorage of the plant is secured by the penetra-
tion of the soil by the roots. The advantage thus
secured is not obtained without difficulty and even
danger. To become fixed in the soil the plant must
penetrate it, a process which it can only carry out by
its gradual growth. The composition of the soil offers
certain difficulties to this penetration: it may be too
dense or too powdery, too dry or too wet ; it may be
slimy like clay, or very hard and strong. The amount
of water in the soil and the degree in which it contains
air are also factors which must be taken into account
in considering this growth. After a plant has once
established itself and secured firm anchorage, it still has
to deal with varying conditions of a similar nature, for
the character of the soil is very liable to changes, depend-
ing on conditions of temperature, weather, and so on.

Besides the advantage of a firm anchorage, the root
depends upon the soil for the supply of certain materials
which ultimately aid in some way in its nutritive pro-
cesses. Certain minerals are necessary to every green
plant, many others are advantageous, some are dele-
terious. We are here face to face with dangers and
advantages which need adjustment to the plant as it
is growing in the soil. Such a struggle can be easily
observed. While all plants need compounds of nitrogen,
some will only flourish on soil which contains as well a
certain, often a large, proportion of chalk, others fail
entirely if the chalk is plentiful. It is much the same
with other constituents of the soil.

If a plant is growing in uncongenial surroundings it
has but little power of adjustment to them. It conse-
quently dies out more or less rapidly. If on the other

INTRODUCTORY n

hand its environment suits its constitution, it has to
adapt its structure to the duty of absorbing from the
soil what the latter will afford. So the two duties of
anchorage and absorption exist together, and the
differentiated root system necessarily discharges both.

If we turn to inquire what dangers beset the part of
the plant we have called the shoot, which grows up
into the air and forms a head that is frequently of
large size, we find them taking shape in the various
atmospheric changes incident to every climate. First
of these we may place wind or tempest. As the shoot
body grows it must offer more and more resistance to
air currents, a resistance which may easily culminate in
a violent uprooting of the plant. This involves such a
subdivision of the plant body as will allow the wind to
penetrate through it without serious disturbance. Here
we see one meaning of the tapering boughs and twigs,
which become more and more flexible as they become
increasingly slender. In the central part of the shoot
system they are rigid and can resist the storm; where
by their dimensions resistance becomes impracticable we
find flexibility, enabling them to bow to the wind often
so completely as to place their long axes parallel to the
direction in which it is blowing.

Yet another reason for this continued subdivision of
the plant body is found in its relation to the absorp-
tion from the soil which we have found associated with
the root. The latter is continually absorbing the water
of the soil ; after separating from such water the mineral
constituents it contains, a very large part indeed of the
water is evaporated, and so passes away to the exterior
again. To favour such evaporation it is advantageous
that the ratio between surface and bulk shall be a large
one, and so the great subdivision of the subaerial part
of the plant is concerned in solving the problem of its
nourishment.

12 BOTANY

Indirectly the composition of the above-ground part
of the plant has a direct application to a danger to which
the underground region is exposed. The pressure of
the wind upon an unyielding surface in the air would be
attended by great danger to the anchoring root, which
might be violently pulled from the ground by the
leverage exerted by such pressure. The great subdivi-
sion of the shoot system and the flexibility of its ultimate
twigs minimises this danger, but even as it is, it is not
unusual after a tempest to find even sturdy trees up-
rooted and thrown down.

The distribution of the water of rain-storms presents
another problem which must be solved by the shoot
system. The water can be led either towards or away
from the centre of the plant. Should the root system
be one which spreads considerably and extends to long
distances below the surface of the soil, it is of great
importance that the rainfall collected on the central
mass of shoots shall be distributed widely so as to reach
as far as the extremities of the roots, watering in this
way a large area of ground. If the root system consists
of a strong main root with comparatively few branches,
this arrangement would largely deprive it of water.
Hence in plants with roots distributed in this way we
find arrangements to conduct the water into the centre
of the mass of shoots.

In some rare cases the duty generally discharged by
the root as an anchoring organ falls upon the shoot,
which then is partly developed underground. Such a
stem bearing in its turn appendages has a special name
—it is called a rhizome.

If we pass to a closer study of the much divided or
branched shoot we find almost invariably that its ulti-
mate twigs put forth certain regularly arranged flattened
expansions. In cases where there is much exposure to
currents of air, these flattened portions are furnished

INTRODUCTORY 13

with stalks of variable length which are extremely
flexible and allow the flattened organs to sway freely
backwards and forwards as the wind blows upon them.
These flattened portions, further, are usually of a vivid
green colour; they are then known as leaves, or, pre-
ferably, foliage leaves.

As almost all plants possess leaves we may inquire
why these organs should so uniformly be thin and flat.

There are several reasons of almost equal importance.
The leaf or other winged part of the shoot portion is in
contact or relation with the air only. Interchanges of
gases between the air and the leaf are continually going
on, and these interchanges are effected most easily and
fully with a large extent of surface. No form gives so
much surface in proportion to its bulk as a thin flat
plate, just such a form indeed as the flattened portion or
blade of the leaf. The interchanges include the absorp-
tion of particular gases from the air, and the giving out
of gases and water vapour. As we shall see later, the
internal structure of the leaf-blade is arranged largely
with a view to the carrying out of these exchanges.

A second reason for the flattening of the leaf is
concerned with the manufacture of the plant's food.
A particular gas known as carbon dioxide, which is taken
in from the air, is ultimately built up into a true food
material, a kind of sugar. Though the formation of
sugar in the plant is not fully understood, it is known to
depend upon the presence of the green colouring matter
and its being properly illuminated. The flattened form
helps to secure the arrangement of the green colouring
matter in such a way that the light, either of direct
sunshine or of the less bright diffused daylight, may
reach it with the least obstruction.

Yet a third reason may be given. The leaves are
very frequently so placed that they extend outwards
from the plant and lie nearly parallel to the surface of

14 BOTANY

the ground. In this way they present their edges to
the wind, and offer as little obstacle as possible to its
passage through the tree, so making as small as possible
the risk of being torn off when the force of the wind is
strong. As the wind passes between them they are
made to rise and fall, but they offer much less resistance
to its force than they would if they were not flattened.

The arrangements of the plant and its parts so far as
we have studied them are such as to secure its firm
attachment to the soil, its stability in storms, with rela-
tion both to wind and rain. They also make possible
the absorption of liquid, containing mineral matters,
from the soil ; the evaporation of the excess of water so
absorbed ; the free interchange of gases between it and
the air; the needed facilities for the manufacture of
sugar from the gases absorbed from the air and the
water from the soil. They are, in fact, suitable to
support and nourish a stationary living organism and
to furnish defences against the most evident dangers to
which it is exposed.

The establishment of such a position by the plant
is carried out by means of growth alone. It is a gradual
process, therefore, and must be accompanied by the
nutritive processes which enable growth to take place
First among these comes the supply of the material for
the increase of size which we associate with growth.
We have seen that the plant absorbs from the soil cer-
tain mineral compounds dissolved in water, and from
the air certain of its constituent gases. The most
important of the materials which the earth yields are
nitrates of potassium, calcium, and other metals, phos-
phates of the same, traces of compounds of iron, a little
silica in some combination, together with the water in
which they are dissolved ; carbon dioxide is supplied by
the air. When the absorption of these substances is
possible, and when light is sufficient and temperature

INTRODUCTORY 15

moderate, the healthy plant is found to increase in size,
and gradually to show all the phenomena of growth.
Hence these various compounds have been regarded as
its food. This is not, however, a correct view, for they
should be considered only as raw materials from which
the green plant can make the food it needs. This is
effected by the agency of the green colouring matter,
the so-called chlorophyll, but only when it receives an
appropriate amount of light. In the absence of chloro-
phyll or in insufficient light the supply of all these
various compounds does not afford any nourishment to
the plant. Plants without chlorophyll are not far to
seek; we find them in the mushrooms, in the moulds
that grow so readily on decaying matter, the mildews of
corn and other crops, and so on. These cannot develop
at all when supplied only with the inorganic compounds
mentioned.

The plant then in order to grow and to establish itself
has to be provided with suitable food. If it has chloro-
phyll and is properly illuminated it makes this food for
itself from the inorganic materials the soil and air pro-
vide. Plants which cannot make their food have to
obtain it from living or dead organic matter. Though
this is difficult it is not impossible, for such matter
abounds almost everywhere — not only in the soil but in
the numerous manufactured products which we meet
with all around us. Living organisms also are often
made to yield food to these non-green plants. The
chlorophyll-containing plants are continuously making
the organic substance which constitutes their food as
long as light shines upon them. We find them growing
at its expense and accumulating large quantities of such
substances as sugar, starch, proteins, and fats in their
own bodies. As they in their turn, or many of them,
ultimately become the food of animals, we may see
their importance in the role of nature. The fact is that

16 BOTANY

the green plant is the only organism which has the
power of forming organic substance from the inorganic
material of the earth and air. As all living beings are
dependent on this organic substance for the maintenance
of life, we see how the continuation of life itself upon
the earth depends on the activity of the green plant.

The establishment of the position of the plant and its
defence when so established may be seen, therefore, to be
subordinate to the manufacture of organic food.

The food so made is complex in character and will be
dealt with in greater detail in a subsequent chapter.
It comprises chiefly three classes of substance: carbo-
hydrates, of which sugar and starch are representatives,
fats, and proteins, which are much more complex in
composition, and are represented by the white of egg
and by the chief constituent of meat and fish. The
proteins are held to be the organic material which
most resembles the living substance itself.

As it is the process of growth at the expense of this
newly constructed food, or of a small supply derived
directly from its parent, by
which the young plant makes
its way into its appropriate
position, it is clear that this is
the action of a living organism
and becomes probable that the
surroundings of the plant affect
FIG. 2. Geotropic curvature j1 in other ways than by afford-
in root and shoot of ing it the material from which

(AfterrGibson.f Ufal ^ to make its food- Careful ob-
servation shows that this is the

case. The root of the plant at even its first appear-
ance grows downwards in the direction of the soil.
If it be made to point in another direction, its plan
of growth slowly changes and it gradually curves
till its tip is pointing downwards again (Fig. 2). If

INTRODUCTORY 17

light reaches it, it bends slowly away from the illu-
minating ray; if anything comes into contact with its
tip, growth causes it to curve so as to leave the obstacle
on one side. The young root shows in these ways
certain sensitivities, reacting to the incidents of its
environment, and behaving as if it were possessed of
rudimentary perceptions of direction, illumination, and
contact. Other features of the environment also affect
it, particularly moisture. The shoot in its behaviour
shows similar phenomena, but its conduct when influ-
enced, or, as it is generally called, stimulated, by gravity,
light, or other disturbing causes, is as a rule the opposite
of that of the root. It grows upwards against gravity;
it curves towards and not away from light ; its behaviour
with regard to contact is not always uniform. Both
parts, however, show what we claimed at the outset as
one of its primal necessities, the power of adjusting
itself to changes in its surroundings.

Consideration of the fourth requirement, the power to
reproduce itself, must be deferred for the present.

We may now with advantage turn to the composition
of the plant and ask what is the distribution in it of the
living matter to which this behaviour is to be attributed.

It is best to begin the study of this point by examin-
ing quite a young plant, or preferably the seed of a
plant, as the structure is then simple,
while it becomes very complex as the plant
grows. If we take a seed (Fig. 3) we find
it contains a young plant or embryo, in
which bv careful dissection we can make J^****^ ^*

J , „,, FIG. 3. Section

out a young root and a young shoot. The Of a seed,
shoot consists of a short axis, to which «> embryo,
are attached either one or two leaves
known as cotyledons, with perhaps traces of more
leaves above them (Fig. 4). When we cut such a
young root or young shoot, we find that it is made up of

B

i8

BOTANY

a large number of very small pieces of living substance,
or protoplasm, each separated from its neighbours by a
thin membrane or cell wall which
surrounds it (Fig. 5). Very fine
connecting threads of protoplasm
extend through the cell walls and
so join the little pieces of proto-
plasm together, but these are so
delicate that it is not possible

t0 See the« With°ut Vefy Skilful

preparation.

The living substance thus extends throughout the
plant in complete continuity, though it is apparently
divided into a number of separate pieces by cell walls or
membranes. These serve at the outset only for pur-
poses of support, and form a kind of
skeleton. Each little piece of proto-
plasm contains a small highly organ-
ised portion called its nucleus; the
whole piece is called a protoplast ; it
is approximately cubical in shape and
has a diameter of about i-30Ooth of
an inch.

As the little protoplast absorbs
water and gets larger, entering into
active life, it finds itself in need of
constantly renewed supplies of water.
Here is its first individual difficulty,
for it is only the external cells
which can come into contact with
the water outside the plant. To
overcome this difficulty the proto-
plast gradually forms a central cavity
in its own substance, in which it
holds a store of water. This cavity is known as a
vacuole ; it is of the greatest importance in the

FIG. 5. Vegetable cells.
h, celr.wall; p, proto-
plasm; k, k, nucleus;
s, vacuole. X7oo.
(After Sachs.)

GERMINATION OF A SEED 19

maintenance of the life and the nutrition of the
protoplast.

As the plant gets older and larger a considerable
amount of differentiation of its internal substance
becomes necessary. This we shall study later. Mean-
time we may say that these changes are accompanied
by the death of some of the protoplasts. The mem-
branes or skeleton of these protoplasts are left in the
interior and subserve certain important purposes ; but
the protoplasts remain in full vigour towards the
exterior and particularly towards the extremities of
both shoots and roots, where new formation of them is
continually taking place.

The living substance is thus situated in greatest
amount towards the outside of the plant and at its
extremities, where its contact with the environment can
be most easily maintained. The subordinate mechanisms
of its life, which are concerned with its mechanical sup-
port and with the efficient working of its body and the
co-ordination of its various forces, are hidden away more
deeply in its interior.

CHAPTER II

THE EARLY DEVELOPMENT OF A PLANT — THE GERMINA-
TION OF A DICOTYLEDONOUS SEED

THERE is a great variety in degree of development
among the plants which exist upon the earth. The
most highly organised of these are the so-called flowering
plants, to which most of the terrestrial forms belong.

These plants have a certain feature in common which
distinguishes them from all others. They form seeds,
which become separated from the parent and after a
period of rest develop into new plants. A seed is essen-
tially a very young plant in a dormant or resting

20 BOTANY

condition, clothed with a separable protective coat, and
supplied with a certain quantity of food stored in it or
around it by the parent from which it came. In its
quiescent condition this young plant is called an embryo.
It consists of a young root and a young shoot, the latter
being composed of a stem on which are borne a certain
number of leaves. These parts are known as the
radicle and plumule respectively, the first-formed leaves
being called cotyledons. The number of cotyledons
varies ; in most cases there are two, in others one, while
in others again there may be several. The number of
cotyledons is constant throughout large groups of plants
and is associated with differences of structure of the
other parts of the plant. The first two groups referred
to are called Dicotyledons and Monocotyledons. In
another group called the Gymnosperms we find a
variable number, sometimes as many as fifteen.

The young embryo is fitted to bear separation from
the parent and transport to different situations by the
fact that its life is in a dormant state and that it is
protected by the skin or testa of the seed. Under
appropriate conditions it can resume active life and
grow into an adult plant, provision having been made
for its nutrition during the early stages of its develop-
ment and until it acquires the power of making its own
food. This necessary food is prepared by the parent
plant and is originally deposited as a relatively bulky
mass around the embryo in its early development in a
particular cell known as the embryo sac. This food con-
stitutes what is known as the endosperm, a collection
of cells which fill up all the space in the embryo sac
which is not occupied by the embryo.

The cells of the endosperm with their contents are all
provided for the nourishment of the embryo. In some
cases the embryo feeds upon this store while very
immature and before it assumes its quiescent state. In

GERMINATION OF A SEED

21

others its quiescence takes place very early, so that the
endosperm remains unabsorbed around it and is not
used till the resumption of active life and growth takes
place. The difference in the time of this absorption
influences the size of the embryo, which is naturally
much larger when it has absorbed the endosperm. The
food so absorbed is always deposited again in some part
of the young embryo, very frequently in the cotyledons
which become large and fleshy. Occasionally, as in the
Brazil nut, it is stored in the axis of the embryo. .

When the endosperm persists till the resumption of
life by the embryo — the process known as
the germination of the seed — the latter is
said to be an albuminous seed (Fig. 6). If
the embryo alone is present inside the skin
(Fig. 7) it is called exalbuminous.

It is best to begin the study of these
seedbearing plants with the largest group,
the Dicotyledons.

They furnish us with examples of both classes of
seeds which are easily accessible and which germinate
readily. We may take first the
common bean. To examine the
seed it is well to soak it for
several hours in water, which
is absorbed by the skin, so that
the whole seed swells and its

FIG. 7. Embryo of pea parts can be easily separated
magnified, r, radicle; nt, from one another. The seed is

plumule; c, cotyledons. . . . , .

somewhat kidney - shaped, and

bears on the concave part a scar at the point at which
it was attached to the fruit from which it came. A
little way from one end of this scar is found an
aperture through the skin, known as the micropyle,
through which the radicle emerges on germination. It
can be localised by gently squeezing the soaked seed,

a, embryo.

22 BOTANY

when a drop of water will ooze out of it. On removing
the testa the body of the seed is found to consist of
a very bulky embryo. The two cotyledons are large
masses placed face to face and easily separated from
each other. On gently moving them apart each is found
to be attached to a very short axis which lies between
them and is almost hidden when their faces are in con-
tact. The lower end of the axis is the radicle and is
bluntly pointed ; the upper end, the plumule, which curls
inwards between the cotyledons, bears two minute leaves.

We may compare with the bean a seed of about the
same size, that of the castor oil plant. It must be
soaked until it swells, when the hard coat it possesses
will crack. On removing the latter a fleshy mass will
be seen which cannot be separated into two portions
without splitting it. If it is divided into two it will be
iound that the embryo plant consists of two very thin
flat cotyledons lying in the centre face to face, with the
very short axis (plumule and radicle) between them.
The fleshy part of the seed surrounds the whole and
adheres firmly to the backs of the delicate cotyledons.
This mass is the endosperm, which has not been absorbed
by the embryo during its early growth.

If the seed is soaked in alcohol this dissection is easier,
as the parts do not then adhere so closely together.

After a period of variable length the embryo awakes
from its quiescent or resting state and develops into a
seedling, which goes on to become an adult plant. The
quickening into this renewed activity, which is techni-
cally called its germination, is only possible when the
external conditions become favourable. The process
demands moisture, a moderate degree of warmth, and
the presence of oxygen. It may be studied easily with
a little care, as it can take place in an ordinary room.
The -absence of light is not essential, although seeds are
usually buried in the soil before they germinate.

GERMINATION OF A SEED 23

Having soaked a bean for several hours till it has
become swollen, remove it from the water and keep it
on damp boiled sawdust or in some moist situation in
an ordinary room for some days. After a short time the
young radicle will be found to protrude from the micro-
pyle and to grow downwards. The cotyledons swell
and the testa cracks and begins to slip off. The plumule,
which was seen to be curved inwards, elongates ; the
curvature becomes more marked and forms a loop
which emerges from between the cotyledons ; it finally
straightens itself and thenceforward grows vertically
upwards. This loop is formed from the part just
above the cotyledons which is known as the epicotyl.
The cotyledons remain much as they were, but as the
seedling grows their contents are gradually absorbed
by the axis and they shrivel away. In their normal
development when the seed is below the surface of the
earth the cotyledons remain buried. The advantage of
the looped epicotyl is seen as it presses upward through
the layer of soil above the seed, for the delicate leaves
•of the plumule are saved from the injury which they
would suffer if they had to force their way through the
earth. The epicotyl in fact opens a passage for them.

Some seeds whose structure is almost identical with
that of the bean behave a little differently in germina-
tion. The part of the axis which elongates and brings
the plumule through the soil is a region a little below the
cotyledons, and it is consequently called the hypocotyl.
The lengthening of this part causes the cotyledons also
to be carried up into the air, and after a short time they
turn green, and take on the work of the foliage leaves
which are developed as the plumule grows.

When the castor oil seed germinates the early stages
are much the same as in those of the bean. The seed
swells and the radicle grows through the micropyle, and
very_soon the young root branches freely. The endo-

24 BOTANY

sperm swells and the flat cotyledons which remain in
contact with it begin to absorb the contents of its
cells. The face of the endosperm becomes very slimy
or mucilaginous and it continues to swell for some
days, ultimately cracking and being loosely attached
to the absorbing cotyledons. The hypocotyl grows up
in the form of a loop and drags the cotyledons out of
the soil with the endosperm clinging to them. They
very quickly change colour, becoming yellow and ulti-
mately green, and as the last traces of the endosperm
are used up they grow out laterally and take on the
appearance and the function of foliage leaves.

CHAPTER III

THE FORMATION OF THE ROOT SYSTEM

THE seeds just described are very useful for observing
also the growth and development of the seedling. Even
better material for this purpose is supplied by the seeds
of the common cress. If several of these seeds are
soaked in water and then scattered over the inside of a
damp flower pot they will germinate very freely if the
pot is kept moist and moderately warm, putting out
their roots in a few hours. As they will have been sown
quite indiscriminately, their positions will be irregular
and the young rootlets will emerge at first in very
different directions. If they are allowed to remain un-
disturbed as they elongate they invariably manage to-
direct their apices downwards, effecting sometimes
curious curvatures to do so. This strange uniformity
of behaviour suggests that the young seedling has a kind
of appreciation of its position or the direction of its
growth. We can test this suggestion by taking several
of them from the positions they have assumed and
placing them so that their roots are at different angles

THE FORMATION OF THE ROOT SYSTEM 25

with the vertical. So long as they are intact, they
gradually modify their growth so as to make their apices
again point vertically downwards (Fig. 8).

If we study the behaviour of the roots under various
conditions we soon find that they manifest other forms
of sensitiveness, all of which are brought to bear upon
the problem of establishing themselves in the soil.
When a root enters the latter and passes between the
particles which compose it, it must sooner or later
come into contact with some of them, and not improb-
ably such contact will hinder the advance of the root
in a straight or nearly straight
line. The growth of the root is
achieved by its advancing in a
kind of corkscrew fashion, the
tip describing a spiral rather
than a straight line. This no
doubt tends to push aside slight

obstacles which may meet the FIG. 8. Geotropic curvature in

advancing tip. If we experi- root and shoot of mustard.

ment upon a seedling bean, size'> (After

which we have seen can be culti-
vated in moist air, we can imitate the conditions met
with in the soil by attaching some small piece of a hard
substance to one side of the root tip, using a little gum
as the attaching medium. By this treatment we can
ensure that the contact shall be prolonged, and hence
the struggle between the root and the obstacle will be
carried to such a point as to exhibit very striking effects.
After a short time the growing region of the root, which
is some little distance behind the apex, will be observed
to curve in such a way as to turn the tip from the object
touching it. As the pressure is not removed under the
conditions of the experiment, this curvature will become
very pronounced and after a day or two the root will be
curled into a loop. In the soil so pronounced a curva-

26 BOTANY

ture is not met with, as a slight change in the direction
of growth causes the root to grow past the obstructing
body, and then the downward direction is resumed.

We can thus show that the young root has not
only an appreciation of direction, but it can in some
way recognise when it is in contact with some solid
obstacle and that it can modify its
growth with a view to getting past such
a body and penetrating further into
the soil.

The root further appreciates the in-
cidence of a lateral light. If the seed-
ling is cultivated in a glass vessel and
so placed that light reaches it only on
one side it very quickly modifies its
growth so that the apex becomes turned
away from the light. In the soil this
behaviour brings it closer to the par-
ticles of the soil, especially a little way
behind the tip. These three rudimen-
tary senses or sensitivities are supple-
™nted by a fourth. It shows an
evident appreciation of the presence
of moisture, and grows towards the dampest parts of the
medium in which it is placed.

If we revert for a moment to the young cress seedling
we find that when it has attained the length of about
half an inch a number of long delicate outgrowths of
its surface may be seen arranged in a broad band all
round the root at a little distance behind the apex (Fig.
9). So long as the root grows this band of outgrowths,
which are known as root hairs, is maintained. New ones
are formed on the side of the apex while the older ones
die and disappear on the hinder margin of the band.
As the root advances in the soil these hairs become so
closely attached to its particles that they cannot be

THE FORMATION OF THE ROOT SYSTEM 27

separated mechanically. While they thus aid materi-
ally in attaching the root to the soil, they carry on the
absorption of the water of the soil with the mineral
compounds dissolved therein.

It is customary to consider the influences we have
spoken of, gravitation, contact, light, and moisture, as
stimuli, and to speak of the behaviour of the root as
•response to stimulation. The power of receiving stimu-
lation indicates the possession of special sensitiveness,
and its response is to a large extent under the control
of the living root. The movements or alterations of
growth are purposeful, and lead us to look upon the
latter as a living sensitive organism engaged in the task
of making the best of its surroundings and varying its
behaviour as the surroundings change.

Seeing the very purposeful behaviour of the root we
may pause to ask what is the most potent factor in the
growth, or what is the determining influence which
causes it to penetrate the ground. The fact that
stability of position is secured strikes us at once, but it
is doubtful if this is the first consideration.

We may dismiss the responses to the stimulation of
light and contact. They are accessory to the effort of
the plant to come into close relatio'n with the soil, but
they by themselves do not minister to any of its needs.
The behaviour of the root suggests that it is seeking
something which the long experience of the race has
shown to be advantageous and which has now become
hereditary in the plant. The object of this search is
the water which the soil contains, which is present as
delicate films surrounding the particles of which it is
composed. Inherited experience has shown to the
vegetable organism that the soil is the source of water,
and its instinctive efforts are directed to the securing of
a position leading to an adequate supply.

The stimulus of gravity, therefore, or the perception

28

BOTANY

of direction, indicates to the root the whereabouts of the
water which it needs. The perception of water aids that
of direction and under normal conditions the two co-
operate. If, however, there be no water in the soil, the
inherited instincts of the plant lead it to penetrate even
the driest sand.

If the plant is in such a position that the two stimu-
lations do not co-operate, but are antagonistic to each
other, the chief instinct of the plant becomes evident,
and it can be shown that its great object is the coming
into relationship with water rather
than with soil.

If some seedlings are allowed to
grow on a sieve which is covered
by a layer of moss they will at the
outset put out their roots through
the holes of the sieve and grow
downwards in a normal way, seek-
ing as their inherited instincts tell
them the soil which should nor-
mally be situated below them. If
the sieve is suspended over a pan of water, so that moist
air is below the roots, they keep on growing downwards as
if growing into earth. But if the conditions be changed
after the roots have attained a length of, say, half an
inch, the air below them being made very dry by artificial
means, while the moss in the sieve is kept well wetted, the
roots soon curve upwards and growing in opposition to
gravity turn towards the water (Fig. 10). They appear
to recognise that their original instinct is deceiving
them and that the true habitat for them is for some
reason above and not below them. If after they have
established this new direction of growth the conditions
be again changed and moisture be restored to the air
below them while the moss is allowed to dry, another
reversal of the direction of growth takes place and

FIG. 10. Hvdrotropism
(After Gibson.)

THE FORMATION OF THE ROOT SYSTEM 29

again the position of the water determines this direction.

The behaviour of the root thus shows it to possess
certain tendencies which are based upon inheritance of
the accumulated experience of the race to which it
belongs, but which are controlled by certain sensitivities
which are its own personal possession. These sensi-
tivities are no doubt hereditary also.

The power of appreciating the influence of these
various stimulating influences has been found to be
confined to a very small region of the root, extending
about one-tenth of an inch from the apex. This region,
which may be called the root tip, may consequently
be regarded as a rudimentary sense organ. There
is, however, nothing in its structure to mark it off
from the region further back. The part receiving
the stimulus is not the part which becomes curved
in the act of responding. The latter is the region of
active growth, where the cells are undergoing elonga-
tion. The cells at the tip only retain their sensitiveness
for a short time. When new cells are formed in front of
them in the process of elongation these are found to be
sensitive, and the original ones, passing into the region
of active growth, lose the power of appreciating stimula-
tion. There is thus no permanent sense-organ in the
root. The protoplasm is sensitive at some particular
stage of its development, and, having passed that stage,
loses its power of appreciating these stimulating changes.

The way in which the stimulus received at the tip
causes a modification of the growth of the cells some
little distance farther back is not at present understood.
Something in the nature of a nervous impulse is thought
to be transmitted from the one region to the other,
passing along the delicate threads of protoplasm which
extend through the separating walls of the cells and put
all the cells in communication with one another.

We noticed in studying certain seedlings, especially

30 BOTANY

those of the castor oil plant, that the root does not
remain single but very speedily begins to give off
branches. By this process of branching a very large
root system is made possible. The main root of Di-
cotyledons usually persists and remains longer and
stronger than its branches. Such a main root is called
a tap root. The branches in turn branch and we get
roots of the second, third, and higher orders. If we
trace the formation of these roots, as we can do by
cultivating a seedling in water, or a dilute solution of
the necessary mineral compounds, we find that they
arise in constant succession as the main root grows,
the youngest thus being always nearest the growing
point of the main root. Each branch root has the same
appearance as the one from which it springs, and
similarly bears near its apex a band or zone of root
hairs. The branches orginate in the interior of the old
root and bore their way outwards. They arise in
definite positions, in relations to certain internal
structures which will be discussed a little later.

The branches are sensitive to the same stimuli as
the main root, but they respond rather differently to
the action of gravity. Instead of growing vertically
downwards, the first branches stand out nearly at right
angles to the main root and persist in growing in this
direction. The branches which in their turn they bear
do not grow in such definite positions, but extend
symmetrically round the one from which they spring.
If by accident the main root is killed, its place is taken
by one of its strongest branches, which alters its response
to gravity and grows vertically downwards.

By this course of development the root system of a
plant comes to occupy considerable space in the earth
and to fill the interstices of the latter very completely.
Two advantages are thus secured: a very firm grip of
the soil is secured by the attachment of the root hairs

THE STRUCTURE OF THE ROOT 31

of the numerous rootlets, spread through so much of
the earth, aided very conspicuously by the large net-
work which the branches form; and a very large area
of water-covered particles is tapped by the absorbing
root hairs the rootlets bear.

As the system gets older, not only is it continually
enlarged by the increased branching, but the individual
roots and branches increase in girth and press more and
more firmly into the soil. They penetrate very deeply
and extend laterally very widely, so that with the in-
creasing size of the above-ground portion of the plant a
firmer and firmer anchorage is afforded, securing the
needed stability.

CHAPTER IV

THE STRUCTURE OF THE ROOT

THE internal structure of the root can be properly
understood only when it is studied from the point of
view of the work which the root has to do. At its first
emergence from the seed its substance is composed of a
large number of the vegetable cells which we have
described, each a little mass of protoplasm separated
from its neighbours by delicate cell walls. They are in
close contact with each other at all points and have no
cavities in them. The chief difference in the mass is
that the external cells at the apex form a kind of cap
over the tip of the radicle, so that its actual apex is not
exposed. This cap protects the true apex from damage
as it penetrates into the soil. When the radicle has
begun to elongate changes in the cells are set up. If a
longitudinal section of it (Fig. n) is examined these
changes will be seen to separate the young root into
roughly three areas. The cap can be seen in front, a
short region behind it shows the cells small and actively

BOTANY

dividing, so increasing their number, and a longer part
still further back is marked by the enlargement of the

cells in all directions, but
most notably longitudinally,
while their vacuoles are being
formed. These regions are
known as the root cap, the
region of cell division, and
the region of cell growth.
Little more can be distin-
guished at this stage.

A little later, when the ex-
ternal band of root hairs
appears, preparation for the
discharge of particular duties
by the different parts begins
to be indicated, while the
requirements of the life of
the organ involve further
adaptations. The first of
these is the admission of air
to the interior to supply the
oxygen all living substance
needs to breathe. The com-
mencement of the formation
of an aerating mechanism
can be traced all through the
young embryo, even at this
age ; as seen in an older root
it consists of the splitting of
the cells apart from one
another at some point of
each, frequently at the angles
their walls make with each
other (Fig. 12). These little splittings make a number
of spaces between particular cells, and as growth goes on

FIG. ii. Longitudinal section of
young root. Xao.

THE STRUCTURE OF THE ROOT 33

these separate spaces become united, so that intercellular
passages run among the cells of every region, being of
different dimensions in different areas. As we shall see
later these passages become open to the exterior in the
upper portion of the plant and so enable air to enter and
circulate in the interior of the tissues.

enpeph x p x px

PIG. 12. Section of central part of root. In the outer region the cells
are separated in places by the intercellular spaces. en, endo-
dermis; pe, pericycle; ph, phloem strand; p, pith; x, xylem
strand; px, protoxylem. Xioo. (After Kny.)

A longitudinal section of the root taken at this age
will show that beside the longitudinal areas or regions
already remarked the internal tissue is beginning to be
differentiated in another direction. The section of the
root is almost conical, but the apex of the cone can be
divided into three layers, each of which is continued
backwards alone: the axis. At the apex each layer can

c

34 BOTANY

be recognised in the zone of cell division. The cells of
these layers can divide and they are called in conse-
quence meristematic layers. The outermost, which is
known as the dermatogen, forms the root cap, and ex-
tending backwards gives rise also to the outermost
layer of the root from which the root hairs grow. The
central one forms a more or less well-marked cylinder
or core, which is known in the meristematic region as
the plerome, while the intermediate one is called the
periblem, and forms the part of the root that lies
between the central cylinder and the external layer. As
we trace these further backwards we find that the
central cylinder becomes very clearly marked off from
the rest by a peculiar layer called the endodermis.

The root hairs are long slender outgrowths of the cells
of the outer layer, which when past the meristematic
region is known as the pili/erous layer, or epiblema.
Each hair has a thin wall of cellulose, which is brought
into close contact with particles of soil as it grows in
among them. On coming into contact with these
particles the outer layers of its walls become changed
into a kind of mucilage, which makes the hair adhere
very closely to the soil. The film of water which sur-
rounds the particles is then absorbed by the root hair.
As there are enormous numbers of these hairs on the
young root, there is soon a great increase in the water
which the root contains. This water passes on from
the hairs into the second region of the root, now called
cortex instead of periblem, and gradually makes its cells
extremely swollen or turgid thereby.

The special mechanism for carrying this water
from the root to the upper parts of the plant is by this
time beginning to appear. It lies in the central region,
now partly shut off from the rest by the endodermis.
Here the growth of the cells is such as to cause them
to become elongated. Certain special areas of these

THE STRUCTURE OF THE ROOT

35

elongated cells form a definite number of columns of
cells which can be traced separately upwards. They
are fitted especially to transport water by changes in
the constitution of their cell walls, which become
gradually changed from cellulose to lignin, the latter
enabling water to pass through it in all directions with
great ease. At the same time the horizontal walls of

st st h TV C'

FIG. 13. Longitudinal section through a vascular bundle of a stem.
5, s', p, p. different types of wood vessel; w, wood fibres; st, sieve
tubes; ph, bast fibres; p', pith; c, cambium.

these cells in great part disappear, so that the columns
of cells become changed into hollow tubes, or vessels,
while their side walls are irregularly thickened by the
deposit of more cell-wall substance upon them in
particular areas. On account of the presence of these
vessels, the collections are known as vascular strands or
vascular bundles (Fig. 13). In the root they are com-
posed, entirely of lignified cells and are therefore called
wood or xylem bundles, to distinguish them from other
vascular strands lying near them. The number of these

36 BOTANY

strands varies in' different roots ; it is very common to
find four, but two is not an infrequent number. They
may extend completely to the centre and all unite there
to form a solid cylinder. If the number is large they
.generally fuse together before extending so far, leaving
a small-celled column as a core. This is known as a
-pith. In form the bundles are wedge-shaped, the apex
of the wedge pointing outwards.

If we trace these conducting strands towards the tip
•of the root they can be distinguished among the soft
•cells of the plerome by their narrow diameters and their
tendency to elongation. The area of each embryonic
strand can be seen distinctly in a transverse section,
their small size and a certain density of their protoplasm
marking them off from their neighbours. The gradual
change from these cells to the mature forms can be
traced; the alteration of the wall and its thickening
appear first along the outer edge of the wedge, known
consequently as the protoxylem, and extending thence
towards the centre of the root.

If these vascular bundles are traced along the root in
the direction opposite to the tip they are seen to be
continuous with similar structures in the stem. In this
way a path is made throughout the plant for the trans-
port of the water after its absorption.

These strands are chiefly concerned with the func-
tion of the root. Others which also are traceable
throughout the plant can be seen to lie one between
each pair of them in the central cylinder. These are
chiefly concerned in the nutrition of the root. They
are equally well defined and lie side by side with the
wood strands, separated from them by a few packing
cells. They differ in texture, their walls remaining
cellulose. They are known as bast or phloem ; and are
made up of vessels known as sieve tubes from . their
terminal walls being somewhat thickened and perforated

THE STRUCTURE OF THE ROOT

37

by a number of holes, so that their protoplasm is con-
tinuous (Fig. 13). With the sieve tubes are a certain
number of slightly elongated cells of the ordinary type.
The bast and wood strands are thus seen to occupy,
with a little supporting tissue, almost the whole central
cylinder of the root (Fig. 14). There is always an outer

FIG. 14. Section of central part of root, b, bast strands; w, wood
bundles. Xioo. (After Kny.)

continuous sheath over the whole, one cell thick as a
rule, which is called the pericycle. Outside the peri-
cycle comes the endodermis.

The endodermis forms a sheath, one cell thick, round
the central cylinder. Its walls in some cases become
uniformly thickened and lignified. In others the
outer and inner walls remain thin, while the side walls
become changed in a different way. The cellulose is
replaced by another material which resists rthe passage

38 BOTANY

of water through it, so that the water of the cortex can
pass directly to the wood strands, but cannot pass from
one endodermal cell to another, being prevented by
bands of a cuticularised substance that pass round the
radial walls (Fig. 15). By their interlocking together
they make the endodermis separate the intercellular
passages of the cortex from those of the cylinder, so that
air cannot penetrate directly to the latter.

As the root grows older and larger and the upper part
or shoot system of the plant develops to a corresponding
extent, this primary structure becomes insufficient for

its requirements. They call
for a greater amount of
conducting tissue as the
branches and leaves of the
shoot multiply, for all the
latter need a supply of
water. The stability of
the whole structure needs

pericycie. ° strengthening, in view of the

greater size being acquired

above ground. There is, as we have seen, a great growth
in thickness of the root and the development of a
system of branches, each behaving like the parent root.
In the stage we have examined the young root shows
no provision for this increase of thickness. It can take
place only by the formation of new cells, and such forma-
tion is not going on except at the apical meristem. A
new departure has accordingly to be made (Fig. 16).
It begins by a curved band of cells of the supporting
tissue lying in front of each strand of bast becoming
meristematic, beginning to divide by walls which are
parallel in direction with the circumference of the root.
These tangential divisions cause the formation of several
rows of cells, one of which, the nearest to the bast,
retains the power of division and is called cambium.

THE STRUCTURE OF THE ROOT

39

The newly formed cells become converted into wood,
so that a strand of wood, called secondary wood, is
formed inside each bast bundle. The cambium layer

FIG. 1 6. Thickening of root ; px, primary wood ; .SAT, secondary
wood; c, cambium; en, endodermis. X8o. (After Kny.)

extends laterally round the bast bundle, so that it tends
to pass up towards the outer edge of the wood bundle on
each side. By the time a little mass of wood has thus
been formed between each bast strand and the centre
of the root, the cells of the pericycle outside the
wood bundles divide by similar tangential walls, so

40 BOTANY

that the pericycle at these points becomes several cells
thick. The innermost of these cells, lying in contact
with the protoxylem, become cambium, and soon extend
to unite the two bands of cambium approaching them
from the two bast strands between which the bundle of
wood is lying, so that a complete ring of cambium is
formed. At first it is necessarily sinuous or wavy, but
as more and more wood is formed inside the bast masses
it is pushed further and further outwards there, till the
waviness of the ring disappears. This cambium ring
then continues to add more and more wood in the same
way to the secondary wood already formed. Behind
the protoxylem groups, which form the outer edge of
the primary wood bundles, no secondary wood is formed,
but only rows of thin-walled cells; consequently the
secondary wood is divided into separate masses by
these rows of cells, which are known as medullary rays.
They are formed with a view to the transport of food
substances from the bast into the interior of the wood.

The cambium produces a little secondary bast out-
side the ring in the same way as it forms wood inside it,
but the quantity of bast is much less than that of wood.
This is natural, as the bast has only to provide a path
of transit for the actual food of the root cells, while the
wood has to furnish a continually increasing amount of
water-transporting tissue.

This woody formation in the centre of the root is dis-
posed very advantageously for maintaining its stability.
A structure with a hard central core is the most suitable
to resist such a vertical pull as would cause uprooting.
This vertical pull is continually being made by the
movement of the storm-tossed upper region of such a
structure as a tree.

The young root as it increases in thickness in the soil
encounters two dangers, one internal, the other external.
The process of thickening compresses very severely

THE STRUCTURE OF THE ROOT 41

its more external layers and in time ruptures them-
The pressure of wet soil against its epiblema is not
unlikely to set up decay. The cortical tissues and the
epiblema are therefore inadequate to protect the gradu-
ally thickening central cylinder. But these difficulties
become obviated as the growth proceeds. By the time
the central cylinder has become only slightly thickened
the zone of the root hairs has been removed to some
distance in advance, by the continuous elongation of the
root. The cortex of the thickened part is consequently
not supplied with water as before, and ceases to play its
original part in transporting the water upwards. The
hairs having disappeared from that region too, the
epiblema has not its first importance there. The pres-
sure of the gradually increasing girth stimulates the
cells of the pericycle and they again show the power of
increasing by tangential divisions. The pericycle be-
comes uniformly several cells thick, one layer of which
remains meristematic. It cuts off repeatedly bands or
shells of cells which remain very regular in shape,,
appearing in transverse sections like rows of bricks.
The outermost ones lose their contents and their walls
are transformed into suberin, a substance closely re-
sembling the cuticularised material of the endodermis.
This band of cells forms what is known as a cork layer.
It extends completely round the root and forms a
strongly protecting sheath. It is perforated here and
there by little rounded masses of cells loosely arranged1
so that air can pass between them. These are known as
lenticels ; they serve to admit air to the interior of the
root. It is quite impervious to water except at these
spots, and hence preserves the root from loss of water by
outward leakage. The cells of the cortex and epiblema
may now rot away without causing any damage to the
root. The latter acquires, in fact, a fresh exterior of
a more resistant and permanent character than the

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PIG. 17. Transverse section of root to show
a rootlet at two stages of development.
rh, root hairs ; ec, cortex ; d, cells in pro-
cess of absorption; en, endodermis; pe,
pericycle; co, conjunctive tissue; ph,
bast ; g, cambium ; x, wood ; c, derma-
togen of rootlet ; p, its periblem ; pi, its
plerome. (After Scott.)

original one. This
corky formation con-
tinues as long as the
root lives and adapts
itself to its increasing
girth. Its outer part
is composed of dead
cells, and together
with the remains of
the layers originally
outside it, constitutes
the bark of the root.
The cortex and epi-
blema continue for a
very short time, so
that in an old root the
bark consists of peri-
cycle tissue and layers
of cork.

We must again re-
turn to the young root
to trace the manner
of formation of its
branches. The latter
originate when it is
quite young, as we
have seen already.
They arise in the peri-
cycle, in very many
cases opposite to the
protoxylem of each
wood bundle, gener-
ally before the strands
are lignified through-
out. There are con-
sequently usually as

THE SHOOT 43

many rows of lateral roots as there are wood strands.
A little group of the cells become marked out by be-
coming meristematic, and dividing chiefly by tangential
walls, so that soon a little mass seems to be growing
outwards. It can shortly afterwards be seen to have a
central plerome covered by a periblem and dermatogen,
which behave just like those of the parent root. The
cells of the cortex which lie in front of the new root
branch are gradually digested and eaten by the latter as
it grows outwards and finally penetrates to the exterior
(Fig. 17).

The cells of the root cap are continually being worn
away by contact with the soil. The cap is added to all
the while by the dermatogen behind it.

CHAPTER V

THE CHARACTERISTIC FEATURES OF THE SHOOT

THE work which falls upon the shoot portion of the
plant is very different from that discharged by the roots,
being very largely the construction of the organic sub-
stance which serves as food, not only for the plant itself
but for the world in general. To understand this con-
struction we must consider the absorption of carbon
dioxide, the utilisation of certain amounts of the water
and mineral constituents furnished by the roots, and
the evaporation of the surplus water. The work in-
volves certain minor or subordinate duties connected
with the distribution of the food after its formation.

The important questions of the breathing of the plant
and the maintenance of a suitable temperature in its
different parts must also engage our attention.

The form and composition of the shoot need careful
study from these points of view, but these are not all.
The relation of its structure, internal as well as external,

44 BOTANY

to its stationary position, and the difficulties and
dangers which the latter presents, must be considered.
The adaptations which it shows and the changes of
climate which it meets are of great importance.
Finally, we have the relation of the shoot system to the
processes of reproduction.

When the young shoot has emerged from the seed and
made its way into the air in the ways already described,
the bent or hooked form gradually changes till an up-
right position is attained. We have already examined
the behaviour of the young root, noting its perception of
direction and its modification of its growth if necessary,
till it can make its way vertically downwards. The
same appreciation of direction is exhibited by the young
shoot and its behaviour is very similar, with the im-
portant difference, however, that it seeks the light and
air and hence grows vertically upwards. We cannot
explain this difference except by recognising the pur-
poseful character of its response to the influence of
gravity. There is no difference in the growing cells, so-
far as we can see, for they have all practically the same
structure whether they are in root or shoot. We see in
this behaviour really a living organism trying in a.
limited way to make the best of the circumstances in
which it finds itself. As we continue to study it we
shall be able to ascertain that it possesses the same
sensitivities and powers of response to changes in its
surroundings that we have found exhibited by the root.

The growth of the shoot, however, is a much more
complicated process than that of the root, in conse-
quence of its more manifold duties, which have called
for a more complicated structure.

The young plumule when it has emerged from the
seed coats consists of a very delicate axis, at the apex
of which a number of minute outgrowths are to be seen.
These are folded in various ways, the outermost covering

THE SHOOT 45

those internal to them. Their number is not uniform,
nor is their method of folding, nor their arrangement,
but they all arch over the apex of the shoot. The
latter does not bear any protective cap, such as is seen
over the root. It is a delicate conical tip, which bears
its outgrowths in regular succession, the latter being
continually developed by the apex as it elongates, so
that the youngest are always nearest to the tip.

These outgrowths are borne upon the axis at definite
points, which show a remarkable difference of behaviour
from the spaces between them, in that they do not
elongate during the processes of growth. All the growth
in length is carried out by these spaces. The points at
which the outgrowths are borne are called nodes, the
spaces between them internodes.

The behaviour of these parts can be studied advan-
tageously on a shoot a little older than the plumule.

It is well to select a tree of some few years' growth and
to examine some of the ultimate endings of its branches.
If from such a tree in the early spring we take a twig
we shall be able to observe that during the previous
summer its internodes elongated, causing the out-
growths to be separated from each other by some little
distance. The year's growth may have caused the shoot
to become perhaps three or four inches long. If we
examine the nodes closely we shall find that between
the original outgrowths and the axis certain small knob-
like bodies occur, almost hidden between the others.
These several parts can always be observed with greater
or less facility on all shoots. The axis is called the
stem, the first-formed outgrowths are the leaves, and the
little knob-like bodies between the two are known as
buds. The angle between the stem and its leaf in which
the bud arises is the axil of the leaf. The apex of the
stem will be seen in the spring to exhibit also the form
of a bud, rather larger than the lateral ones in the axils

46 BOTANY

of the leaves. The plumule is really the first bud of the
seedling, and it shows fundamentally the same structure
as the others appearing later on the stem.

As the seasons of the year in our climate render
growth intermittent, confined to little more than half
the time, and as the growing shoots are exposed to very
unfavourable conditions during the remainder, it is easy
to understand that special precautions are called for,

A B

FIG. 1 8. Buds of lilac. A, shows the external appearance;
B, a slightly magnified section ; C, the bud-scales are reflexed
and the leafy shoot has begun to elongate. (After Marshall
Ward.)

that they may develop. If we cut a longitudinal
section through one of these buds in the spring before
growth is resumed we shall find evidence of such (Fig.
1 8 B). The delicate growing cone in the centre will be
found to be surrounded by a varying number of leaves,
each of which arches over it and is in turn arched over
by the next one external to it. The most internal ones
are extremely delicate and almost unformed, while the
cone itself if magnified will be seen in many cases to bear
upon its surface small swellings which indicate that
other leaves are in course of formation there. Over

THE SHOOT 47

these delicate leaves are others more sturdy, while the
exterior ones are frequently quite dry and hard and in
many cases covered over by a sticky substance. Some
of those in the interior are in many cases covered with
thick coatings of hairs, forming a downy pad of material
calculated by its non-conductivity to keep out the cold.

If the bud is small, it will be found to contain only a
few leaves, perhaps only two or three; even in this
case, however, the general arrangements are the same.

If we compare the apices of stem and root, we see
how the surroundings in each case have influenced the
structure. The root apex is specially protected against
damage from contact with hard or rough materials while
penetrating through the soil ; the stem is exposed to no
such danger, but shows a careful protection from frost
and wet, and undue evaporation.

The young leaves are thus merely flattened boat-
shaped expansions curling over the apex of the stem.
Later, when their protective powers are no longer
called for, their adult forms are assumed.

The leaves bring about their curving over the apex of
the stem in the bud by an irregularity of growth. When
the little swelling first appears on the growing cone it
is itself rounded or conical; it soon becomes laterally
flattened and for a time, so long as it is in the bud, its
under surface grows faster than its upper one, so that it
is made to curve forwards. When it escapes from the
bud later it reverses this distribution of growth and
grows more rapidly on its upper face, so becoming flat.

Buds always terminate the ends of normal growing
shoots ; indeed the bud-form is always assumed by the
apex of the shoot as soon as its growth is suspended by
unfavourable conditions. The buds which appear in
the axils of the leaves lower down on the stem are the
commencements of the secondary shoots or branches,
which will elongate in due course.

48 BOTANY

In many cases the bud is the foreshadowing of the
growth of the stem or branch of the next year. It has
"been formed by the shoot as its last effort for the year,
and its development during the succeeding year will
only involve the elongation of the internodes, the
assumption of the adult forms of the leaves, and the
preparation of the buds for the year following. In
other cases it is not so simple. During the growing
period more leaves will be produced than the bud in its
resting state exhibits, and growth will be prolonged
accordingly. But even in these cases as soon as growth
in length stops, the development of another terminal
bud with its potentialities can be noticed.

The growth of the shoot thus shows considerable
•differences from that of the root. In the case of the
latter it is not at all easy to say what are the limits of
the year's elongation, while in that of the shoot they
may be fairly accurately determined.

When the next growing season sets in, the bud begins
to swell owing to the upward pressure of the elongating
axis. The outer leaves are loosened and pressed apart,
•so that the bud bursts open at the apex. When the
external leaves are hard scales they are generally cast
off entirety, and the internal leaves emerge. The
^elongation of the several internodes rapidly follows and
the shoot takes on its proper form.

As this change proceeds certain other facts can be
determined. The external scales have no buds in their
axils, nor do all the leaves develop into foliage leaves.
The external ones, and often some just internal to them,
do not change their form, and frequently only persist
lor a short time, soon falling away. All these are
classed together as bud-scales ; they really represent
only the bases of leaves (Fig. 18 C).

As growth goes on other differences appear. The
internodes between the bud-scales do not elongate, so

THE SHOOT

49

that while the scales persist the young shoot seems sur-
rounded by a number of small leaves at its base (Fig.
1 8 C). When the bud-scales fall off, they leave the base
of the shoot surrounded by scars, which mark the places
of their original attachment. At the close of growth on the
onset of winter, the
shoot, now become
what is technically
called a twig (Fig.
19), shows these
scars closely placed
together round its
base. In the winter
it is easy to recog-
nise the amount of
growth of a twig
during the preced-
ing year, by noting
the distance be-
tween this collec-
tion of scars and
the apex. In an
older twig or young
branch several
such collections of
scars can be de-
tected, and so the
limits of each year's growth can be easily ascertained.
With the opening of the bud and the expansion of its
leaves as its stem elongates we can trace the sensitive-
ness of the shoot to the various influences that sur-
round it. We have pointed out the response its axis
makes to the influence of gravity and lateral light. We
have also incidently mentioned the change of the cur-
vature of the leaves which sets in as soon as the bud
opens. The change is a response to the access of light

D

FIG. 19. Twig of 3 years' growth, bs, scars
of bud-scales of each year. The twig shows
racemose branching. (After Ward.)

50 BOTANY

which accompanies this opening. The water in the cells
of the leaf was in the early stages of development dis-
tributed mainly to those of the under side, making them
most turgid and causing them to grow most freely.
The access of light disturbs this relationship and the
cells of the upper side become most turgid; the conse-
quent growth causes them to lose the concavity of their
upper sides; they become flat or sometimes slightly
curved in the other direction.

If light is not allowed access to them this growth of
the upper side is very much interfered with, and the
leaves show but little change of curvature, lying close
to the stem as it elongates, and in some cases not
becoming even flat.

CHAPTER VI

THE CONSTRUCTION OF THE SHOOT SYSTEM

THE behaviour of the plumule in elongating and ex-
panding gives rise to the primary shoot. Every succes-
sive bud and branch which spring from it increases its
dimensions by multiplying the number of twigs it bears.
As the number of such buds upon each twig is fairly
large, we see that the young branches increase in a kind
of geometrical progression, causing the formation of a
large shoot system, which constitutes the body of a
shrub or the head of a tree. We must next study the
construction of such a head.

To understand it we must inquire what are the
purposes for which it exists, and what are the dangers
against which it must protect itself.

We have already drawn attention to the fact that the
functions of the root and the shoot are fundamentally
different. That being so, it seems clear that the mode
of arrangement of the parts of the one need form no

CONSTRUCTION OF THE SHOOT SYSTEM 51

rule for the other. There is nevertheless a general
agreement between the two, though careful observa-
tion shows that similarity of arrangement subserves
very different purposes. The arrangements of the shoot
all bear a certain relationship to life in air and its conse-
quent requirements, and show further a co-ordination
with the needs which are cared for by the roots.

We have seen that one of the primary objects of the
latter is to secure a firm anchorage for the plant that it
may be able to maintain its erect position. The de-
velopment of a large head or upper part makes against
such anchorage, by offering a large area to the pressure
of wind and the beating of rain — forces likely to lead
to uprooting from the soil.

We may ask why such a risk should be undertaken —
why the sub-aerial portion of the plant need attain the
large dimensions it possesses. What are the advantages
which are afforded by a widespreading head rearing
itself into the air? Are they commensurate with the
risk, and what are the precautions which protect the
plant in face of the dangers it involves ?

In seeking answers to these questions we must look a
little more closely at the chief features of the upper
portions of the shoot system. We soon see that one of
the objects secured by the method of development which
it follows is the great amount of surface in proportion to
bulk which the shoot presents. The twigs are thin, the
leaves flat. We have indeed, as we have in the root,
and as we notice in the case of the large seaweeds, the
bringing of the structure of the plant into relationship
with as large a portion of the environment as possible.
Here is clearly an indication or suggestion of an inter-
change of material between the two.

We have already assumed that there is such an inter-
change, and may now examine more closely its nature.
A few simple observations will enable us to prove it.

52 BOTANY

Let us remove a twig with its expanded leaves to a con-
fined space, so that we examine the conditions of the air
around it to see what changes, if any, take place. Let
us shut it up in a well-dried bottle and keep it at its
accustomed temperature. We shall find after a short
time that the sides of the bottle become bedewed with
moisture, and a little later we shall see that the leaves
upon the twig and at least its upper part become wilted
and drooping. Part of the work of the shoot is clearly
to exhale watery vapour from its surface.

FIG. 20. Section of leaf showing intercellular spaces and stomata.
The cells contain chloroplasts. X8o.

If careful measurements are made of the total water
a plant gives off, it is found to be very considerable in
amount, and to be given off during the whole of the day
in quantities varying with the changing conditions sur-
rounding the plant. The structure of the leaf, to which
we must give later some careful attention, shows us
that the intercellular spaces which we observed in the
body of the root exist in even greater degree in the leaf
(Fig. 20) and yield an evaporating surface much larger
than the external surface of the twigs and leaves.
These internal channels communicate with the exterior
through small openings in the limiting membrane of the

CONSTRUCTION OF THE SHOOT SYSTEM 53

leaves and the more delicate parts of the twig. These
openings, which are known as stomata, are themselves
co-ordinated with the regulation of this exhalation of
vapour, the width of the opening being capable of
variation according to different conditions. We must
associate the evaporation of so much vapour by the
leaves with the very large absorption of water we
observed in the root, and we can see that the structure
of the leaf is as well adapted to evaporation as that of
the root is to absorption. Further structural adapta-
tions to this maintenance of a stream of water through
the plant will become evident later, but in the mean-
time we can see in the features already alluded to a
definite relation to this particular interchange between
the plant and its surroundings or environment.

Still pursuing our inquiry, we may notice that while
the general colour of the shoot is green, the depth of the
green tint is not uniform. The flattened parts or leaves
are of a brighter green than the cylindrical axes, and in
general it soon appears that the more exposed any part
of the shoot is in its young and most delicate condition,
the more prominent the green colour becomes. We
have consequently a suggestion of some co-ordination
between exposure and colour. Comparing two shoots
growing in different places we can soon associate the
optimum brightness with the best illumination, and we
are led to infer that one reason for the flatness of certain
parts of the shoot is the desirability of exposing as much
of their surface as possible to light.

We have already called attention to the fact that the
greater part of the plant's food is manufactured in the
leaves, and that the green colouring matter — chlorophyll
— is chiefly concerned in making it. The chlorophyll is
not diffused throughout the living substance, but is
confined to a number of small ovoid bodies which are
embedded in it, and these green bodies are placed very

54 BOTANY

little below the surface of the leaves, being thus covered
only by a thin transparent layer of cells. The dis-
tribution of these green bodies, which are known as
chloroplasts, so bears a very definite relation to the
incidence of the light, and suggests to us that while one
duty of the leaf is to exhale watery vapour, another
is to secure the illumination of a definite part of its
mechanism, which is concerned with the most intimate
questions of nutrition.

As the two functions thus suggested are found upon
further inquiry to be intimately bound up with the well-
being of the plant, we must examine them a little more
closely before looking for the ways in which they in-
fluence the form and position of the shoot system.

There are two reasons for the copious evaporation of
water which we have pointed out. The first is con-
nected with the problem of feeding, as we noticed in
our introductory chapter. Certain constituents, either
entering into the composition of the food itself or
necessary factors in its construction, are only to be
found in the soil and are procured therefrom by the
roots. These compounds are absorbed from the soil in
solution in the water entering the root hairs, and the
solutions are necessarily very dilute to facilitate their
passage through the living substance of the hairs. As
with a rapidly-growing plant continuously increasing
quantities of these substances are needed for nutritive
purposes, it follows that large quantities of the solution
must be absorbed. In the plant these mineral com-
pounds are taken from the water, and the great bulk of
the latter is evaporated into the intercellular passages
and the vapour subsequently passed out of the stomata.
Hence, speaking broadly, the more water that is taken
up and subsequently evaporated, the more mineral
matter is secured for the use of the organism.

But there is another and equally important function

CONSTRUCTION OF THE SHOOT SYSTEM 55

that this evaporation discharges. In the time of sun-,
shine a great deal of the sun's energy in the form of heat
and light is falling on the plant. It has been computed
that the amount is so great that it would raise its
temperature to such a dangerous extent that if no
counter-influence were at work it must speedily perish.
Now the evaporation of water always requires the
expenditure of a considerable amount of heat, and we
find that the greater part of the heat reaching the
plant from the sun is devoted to the vaporisation of
the water in the intercellular passages of the leaves and
other parts. The normal temperature of the plant is
thus maintained in the face of the enormous absorption
of solar heat which its exposed and often unprotected
position renders inevitable.

The study of the behaviour of the chloroplasts shows
us that their position is definitely associated with the
duty which we have attributed to them. Not only is
their colour dependent on their exposure to light, but
the part they play in the construction of food is equally
related to the illumination they receive. We have
already spoken of the work done by the chloroplasts,
and have seen that they construct organic food in the
form of sugar and .similar compounds from the carbon
dioxide of the air, together with a portion of the water
supplied them by the root. Carbon dioxide is present
in very small proportion in the air, only some 3 or 4
parts in 10,000. The construction of food from such
antecedents is only possible in the presence of light;
two things therefore must be secured — a wide surface
and preferably a copiously subdivided one, to bring as
much air as possible into contact with it, and as com-
plete an exposure as possible of the chloroplasts to
light to enable the construction of the sugar to go on.

The form and disposition of the shoot system must
be regarded from the point of view of these require-

56 BOTANY

ments. True, at first sight they seem a little antagonistic
to other needs. The evaporation of the water and the
illumination of the chloroplasts demand a large and
increasing shoot-body, but its increase in size brings
with it a distinct danger to the stability which we have
seen is one of the first necessities of the plant as a whole.
The reconciling of these demands must add to the
interest with which we study the form and distribution
of the members of the shoot system.

We have seen that the axis of the latter is very much
subdivided, the ultimate divisions, the branches, taper-
ing to points, in some cases extremely gradually, in
others more abruptly. These cylindrical or conical
divisions bear a number of flattened organs, the leaves,
which are usually attached to the axis by flexible stalks
or particles. We can now see the reason for this sub-
divided conformation. It secures strength by the
cylindrical form of the twigs, surface by the flattened
form of the leaves. The winds can blow freely through
the mass of twigs, while the long leaf stalks allow of
sufficient displacement of the flattened parts when the
pressure of the wind is brought to bear upon them.
Moreover, the parts concerned are all extremely flexible
and elastic, so that they can yield to pressure and
regain their positions as soon as it is removed.

The form of the shoot system of a plant will depend
upon the manner of its branching, and the number, size,
and arrangement of the leaves its branches bear.

The branching will be affected by two main factors :
firstly, the number of branches produced at a node ;
secondly, the relative degree of growth of each main
branch and those to which it gives rise.

The first of these does not show as much variety as
might be expected. Usually one branch, not infre-
quently two, appear at a node, but seldom more.

The second factor, however, plays a much more

CONSTRUCTION OF THE SHOOT SYSTEM 57

prominent part in the construction of the head. If the
first axis grows more vigorously than its branches — a
behaviour we found to lead to the formation of a tap
root in the root system — and if each of the branches in
turn is longer and stronger than those arising from it,
the ultimate form of the head is pyramidal, for the
successive branches arise nearer and nearer to the apex,
and so long as the growth is regular the lowest will be
generally the most widespreading. This is true of the
series of branches which each of them bears. This type
of branching is said to be indefinite or racemose (Fig. 19),
and it is illustrated by such trees as the spruce fir.

If, on the other hand, the growth of each axis or
branch is soon checked and so its development becomes
exceeded by the growth of the daughter-axes to which
it gives rise, the head will be sub-globular or rounded.
The exact shape, however, will largely depend on the
number of branches springing from below the apex of
each in turn, for these all arise at the same node. They
are therefore on the same level, and do not grow in
what is called acropetal succession, as in the first case.
A very common form is that in which each branch is
solitary. This form of branching is called definite or
cymose. Examples are afforded by the elms, oaks, and
many other forest trees (Fig. 21).

Another factor in the shape which the branching
helps to give to the shoot system is the non-develop-
ment of some of the buds. We have seen that a bud is
produced in the axil of every foliage leaf. It often
happens, however, that a twig cannot adequately feed
all the buds it bears. Hence some perish and others
remain dormant for some time, circumstances which
cause a good deal of irregularity.

Before we study the influence of the arrangement of
the leaves upon the form of the shoot and the shoot
system we must look a little more closely at the peculi-

58 BOTANY

arities of their flattened form. We have seen wherein
lie its advantages, but we must consider also the diffi-
culties and even dangers which it involves. Difficulties
arise from the certainty that the leaves must encounter
rough weather in the course of the year. Rain may
soak them through, wind may tear them apart, or even
strip them from the twigs. How are these perils met ?

FIG. 21. Diagram of forms of cymose branching.

There are two reasons why rain falling upon them
does not affect them seriously for a long time. Gener-
ally the shape of each is such that there is a longitudinal
groove all along its upper surface, running from apex to
base in the centre of the flattened blades. This con-
ducts the water away as fast as it falls upon the leaf,
either towards the apex or towards the base. In the
latter case the groove is continued along the leaf stalk
so that the water is taken to the ground. The second
reason is that the outer layers of the walls of the cells

CONSTRUCTION OF THE SHOOT SYSTEM 59

FIG. 22. Venation of
leaf.

of the upper surface become almost impermeable by
water. It is only after long soaking, therefore, that
any can find entrance.

The danger from wind is perhaps greater than that
from rain. The leaf-blade, however,
though delicate and thin, is never-
theless very strong and not easily
torn. Running through it are the
ultimate endings of the vascular
strands we have already noted in the
root, the conducting tissue (Fig. 22).
These strands form the so-called veins
of the leaf and they constitute a net-
work of very tough fibrous bands
upon which the delicate tearable
tissue is supported. They generally
strengthen particularly the margins
and the apex of the leaf-blade and protect it from being
torn. The blade, therefore, when acted on by wind is
seldom either bent or curled, but is made to play as a
single rigid piece moving up and down without losing its
flatness for a moment.

The danger of stripping from the twig is dealt
with differently. When the plant is of a sturdy, rigid
habit, the leaves are usually attached to the stem very
strongly, and are bent upwards so that the direction
of the wind must drive them towards the stem, and its
force cannot be felt between the latter and the leaf's
upper surface. More frequently, however, we find that
the blade of the leaf is attached to the twig by means of
a tough, flexible stalk, capable of movement in almost
every direction on its point of attachment. The elasti-
city is so great and so readily called into play that with
even the lightest breezes the leaves of most trees are
seen to swing to and fro with the greatest freedom.

The form of the head of the tree is influenced by the

6o

BOTANY

shape as well as the arrangement of the leaves. Usually
leaves consist of three regions, a flattened part or blade,
a leaf-stalk or petiole, and a leaf base by which it is
attached to the stem. If we regard it as an outgrowth
from the stem, we find that it assumes
its flattened form by developing a wing
on each side, the outgrowth itself also
becoming flat. If the outgrowth
branches and only its branches develop
wings we have what is commonly
termed a compound leaf (Fig. 23).

The leaf-stalk is the lower part of the
axis of the leaf and it is continued
forwards to the tip, the part which has
become winged being called the mid-rib.
In some cases the whole of the axis of
the leaf becomes winged. The leaf is
said then to be sessile or to have no

stalk> M ^ bage of the kaf are yery

frequently two small outgrowths, of the
nature of leaf branches. These are known as stipules.
They vary a good deal in shape and size.

The object aimed at in the distribution of leaves on a
tree is the covering of the framework of its head as com-
pletely as possible by a thin curtain of leaves, as free
from unoccupied gaps as possible ; the leaves themselves
must be so arranged that little shading of one part by
another shall occur. If we stand under a tree and look
up through its branches we find the leaves are not dis-
tributed all about the interior of the space occupied by
the boughs and branches ; they are seen to be a more or
less complete covering to the head. In a humbler type,
such as a thistle or a sunflower, the leaves overlap very
little, so that practically the whole leaf-surface is ex-
posed to the light during at any rate some part of the day.

The leaves are arranged in various ways upon the

FlG' ^Compound

CONSTRUCTION OF THE SHOOT SYSTEM 6r

stem, but always occur in vertical or nearly vertical
rows. Sometimes only one leaf originates at each node,
sometimes two, or occasionally more. When only one
occurs the leaves are found to be arranged spirally or
alternately up the stem. When more than one there is
said to be a whorl of leaves at each node. Frequently
the whorl consists of two leaves only. Successive
whorls, whatever the number of leaves, have their
separate leaves placed opposite the spaces between
the leaves of the whorls above and below them.

The number of the vertical rows is correlated with the
shape and size of the leaves which compose them.
Leaves with very broad bases, often indented, and
tapering fairly rapidly to a pointed apex, known techni-
cally as ovate or cordate leaves, generally occur opposite
to one another on the stem, there being only two rows.
Sometimes they have short stalks, sometimes none.
When the leaf has its broadest part near the middle and
tapers to both apex and base it is termed an oval or ellip-
tical leaf; such are generally arranged in three rows.
When still narrower, becoming what are known as
lanceolate leaves, the number of rows increases to five or
eight. Still narrower leaves occur in greater numbers of
ranks still. We see thus a co-ordination between the
shapes and dimensions of the leaves and their mode of
attachment to the stem, just such a co-ordination as we
should expect when we remember the disadvantages
which would arise from a crowding together of large
ovate leaves in several ranks, or the sparse scattering of
linear or narrow leaves in few rows.

When we study in this way the shoot systems of
different plants we find them to be in harmony with
their surroundings as fully as are the root systems. The
surroundings influence the plant very forcibly while it
is developing, and many of the results of its development
can only be understood by observing that they are

62 BOTANY

essentially purposeful. The only mode of securing this
adjustment with the environment which is possible is
that of regulating its growth.

During the early development and growth the plant
exhibits in its shoots as in its roots powers of purposeful
response to certain features of the environment which it
is capable of appreciating. If we examine the plumule
or young bud of the seedling as soon as it begins to grow,
we shall notice the same perception of direction as we
observed in the root. As the latter would persist in
growing downwards, curving itself if its apex pointed
in any other direction, so the shoot persists in growing
upwards. The sensitive part is not so easy to localise
as in the root, but careful experiments made on various
plants have proved that the perceptive part of the
shoot is the tip and that the sensitive zone does not
extend far downwards. The response to the stimulus is
brought about in the same way in the two cases, viz., by
a modification of the growth, and it is clearly purposeful,
to plant the root in the earth and the shoot in the air.
There is a close resemblance again in their behaviour
between the primary branches of the stem and root.
None of them grows in the same direction as the axis
from which it springs, but usually they stretch out nearly
at right angles to it. This is a response to the stimulation
of gravitation in both.

Another factor which is of much greater importance
in the case of the shoot than in that of the root is the
incidence of lateral light, which helps to determine the
position of the branches as well as of the leaves. If the
light falls on a shoot more intensely on one side than
another, the rate of growth very speedily changes so as
to cause the growing region of the stem to bend or curve
till its apex is directed towards the point from which the
strongest light is coming.1 The plant exhibits a power

1 A figure illustrating this is given in the Biology primer, p. 71.

THE STRUCTURE OF THE SHOOT 63

of perceiving or appreciating differences of intensity o^
illumination. This sensitiveness is of the greatest value
io the shoot, for as the stem bends towards the source of
the light the leaves which are expanded nearly at right
angles to it are exposed to the rays which they need for
the manufacture of sugar.

The leaves also manifest an independent sensitiveness
to light. They are generally so expanded as to expose
their upper surfaces to the sunshine. If this position
•cannot be attained without a movement of the leaf this
movement is effected and supplements the other. The
leaf-blade twists on its petiole, or the petiole twists in
such a way as to expose the surface of the blade.

With the same sensitiveness to light we see thus that
the different members of the plant respond differently,
but always purposefully, to it. The root grows away
from the incident rays, penetrating into the deeper
crevices of the soil ; the stem grows towards them,
while the leaf places itself across their path.

The positions assumed by the stem, branches, and
leaves are greatly influenced by the various stimulations
they receive ; some respond more actively to one, others
to another; but all show both perception and response
as they adapt themselves to their environment.

CHAPTER VII

THE STRUCTURE OF THE SHOOT

WE must now examine what are the arrangements of
the internal structures of the shoot which enable it to
carry on these duties. Though the shoot is to be
regarded as a single system comparable with the root,
the duties discharged by its cylindrical and its flattened
parts are so far distinct that it will be well to consider
them separately from our present point of view.

64 BOTANY

When we examine the plumule we find it to be com-
posed of cells resembling those of the radicle. They are
at first all alike, and only slowly do differences become
apparent. At the apex we find them meristematic,
that is, each cell has the power to divide into two. A
little farther back they increase in size and become
vacuolated. If we take a longitudinal section at this.
d pe pi a£e> we fi11^ that, as in the

root, we can distinguish
three regions which are
faintlY indicated (Fig. 24).
The central strand or
plerome is visible, appear-
ing conical in shape as in
the root. Outside it lies
a periblem, and this is
covered by a dermatogen,
a layer of a single cell in
thickness. These two are
not conical, but are thrown
into irregularity by the
outgrowth of the leaves.
The leaves and branches
FIG. 24. Growing point of stem of differ in their origin from

Dicotyledon, d, dermatogen ; pe, the branches of the root as

young they begin with the Ollt"

growth of the periblem,
which pushes the dermatogen before it. The plerome
takes no part in their formation. As the plumule gets
older its elongation proceeds by the continued formation
of new cells and their subsequent growth. This goes on
for some time, and extends as a rule further back than it
does in the root. The growing region is a little more
complex in the stem than in the root, because the cells
do not all grow alike, those of the nodes, or places
.where the leaves arise, elongating scarcely at all, while

THE STRUCTURE OF THE SHOOT

those of the internodes are very vigorous. The leaves
on the nodes elongate from the first, but the branches
in their axils appear much later.

As the seedling grows, it prepares for the discharge of
the duties which devolve upon it. What we are about
to describe of its structure corresponds almost exactly
with the structure of each year's
twigs of the tree or shrub into
which ultimately it develops.

The two main duties of the
stem we have seen to be the
support of the head or leaf-bear-
ing part of the shoot and the
transport of the water and
mineral compounds absorbed
by the roots to the seat of con-
struction of organic substance.
Both these objects are carried
out by the arrangements in the
central cylinder, and both de-
pend upon the development of
vascular strands connected with
those of the root. If we look
at a longitudinal section of a
whole plant we find that these FlG-
strands are continuous through-
out it though they are arranged
differently in its different regions. In the root we found
the strands of wood lying sometimes separately in a
central ring, sometimes joined to form a solid cone of
wood. Other strands, soft in nature, known as bast, lie
between them or between their outer limbs when they
are fused in the centre. As we pass upwards we find
that in the region just below the cotyledons a certain
rearrangement of the strands takes place. The bundles
shift their relative positions and the wood strands come

25. Diagram showing
the general structure of a
dicotyledonous plant.

66 BOTANY

to lie exactly inside the bast strands, the two being
separated only by a layer of meristematic cells known as
cambium. The bundles in the shoot are known as con-
joint bundles from this association of the wood and bast.
The wood strands, further, become twisted on their long
axes in this same region, so that the protoxylem, which in
the root is on the outside, is in the stem on the inner face.
The bundles are wedge-shaped much as they are in the
root. Each seems thus to have turned completely round
so as to face in the opposite direction. Instead of the
cylinder being solid in the centre, the conjoint strands
always stand round its periphery so that there is a large
unoccupied space in the centre, known as the pith.

Following them to the growing end of the stem we
find that they do not terminate in its growing cells, but
can be traced into the leaves through their petioles.
In the latter they usually form a half cylinder open on
the upper side, instead of a complete hollow cylinder as
in the stem. From the petiole they can be traced into
the flattened portion of the leaf, where they form the
network which we call the veins of the leaf.

While the leaf and stem are very young we find in
them the first traces of the origination of these strands.
They appear in the growing point a little way back,
as separate strands in the plerome, made up of small
cells, longer than broad, defined from the rest chiefly by
their smaller transverse diameters. They are all meri-
stematic and only slowly lose the power of dividing. A
transverse section (Fig. 26) of the plerome shows these
little strands as wedge-shaped areas, the procambial
strands, arranged in a circle near the outside of the
plerome, separated by narrow areas known as medullary
rays. As they get older the cells become changed into
their adult form. The change in the wood cells is asso-
ciated with growth in diameter and irregular thickening
of the walls, making them appear as if marked out into

THE STRUCTURE OF THE SHOOT 67

FIG. 26. Diagram of sections of stem of dicotyledon at three ages.
A, young condition, showing commencement of differentiation of
the plerome and its vascular strands: a, strand; b, limits of the
plerome; c, periblem; m.r., medullary ray; pi, pith. B, a little
older stage: p, bast; x, wood; c, cambium; i.e., interfascicular cam-
bium: (one of the strands has been shaded). C, older stage, after the
commencement of secondary thickening: px, protoxylem or first-
formed wood; x, secondary wood; ph, secondary bast. (After Sachs.)

68 BOTANY

curious patterns ; with substitution of lignin for cellu-
lose as the material of which they are composed; and
with the disappearance of many of the transverse
separating walls, causing a vertical row of cells to be-
come a vessel. The cells to show the change first are
those on the inside of the wedge-shaped strand — the
protoxylem. In these the thickening of the walls is
laid down in the shape of a spiral band, or a series of
rings. These vessels remain of small diameter. The
other wood cells and vessels are thickened more irre-
gularly and are called reticulated ; in some cases when
the thickening deposit leaves only very small thin spots
they are known as pitted elements (Fig. 13, p. 35).

The bast of the strand begins to be differentiated on
the side nearest the periphery, where the cells are called
the protophloem. The vessels of the bast are sieve tubes
(Fig. 27) as in the root. The other elements are mainly
elongated cells with thin cellulose walls.

As the differentiation begins at the front and back of
the bundle and advances in each direction the wood and
bast are not very long in meeting. In plants that only
live for a few weeks or months they come into actual
contact, but to those whose lives are longer provision is
made for further development by the last layer left
between them remaining meristematic or capable of con-
tinuous dividing. This is the cambium layer of which
we have spoken. It is only a single cell in thickness.

This arrangement of the supporting tissue is very
strong and most economical. The hollow cylinder or
tube is one of the strongest forms of support that a
structure can possess. It has, too, a certain flexibility,
for while the strands are gradually hardening they can
bend freely without breaking. The young stem thus
shows itself built for toughness and elasticity, so pos-
sessing a power of bending to wind and recovering as
the force of the air passes it. The continuity of the

THE STRUCTURE OF THE SHOOT

69

vascular strands throughout the plant ensures the proper
distribution of the water absorbed from the soil.

Certain other features of the framework of the plant
next call for attention. If we examine the outer layer,
called in the stem and leaf the epidermis, we find it as a
continuous sheet over the whole, and in most cases a

FIG. 27. Sieve tube from stem of Cucurbita. A, transverse; B, longitu-
dinal section ; s.p., sieve plate ; c, companion cell. (After Strasburger.)
X5oo.

single cell in thickness. A delicate structure like a seed-
ling, whose cells are filled with water, is exposed to a
general evaporation at the surface. This, if not guarded
against, would lead to a loss of water beyond the control
of the plant, and would interfere with the proper con-
duction of the water to the places where the construction
of sugar takes place. We find a very simple but very
effective protective mechanism. The outer walls of the
cells of the epidermis become thickened and their
external layers are changed into a very impermeable

70 BOTANY

material called cutin. These external layers can be
stripped off from large pieces of the surface in a kind of
pellicle, which is known as the cuticle. It is developed
more freely over the leaves than over the stem.

This layer serves too as a protection against cold.
For this purpose many plants have an additional safe-
guard, in the shape of hairs, or outgrowths of the
epidermal cells, forming a fine felt work over their sur-
faces, clothing them indeed in a kind of cotton garment.

Both cuticle and hairy coating serve also to protect
the delicate surface from injury by rain.

The outer coating or cuticle, covering as it does the
whqle exterior, would be a source of danger to the plant
by preventing almost all evaporation, if it were altogether
intact. The epidermis is pierced by small apertures,
which are the openings of the system of intercellular
spaces or passages we saw to be developed in the root
and which we now find to extend throughout the whole
of the shoot as well. These stomata, as they are called,
are more numerous in the leaf than in the stem, but they
are present in the latter so long as it is young. The
aperture or stoma is surrounded by two cells called
guard-cells, which are attached together at their ends
but not in their centre. They are kidney-shaped in
appearance, and when filled with water they stretch so
as to draw apart in the centre, opening the stoma (Fig.
28). When the water is withdrawn from them they fall
together and close the aperture. This arrangement
thus allows the necessary evaporation of water to take
place. The vapour is formed in the intercellular
passages and passes out through the stomata, the
width of the apertures being regulated by the amount of
water in the guard-cells, which in turn depends on the
amount of water in the plant.

The layer of cells between the central cylinder and
the epidermis, which is the continuation backwards of

THE STRUCTURE OF THE SHOOT 71

the periblem, is the cortex. Its composition is very
varied as the plants grow older. In the young condition
it is only noteworthy because its outer layers of cells
contain the green bodies we have called chloroplasts.
The great development of branching which takes

FIG. 28. Epidermal cells of leaf showing three stomata in
various stages of opening.

place necessitates a considerable enlargement of this
primary structure. The increase in number of the
leaves makes it important to increase the means of
transport of water; the slender cylindrical tube of the
young stem soon becomes unable to support the weight
resulting from its greater size and the number of its
branches. The transport of food to its different parts
makes increasing demands upon its bast. We must
examine the way in which these necessities are supplied.

72 BOTANY

As the stem grows we find additional vascular strands
continually being developed, a change directed especially
to the strengthening of the stem, as the new strands are
not directly connected with the leaves. The original
strands also are much enlarged and strengthened.

All this work is done by the cambium layer. Part of
the original bundles, as we have seen, consists of this
tissue, which is hence called fascicular cambium. By
continual division of its cells, mainly in a direction
parallel with the outside of the stem, masses of cells are
produced between the wood and the bast. One layer of
these remains cambium — those on the inside of it are
changed into wood, those on its outside into bast.

Very soon after this process has been set up in the
bundle the cells which lie between the strands, known as
the medullary rays, are the seat of change. No doubt the
multiplication of the cells of the cambium sets up a
strain in the ray cells adjoining them, stretching them,
or dragging upon them. The stimulus of this strain
makes certain of these cells, extending across the ray,
begin to divide in their turn, and soon the rays are all
crossed by layers of meristematic cells, joining up into
a ring the isolated cambiums of the bundles. These
new portions, which complete the ring, are known as
inter fascicular cambium. The whole ring now behaves
as the original cambium of the bundles does, and soon a
ring of wood is formed in front of and a ring of bast behind
it. The parts of the ring formed by the interfascicular
cambium have no connection directly with the leaves.

As new leaves arise at the apices of the twigs the
vascular strands belonging to them are connected with
this vascular cylinder as were the primary ones, for the
structure of the young twigs resembles in all points that
which we* have described for the seedling.

At the end of each year, in our climate, the growth of
most trees ceases, owing to the fact that the leaves fall

THE STRUCTURE OF THE SHOOT

73

off. When it is resumed on the putting out of the
leaves of the next year this process recommences.
There is a difference in appearance between the wood

FK

. 29. Section of twig of lime-tree, 3 years old. ix, 2x,
the successive annual rings of wood. (After Kny.)

formed at the end of the season and at the beginning, so
that the formation of each year stands out distinct from
that of the next, when a transverse section of a twig or
branch is examined. Each year's formation is spoken
of as an annual ring. The rings of wood are easily seen,
but those of bast are not so conspicuous (Fig. 29).

74

BOTANY

When a tree gets old the central part of its wood
usually dies and becomes very hard. The only living
wood is a narrow area close to the cambium. This is
known as the sap-wood, or alburnum, the dead centre
being the heartwood or duramen.

This substitution of a solid core for a hollow cylinder
of wood is necessary for the strengthening of the trunk
and branches. In this well-developed form the mass of
the body of both shoot and root is made up of hard wood.
As the trunk and branches gradually thicken, their
outer regions are strengthened by the production of bark.
This begins in the stem as in the root by the forma-
tion of sheets of cork. In
the root these layers arise
in the pericycle ; in the stem
they begin in the cortex just
below the epidermis (Fig.
30). As the years pass,
more and more layers of
cork are formed deeper and deeper in this region. The
outer ones are pierced by lenticels (Fig. 31). They are
all formed in the same way, by the formation of meri-
stematic layers, which
produce cork in their out-
side and add to the cortex
on their inner faces, be-
having in much the same
way as the cambium,
though the cells they
form do not give rise
to bast and wood. The n

, , FIG. 31. Section of a lenticel, /; per,

cork is impermeable to cork layer.

water and consequently

all the cells outside the innermost layer of it die ; the

tissue thus formed constitutes the bark (Fig. 32). As the

years go on it becomes thicker and thicker and much

FIG. 30. Commencement of cork
formation in stem.

THE STRUCTURE OF THE SHOOT

75

crinkled and split up through the action of the weather
and the storms the tree is exposed to.

We must now examine the interior arrangements of
the leaf. We have already learnt that its special work
is mainly twofold. It is the chief agent in transpira-

FIG. 33. Section of bark of Quercus. pe, cork layers arising at
different depths in the cortex. (After Kny.)

tion or the evaporation of the water which the plant
does not permanently retain; while it also is the chief
seat of the construction of organic food substance.
These two duties are discharged mainly by the cells of
the lower and of the upper halves of the leaf respectively.
The petiole or stalk of the leaf has a general structure
not unlike that of the stem, except that its vascular
strands do not form a complete cylinder, but only half

76

BOTANY

a one, being open on the upper side (Fig. 33). The
petiole is generally not cylindrical in shape, but flattened
on the upper face. It is continued with little change of
structure into the blade, where it appears as the mid-rib.
The blade of a stalked leaf is the ultimate portion of
the outgrowth with the flattened wing which has been

" ep

-co

FIG. 33. Section of petiole of Primula sinensis. ep, epidermis; co,
cortex; en, endodermis ; pe, pericycle; ph, bast; x, wood.

developed along its two edges. The part between the
two wings is generally known as the mid-rib. If we cut
a section through the blade (Fig. 20) we find its structure
adapted to the work for which the flat part was de-
veloped. On both surfaces we find an epidermis, the
lower one especially pierced with stomata. Under the
upper epidermis the cells are long and narrow and
arranged side by side much like the vertical railings
of a fence. They touch each other along nearly their

THE MONOCOTYLEDONOUS PLANT 77

whole length, intercellular spaces being small and not
numerous. There may be only one layer or several
layers of these cells, which constitute what is known as
the palissade tissue. The cells contain numerous chloro-
plasts, which are embedded in their protoplasm. Each
has as usual a central vacuole filled with water. These
chloroplasts are capable of a little movement in the celL
It is in this layer, exposed to the light most freely, that
the sugar is constructed. (See Fig. 20, p. 52.)

The lower half of the leaf is made of cells which are
spherical, cubical, or oblong, and are arranged so as to-
touch each other only at few points ; consequently the
intercellular passages between them are very large,
taking up sometimes more space than the actual cells.
This is often called the spongy tissue of the leaf. The
cells contain some chloroplasts, but not nearly so many
as the palissade cells. This layer is the layer in which
evaporation occurs. The veins generally run in the
centre of the blade, between these two layers of cells.

All the structures of both petiole and blade thus show-
exact adaptation to the two main duties of the leaf.

CHAPTER VIII

THE MONOCOTYLEDONOUS PLANT

OUR attention has been mainly directed so far to the
peculiarities of the dicotyledonous plant. We must
now turn for a little while to study another form, in
which the embryo has only one cotyledon. The plants
of this type are not so numerous as the former class, but
they are still very widespread. The most easily acces-
sible of them in this country are the grasses and the
group which is represented by the common white lily.

If we take a grain of wheat we have what is very
generally spoken of as the seed of the plant. This is

78 BOTANY

not strictly accurate ; it is really the fruit and contains
the seed, but the testa of the seed and the wall of the fruit
are so closely united that we cannot separate them. The
grain of wheat is a small ovoid body with one side flattened
and grooved down its length. At the back,
quite at the lower end of the grain, is a little
. wrinkled area, which marks the position of the
embryo, above which and forming the greater
part of the grain is the endosperm, filled with
food for the young plant during its early growth
or germination. A section of the grain is shown
in Fig. 34.

The grass embryo possesses a single large
cotyledon which is at first terminal and con-
tinuous in a straight line with the radicle,
while the plumule grows out laterally some
plG little distance below the apex. As it grows

Longitudi- the cotyledon becomes forced over to one side,
n^ s®?"0^ and the plumule and radicle come to lie in a
oatgrai ^ straight line, as in the dicotyledons. The
cotyledon then develops along the side of the
rest of the embryo, separating it from the endosperm.
The side of it which is in contact with the latter is the
part which absorbs the food in the endosperm cells. In
other seeds the cotyledon remains in a line with the radicle,
the whole embryo being surrounded by the endosperm.
In germination, however, the upper end of the cotyledon
is the last to leave the seed coat, remaining there and
absorbing the endosperm as long as any persists.

If we soak some grains of wheat or barley and keep
them warm germination soon begins. The radicle pro-
trudes as a little white body from the micropyle ; looking
along the back of the grain we can notice a little pointed
prominence gradually making its way in the other
direction under the skin and ultimately emerging at the
other end ; this is the plumule. As it gets larger it dis-

THE MONOCOTYLEDONOUS PLANT 79

places the rest of the grain, which comes to lie on the side
of the young plantlet. The grain remains underground.

The further development is differ-
ent from that of the dicotyledon. The
root does not form a tap root, but
branches almost at once; indeed in
the grasses generally it begins to do
so before it escapes from the grain.
The main root grows scarcely at all,
but a number of branches arise behind
its apex, making in the grasses a
cluster of delicate fibrous rootlets.
The growth of the young stem is seen
more advantageously in a larger grass
—the maize. At first it is very
slender, but as development proceeds
its growing point becomes continu-
ally larger and more vigorous, so that
each node and internode become
larger than the preceding ones. The
young stem has thus the form of an
inverted cone (Fig. 35). This goes
on till the plant reaches a certain
height, when this continuous enlarge-
ment ceases and the later parts of the
stem are cylindrical. Several roots
are developed from the nodes of the
lower part of the stem in the case
of most monocotyledons ; as they
arise out of the normal order they are called adventitious
roots.

The stem of the monocotyledon produces as a rule
one leaf at each node. This leaf has a very broad base,
which encircles the stem in large part, or sometimes
entirely. The leaves are said to be sheathing.

The general requirements of the plant are not very
different from those of the dicotyledon and need not

G. 35- Diagram of
mode of growth of
stem of monocoty-
ledon (Zea). (After
Sachs.)

8o

BOTANY

therefore be discussed at any length. The distribution
of the conducting tissues is, however, materially different
so far as the stem is concerned. The root has its strands
placed like those of the dicotyledon, but it never increases
in thickness and does not show, therefore, any develop-
ment of cambium or secondary woody elements. The
strands in the stem are confined to the central cylinder,
but each conjoint strand of bast and wood passes up the

stem separately. Each is
surrounded by a protecting
sheath of hardened cells
and never contains any
cambium. The strands
are numerous and are
arranged in a series of
circles. When the number
is very great this circular
arrangement cannot easily
be seen and the strands
appear to be scattered
thickly through the central
cylinder (Fig. 36). They
are found to be continuous
with similar strands in the leaf, which as before are called
the veins. These run in the main parallel to one another,
and do not form the complex network which is seen in
the leaf of a dicotyledon.

The relations of the leaf to the stem are a little
different from those of most dicotyledons. The bases of
the leaves sheathe the stem and do not as a rule fall off
in autumn. The leaf, however, sooner or later dies, but
its base remains where it was. As the stem grows
older, as in the case of many palms, these leaf bases
cover nearly the whole of the trunk, causing the latter
to appear much thicker than it really is.

The leaf has palissade parenchyma in a narrow layer
under both surfaces.

FIG. 36. Diagram of transverse sec-
tion of stem of monocotyledon.

THE FOOD OF PLANTS 81

CHAPTER IX

THE FOOD OF PLANTS

WE have in our introductory chapter considered in out-
line the important question of the nutrition of plants, a
subject which has been treated of also in Chapter VI. of
the primer of Biology. It is necessary now to return to
this subject and examine it a little more fully.

The substances that are taken in by a plant are the
carbon dioxide which is present in the air, and the water
and dissolved mineral matters which the roots obtain
from the soil. We must again emphasise the fact that
these materials are not capable of serving as food in the
condition in which they are absorbed, but that a great
deal of work has to be carried out to convert them into
nutritive material. It is only the green plant which can
build them up into such compounds as the living substance
can incorporate into itself, the work being effected by
the chloroplasts, the little ovoid bodies which are the
seat of the green colouring matter. Further, the chloro-
plasts can only work when they are properly illuminated.

The carbon dioxide exists in very small proportion in
the air, not more than about 3 parts being present in
10,000. The gas enters the plant by way of the stomata,
and so makes its way into the intercellular spaces
whence it obtains access to the cells in which the chloro-
plasts are present. The water from the soil is conducted
in the way we have described to the same cells, continu-
ally replenishing the supply in their vacuoles. We
have thus present in the cells of the parenchyma of the
leaf a supply of carbon dioxide, water, and the chloro-
plasts themselves. When sunlight shines upon the
leaves in appropriate intensity the constructive action
commences. The stages are not yet fully understood,
but there appears to be no doubt that in some way the
chloroplasts cause a certain chemical action to be set

F

82 BOTANY

up ; the carbon dioxide and some of the water disappear
and are replaced by a simple organic substance known
as formaldehyde, while a quantity of oxygen, equal in
volume to the carbon -dioxide, is set free. The oxygen
finds its way from the cells into the intercellular spaces
and passes out of the plant by way of the stomata.
Formaldehyde is thus the first organic product which is
formed; it is a gaseous body and probably is never
present in any but very small quantities, for it is almost
immediately transformed into a kind of sugar. The
manufacture of sugar is thus the first stage in the
preparation of the food of the plant.

This construction of sugar cannot be carried out
without the application of energy. We are familiar,
from our ordinary experience of things, with the fact
that a machine cannot be made to do work without a
supply of energy. A steam engine cannot work without
the expenditure of a certain amount of fuel. Whence
then does the chloroplast obtain the energy which it
applies to sugar-making? The answer to this question
explains the necessity for the proper illumination which
we have spoken of as a condition of its activity. The
rays from the sun, which we speak of as the rays of
light, are absorbed by the green colouring matter of the
chloroplast. We can prove this by the use of an instru-
ment known as the spectroscope, which is an arrange-
ment of glass prisms. If we let a beam of white light
from the sun fall upon such a prism the rays of which it
is composed are bent or deflected unequally on entering
and leaving the glass, so that if they are allowed on
emerging to fall upon a plane surface they appear as a
broad band of light showing a series of colours ranging
in order from red to orange, yellow, green, blue, and
violet. If now we place a thin film of a solution of
chlorophyll between the source of light and the prism
we find all the rays do not reach the glass, so that the

THE FOOD OF PLANTS 83

coloured band which emerges is not continuous, certain
parts of it being blotted out. The spectrum, as the
band is called, is consequently crossed vertically by a
number of dark bands, corresponding to the position of
the missing rays. In the living cell these rays are
absorbed by the chlorophyll exactly as they are by the
solution used in the experiment, and it is from them the
plant derives the energy which is used. The rays which
are most active are a certain number of the red ones;
these correspond in position with a broad black band
which is one of those described.

This process of sugar construction can only take
place at a moderate temperature.

Another very important constituent of the plant's
food is protein, which differs from the group of food-
stuffs to which sugar belongs by containing nitrogen in
combination. Very little is at present known of the
processes by which protein is made. Compounds of
nitrogen, preferably nitrates of potassium, calcium,
magnesium, or ammonia, are absorbed by the roots, dis-
solved in the water which they take in. The changes
they undergo lead in some still unexplained manner to
the formation of much more complex nitrogen com-
pounds, which are generally though not strictly accur-
ately described as amides. Among them may be
mentioned asparagin, leucin, and tyrosin.

By still further changes these are converted into
proteins, but the chemistry of the process is still obscure.

A third constituent of the food of plants is fat or oil.
This is less widely distributed and appears for the most
part only in places of storage. It is formed directly
from the living substance there and does not appear to
be built up from simple substances in the plant.

Sugar is a member of a group of substances which are
called carbohydrates. It is formed in large quantities
when the chloroplasts are properly illuminated; much

84 BOTANY

more is made than is needed for immediate use. The
surplus is immediately deposited in the chloroplasts in
the form of grains of starch. Starch is another carbo-
hydrate which very readily becomes transformed into
sugar by the action of dilute mineral acids or of a
peculiar body known as diastase, which is a digestive
ferment or enzyme. Probably an excess of protein is
also made while the conditions are favourable.

The cells in which these foodstuffs are made have to
manufacture sufficient food not for themselves only, but
for the whole plant, many of whose cells, as we have
seen, are set apart for other purposes altogether. It is
necessary accordingly for the food to be made in large
quantities and for the surplus to be transported from
the cells in which it is made to the other cells of the
organism. Part of it is devoted to the nutrition and
growth of the body of the plant, and a large amount is
stored in various places to nourish the reproductive organs.

There is consequently a continuous movement of the
manufactured food substances about the plant. The
sugar temporarily deposited as starch grains in the
chloroplasts is taken away as soon as darkness falls.
The ferment or enzyme diastase, which is present in the
cells, converts the starch into another form of sugar,
which diffuses outwards. A stream of such sugar is thus
leaving the leaf during the night and passing to other
cells of the plant, either to be consumed in the growth
processes or to be stored up till wanted. Probably this
stream of sugar is passing also during the day, but while
the manufacture is going on it is not easy to detect it.

When proteins leave the cell they are first converted
by a similar ferment into the amides we have spoken of
and pass through the plant's tissues as such.

Sugar and either proteins or amides are taken up by the
living protoplasm and incorporated into its substance.
This is the true assimilation, or nutrition of the plant.

THE RESPIRATION 'OF PLANTS 85

The foodstuffs that are stored instead of being
directly consumed, undergo a transformation which is
the opposite of that to which the enzymes give rise.
The sugar is converted again into starch, a process
carried out by certain plastids much like chloroplasts,
but without any green pigment. These leucoplasts, as
they are called, are present in the cells in which the
storage takes place. The amides are built up again into
proteins and deposited in the cells. In seeds they appear
as peculiar grains of protein matter, which have long been
known under the name of aleurone grains. They are formed
by the protoplasm of the cell and not by any plastid.

The transport of these streams of food material is
effected chiefly by those soft parts of the vascular
strands of which we have spoken as bast or phloem.

CHAPTER X

THE RESPIRATION OF PLANTS

IT is a matter of common experience with us all that a
certain process called breathing must take place. We
know that we are continually taking air into our bodies
and passing it out again. What we are not perhaps
aware of is that the air we give out differs in two im-
portant particulars from that which we take in — it has
gained some carbon dioxide and it has lost some oxygen.
What is true of ourselves is true also of plants.
During the whole time of their lives they are absorbing
oxygen and giving out carbon dioxide, two processes
which constitute the beginning and the end of another
very complex internal one which is known as respiration.
So long as active life lasts this interchange of gases can
be detected by appropriate methods, though it is
observed with difficulty during the daytime on account
of the absorption of carbon dioxide and liberation of

86 BOTANY

oxygen, which we have seen to be associated with the
manufacture of sugar. It can, however, be detected by
experiments. To prove the absorption of oxygen, take
a flask and fit it with an india-rubber stopper through
which a glass tube bent twice at right angles can be
passed; let the end of the tube dip into mercury in a
small vessel standing side by side with the flask. Fill
the flask with healthy leaves. In addition to the leaves
place in the flask a test tube containing a solution of
caustic potash and cork it up so that the outlet tube
dips into the mercury. Keep it at a constant tempera-
ture for some hours. The mercury will rise gradually
into the tube, and will continue to do so for some time,
so showing that a diminution of the volume of the air in
the flask has taken place. Analysis of the air then left
in the flask will show that the volume of nitrogen is un-
changed, while that of oxygen has diminished. Exami-
nation of the caustic potash will show that it has
absorbed an amount of carbon dioxide.

The reason for employing the potash is that the
carbon dioxide which is exhaled in respiration is about
equal in volume to the oxygen absorbed, so that unless
it is removed the diminution of the volume of the
oxygen will be compensated for by the addition of the
carbon dioxide and the mercury will not rise in the tube.

We can show the exhalation of carbon dioxide by the
living plant by means of another experiment. Place a
number of leaves in a flask through the stopper of which
air can be made to enter by one glass tube and can be
removed by another. Make the air as it enters pass
through a bottle containing a solution of caustic potash
which will free it from all traces of carbon dioxide.
Pump the air through the flask by means of some form
of air pump or aspirator and lead it through a bottle of
lime-water. It will soon make the lime-water milky in
consequence of the formation of carbonate of calcium,

THE EVOLUTION OF PLANTS 87

brought about by the reaction of carbon dioxide with
the lime. As all carbon dioxide was kept from entering
the flask, and as the lime-water shows by becoming
milky that some of this gas reached it in the stream of
air pumped through it, it becomes perfectly clear that
the plant has exhaled it.1

Besides carbon dioxide^ certain amount of water vapour
is exhaled by the plant in the process of respiration.

To understand the purpose of respiration we must go
back a little and consider some of the features of sugar
manufacture. We saw that the energy for this work is
derived from the sun. The work it does is to build up
sugar and subsequently other compounds, which remain
in the plant. These substances retain in themselves the
energy which was expended in making them. When
they are decomposed or reduced to simple compounds
like those from which they were made, this energy can
be liberated again. If we burn them, for instance, we
get the liberation of a large amount of heat, which can
be made to do work in other directions. So we see
that the construction of the plant, both living and non-
living substance, involves the fixation of large amounts
of energy as well as of material.

The purpose of respiration, which involves the break-
ing down of the living substance incidental to its wear
and tear and the work it does, is in this way to liberate
the energy without which these operations could not
take place.

CHAPTER XI

THE EVOLUTION OF THE FORMS OF PLANTS— ALG^

THE considerations set out in the foregoing chapters
apply in great part only to the higher terrestrial plants.
But the whole range of vegetation is much more exten-

1 A form of the apparatus is shown in the primer of Biology, p. 56.

88 BOTANY

sive than this. There are many other types of plants
which differ from the land plant we have considered
and differ too among themselves in many respects.
They live in various situations, they attain to very
different dimensions, and they show a great variety in
the details of their internal structure. A very large
number of plants, some very humble, others very
elaborate, in degree of development, live altogether or
almost entirely in water. Many others of simple struc-
ture occupy moist situations, such as rocks on the banks
of streams, damp earth, or trunks of trees near the
ground; others again dwell in hot, arid regions where
little water can reach them. Nearly all these forms
resemble the higher plants in possessing the green
colouring matter; but there are others which have lost
it, and which live, therefore, on decaying matter or in
the bodies of other plants or of animals.

If we consider the whole mass of vegetation and com-
pare the simple forms we find with others of much
complexity, we come to see that there is and has been
throughout it a continual advance, though a very slow
one, in the direction of greater complexity. This leads
us to believe that the most highly organised plants we
see to-day have been developed from extremely simple
ones during the long ages of the past.

If we try to determine how this has taken place we
are able to form some idea of its course by studying the
simple plants existing at the present time and the
gradual increase in complexity we can find among them.
Very probably the different forms show us the different
stages through which development has passed.

The simplest plant we find to-day is a very small
structure living in water. It consists of a single piece
of protoplasm with its nucleus ; it is clothed by a thin
cell wall and contains the green colouring matter. We
can suppose without much fear of mistake that the first

THE EVOLUTION OF PLANTS 89

plant that existed was not very different from this, and
like it dwelt in water. Nor are we likely to be wrong
in thinking that the whole group of the seaweeds arose
by gradual development from such a form, perhaps
before any plants grew upon the surface of the land.
If we examine this group of seaweeds we find a
succession of forms which are step by step more
elaborate, and we see therein some justification for
thinking that development or evolution has proceeded
on similar lines. There are two points that may well
be insisted upon as a preliminary to thinking about
these changes: first, there is a predisposition in the
living substance to become more highly organised;
second, this organisation is brought about by the action,
direct or indirect, of the surroundings, the plant suc-
ceeding best which is in the most complete harmony
with them. We have already illustrated the second of
these points so far as the higher land plant is concerned.

It is possible that the original ancestors of the sea-
weeds were even simpler than the one we have described,
not possessing any cell wall. If so, the exposure of the
living substance to the changes in the water and to
contact with obstacles in it no doubt led to the formation
of this cell wall for purposes of protection.

The development of this membrane round it, how-
ever, introduced a difficulty in its relations with the
water, free access to which for so many reasons was very
necessary to its welfare. We can understand that the
development of the vacuole, with its little store of water,
was speedily effected to obviate this difficulty.

Increase of size of the protoplast was no doubt one of
the. early features of the life of the plant. This in the
condition of a cell clothed by a membrane was only
possible up to a certain point without weakening the
membrane and so disturbing the protection it had
secured. Growth was consequently followed by divi-

90 BOTANY

sion into two and the gradual separation of the two from
one another. Here we see the simplest form of repro-
duction following growth, each of the two protoplasts
resulting from the division possessing the same proper-
ties as the original cell. When in some cases the
processes of growth and division became very rapid, a
second division might well take place in the two new
cells before they had become separated. There would
arise in this way by degrees a new form of plant, one in
which the processes of division continued, but the cells
did not separate at all, so that a chain or filament of
cells resulted. They would keep their individuality at
first at any rate, being almost as independent of each
other as if they had separated, their attachment being
mainly mechanical. We find a very large number of
water-weeds, or algae, existing in our ponds and streams
to-day which have just this structure.

The maintenance of this thread-like structure depends
on the cells always dividing in such a way that the new
cell walls arise across the length of the thread. It seems
certain that this course was soon departed from by some
of the plants and that some divisions arose at right
angles to the others. This led to the formation of a flat
plate of cells, still only one cell thick. A plant with this
structure would meet with very little greater difficulty
than the thread-like forms ; the requirements of the
cells would be the same, though the plant might easily
grow very much larger, the number of its protoplasts or
cells often amounting to many hundreds. Still the cells
would be all alike and probably all independent, for
water would have access to each and no further pro-
vision would be called for. Plants of this type of
structure are still common among marine seaweeds.

A great complexity, however, would arise if the
protoplasts began to divide in three planes, each at
right angles to the other two. No doubt this was not

THE EVOLUTION OF PLANTS 91

long delayed, and plants began to possess a bulk or
mass, instead of being filaments or one-layered plates.
This was a most important change, for it altered the
relations between the cells or protoplasts of the plant
and the surrounding water. Only the external cells in
such a mass were able to absorb water and the inner ones
had to depend on them for a supply. The external
cells, too, were those which ran the greatest risks from
changes of temperature in the water and from contact
with particles in it, or the many dangers which the
environment brings to the plant. These dangers and
difficulties must have increased as the plant itself grew
larger. The difficulties of the internal cells were
different, but no less serious. They were compelled to
draw upon the external ones for the renewal of the
water in their vacuoles, for the oxygen necessary for
breathing, for their food or its constituents, and for the
removal of any waste products they might produce. So
the two became gradually less and less like each other, or
more specialised. The gradual change of structure and
form of plants that followed can be traced in this way to
the need for adapting them to their conditions of life.

We can follow with some probability the further
course of events. As the bulk increased the external
cells became still more devoted to protection and absorp-
tion ; the internal ones ceased to develop chlorophyll as
less light reached them, and the work of food construc-
tion which is the function of the chlorophyll was thus
thrown upon the outer layers. Among the internal
mass certain cells became concerned mainly in conduct-
ing the water to the rest, so supplying them with oxygen
and food. As the size became still greater the exterior
surface of the still symmetrical spheroidal or spherical
plant became inadequate to supply all that the inner
mass required; the spherical plant no longer had suffi-
cient surface in proportion to its bulk. This involved

92 BOTANY

the restriction of growth mainly to certain regions of the
surface, which became what we now call growing points ;
here the multiplication of cells led to the formation of
conical outgrowths, and these in turn were soon recognis-
able as branches. The larger the plant became the
more necessary was it for it to become branched in view
of the dangers from storms, and currents in the water,
as it would oppose much less resistance to the move-
ments of the stream or tide.

The problem of the passage of the absorbed water, as
it made its way from exterior to interior cells, involved
the question of its being able to pass through the cell
walls that lay in its way. As the length of the body of
the plant increased with the putting out of branches, the
number of the cell walls became an inconvenient obstacle
to its passage. The flow being in a particular direction
caused the cells to stretch a little accordingly and so to
make them become a little longer than broad. This
elongation soon became advantageous, as with cells of
that shape there were fewer walls to pass in a given
distance. So gradually in the regions of transport
elongated cells became most usual. After a time the
end walls became perforated and later still dissolved
altogether, so that the water could pass easily along a
tract which was originally a number of columns of cells.
We can observe this change of internal structure taking
place in many plants to-day. Each column when it
has lost its end walls forms a vessel. We find them
fully developed only in terrestrial plants, but some of
the larger seaweeds contain structures much like them,
the end walls being perforated very freely.

During the time these changes were taking place
in the arrangements of the interior, other signs of
specialisation of exterior parts made themselves visible.
With increase of size and great flexibility of body as in
the filamentous forms it became advantageous to be

DEVELOPMENT OF SEAWEEDS 93

attached to some substratum and no longer passively
floated about. So attaching organs were developed;
in some of the simple filaments they were only a modi-
fied cell at the basal end of the thread ; in the bulky
plants the whole of the unbranched end was often
specially modified to form a kind of grasping organ
capable of growing round stones or into crevices.

We find such anchorage mechanisms still in the larger
seaweeds of our coasts. These organs are not true
roots, for they anchor the plant only, and do not absorb
nutritive or mineral compounds for its use.

Such a development of the body of the plant was
sufficient for aquatic plants such as seaweeds. Even in
the largest of them there is only a very slight specialisa-
tion of structure compared with that which is necessary
for plants when they come to live on land. We can,
however, find in many present day forms three systems
of tissues, an external protective and absorbing coat, a
conducting system, and between them a system of cells
belonging to neither, whose function so far has not been
very apparent to us. We see that the division of the
body of the plant into members does not go so far as to
enable us to recognise what we call stem, leaf, or root.
It is in some cases undivided, in others very much
branched, but its structure is practically the same
throughout. We speak of such a form of plant as a
thallus, whether branched or simple, massive or minute.

CHAPTER XII

THE DEVELOPMENT OF THE REPRODUCTIVE
PROCESSES IN THE ALG^E

WHILE these changes were occurring in the form and
structure of the body of the plant, other developments
were taking place in the ways in which new individuals

94 BOTANY

arose. While plants had only a unicellular body, the
division of the cell caused the appearance of a new in-
dividual. In fact, the individual was the same thing as
the cell. But when the cells failed to separate this
ceased to be the case, and the individual plant became
identified with the chain of cells. Division of a cell so
added another link to the chain, but it did not produce a
new individual. For a time the simple method of re-
production by cell division was replaced by the break-
ing of the chain into a number of segments, each of
which grew into a new filament like the first. This
method of reproduction is shown now by certain blue-
green algae and by many fungi.

These simple methods, however, led to the production
of comparatively few offspring. A larger number
became desirable if a species was to hold its own in the
competition with its neighbours for what the surround-
ings afforded, particularly as the life of any individual
was but short. So there arose in the plant body special
cells, usually in great numbers, which could be detached
and give rise to new individuals. Various forms of
these cells are still met with ; some with no membrane,
able to swim about by means of little protoplasmic
thread-like outgrowths, known as cilia, which by rapid
vibrations set up currents in the water ; some motion-
less and well protected by firm cell walls, so as to be able
to resist heat or cold, and even some degree of drying
up. Such little specialised reproductive cells are now
generally called gonidia. They are still produced in
various ways and usually in very large numbers by
many of the seaweeds and another group related to
them — the fungi. This showed a great advance in the
spread of the species, as each of the large number
produced could give rise to a new plant.

But such rapid reproduction tends to weaken the race,
and no doubt it told its tale in the ancient times. It

DEVELOPMENT OF SEAWEEDS 95

was supplemented by another method which gave rise
to a renewal of vigour in the offspring. How the new
method was first brought about it is hard to say. It
can still be seen to play its part among the free swim-
ming gonidia of the seaweeds. Two of them, instead of
developing into two plants, fuse completely together to
form a single protoplast, which after a period of rest
grows into a single new plant. This union of the two is
called conjugation and the fused product is a zygote.

We have in this process an indication of the way in
which sex in plants began to be developed. We cannot
speak of either of the two conjugating cells in this stage
as male or female, but the further development into
well-recognisable sexes can be easily traced. One of the
cells of the pair became larger and more sluggish than
the other ; then a stage was reached in which the larger
of the two was not motile at all. By and by, instead of
being developed from any cell of the plant they came
to be produced in particular cells or groups of cells —
organs for their development. The smaller active cells
came to be recognised as male, the sluggish ones as
female. At first they were equally numerous, but as the
females became larger fewer of them were formed in the
specialised cells. The number varied indeed inversely
as the size. While the males continued to be produced
in large numbers the females became ultimately solitary
in the organs bearing them. Finally the female never
escaped from the organ but was joined there by a male.
The act of fusion of the two has come to be spoken of as
fertilisation. The female cell is now known as an ovum ;
the male cells are called sperms. There is an almost
infinite variety in the modifications which these lowly
plants have shown and show to-day in the ways they
develop their gonidia and their ova and sperms. We
cannot go more deeply into the matter here.

From the elaborate nature of its mechanism, this

96 BOTANY

sexual process of reproduction led to the appearance of
fewer offspring than the method of gonidia production,
but those so formed were much more vigorous and better
qualified in every way to carry out their vital processes,
and hence this process has become almost universal.

This stage does not, however, mark finality among
plants. It involved a good deal of uncertainty as to
whether fertilisation would take place, as both the
parents of the sperm cells and the ova were unable to
move except as they were drifted about in the water.
The sperm retained the power of swimming when
liberated, but the ovum in the higher types remained
enclosed in the oogonium or cell in which it was formed.
This uncertainty led to a further development, by which
one act of fertilisation was made to produce several
young plants instead of one. This was brought about
by the zygote — the fertilised female cell — dividing up
into a number of cells, which were liberated by the
bursting of the zygote wall. These cells in most cases
were furnished with cilia and in general appearance and
behaviour were hardly if at all distinguishable from
the gonidia described before. Such reproductive cells
can be seen to-day in the alga (Edogonium. To distin-
guish them from the gonidia they are spoken of as
spores. One act of fertilisation results in this way in
the production of a number of new individuals.

A little further advance still was made among the
algae, which became, however, much more marked
among the plants which established themselves on land.
Instead of the fertilised cell dividing at once to produce
the spores, it developed into a relatively bulky multi-
cellular structure, which became differentiated so that
only part of it gave rise to the spores, while the rest
served to protect them and to minister to their nutrition.
Such a structure can be seen to-day in many of the red
seaweeds, in which it is known as a sporocarp.

EVOLUTION OF THE LAND PLANT 97

In the land plants, as we shall see presently, this line
of development became much more extended. The
zygote gave rise to a much more highly organised sporo-
carp, in which the cells which formed the spores became
more restricted in their numbers and disposition. The
further progress of development led to much greater
differentiation of the sporocarp, which gave it the power
of living independently. The sterile part, or the region
which did not directly form spores, became much en-
larged and grew to dimensions much exceeding those of
the original thallus. Its body became differentiated
into root, stem, and leaves, and upon the sub-aerial
parts the spores were formed in structures known as
sporangia. This structure, known as the sporophyte, is
now the dominant form of all terrestrial plants. We
shall consider this change in the next chapter.

CHAPTER XIII

THE ORIGIN OF TERRESTRIAL PLANTS — EVOLUTION
OF MOSSES AND FERNS

WE must now consider what has been perhaps the most
important step we find in the development of vegetation,
the transference of plants from water to land. At first
all were aquatic, but in the natural course of events no
doubt many were washed on to the moist earth by the
side of the water in which they were living. Being
adapted only to life in water no doubt most perished,
and it was only gradually that some established them-
selves. A comparison of aquatic with terrestrial forms
as we find them to-day shows how complete a change
in almost every respect took place. In the water the
direction of their growth was comparatively unimportant,
and a large number grew in a horizontal position. On
land we find this extremely rare; most plants, as we

G

98 BOTANY

have seen, grow vertically upwards and downwards, and
show very different peculiarities in the parts which take
these opposite directions. The transference probably
did not take place till the habits of reproduction we
have briefly described had been acquired. Most likely
the first form which secured a footing in the soil was a
flattened thallus, consisting perhaps of only a few cells.
Its small size would enable it to touch the moist earth
over a large part of its surface and so to be able to
absorb the water it required. But with multiplication
of cells, and consequent increase of size, this became dim-
cult or impossible. It could only reach the moisture in
places and the supply so became insufficient. We find
such plants now on the earth, and we see that to secure
the water supply they have developed outgrowths of
their surface cells which resemble root hairs in struc-
ture; these — the rhizoids — adhere very closely to the
surface and gradually penetrate between the particles of
earth, so burying themselves in the soil. They become
very closely attached to its particles, so that they are
able to absorb the film of water by which each particle
is surrounded. They are produced in great numbers
and are continually being renewed. It is probable that
it was by such an arrangement that the early terrestrial
plants were enabled to establish themselves.

The difficulties which they encountered caused them
to produce as many and as vigorous offspring as possible,
and as in consequence of the scarcity of water fertilisa-
tion became more and more difficult, the sperms not
being brought into the neighbourhood of the ova, they
gradually came to develop an increasing number of
spores from each fertilised cell. This was advantageous
in another way as the spores were more capable of resist-
ing the adverse conditions than their parent, owing to
their simpler structure, their more moderate require-
ments, and their thicker outer membrane. On the

EVOLUTION OF THE LAND PLANT 99

other hand, the multiplication of the young plants to
which the spores gave rise made competition much more
severe. The young plants were produced so close to
each other as to cause great overcrowding, making it
difficult for each to be properly illuminated and to
secure sufficient nutritive material from the soil.

The flattened form of plant under these conditions
became unsuitable, and each plant was compelled to
grow upwards or perish. The rhizoids in this way
tended to accumulate on the comparatively small part
of the thallus which was left nearest to the soil so that
the rest might raise itself into the air. Gradually thus
the plant came to show the descending portion adapted
to live buried in the soil and the ascending portion
freely exposed to light and air. We find thus an indica-
tion of the parts we have spoken of as the root and the
shoot. It must not be concluded, however, that these
early plants showed either in such a state of development
as we recognise in the land plants of to-day.

The methods of reproduction gradually underwent
much modification, as those of the aquatic plants were
but slightly suitable for erect plants, only part of which
could have free relations with water or moist earth.
The change of position rendered fertilisation uncertain,
and it became more and more difficult for the sperms to
get to the ova. To help the process to take place, more
elaborate structures were formed in which to develop
the sperms and the ova. These, known as antheridia
and archegonia, were produced in the neighbourhood of
moisture as far as possible, often on the under side of
the flattened thallus, or, in the case of an erect form, on
the parts liable to be moistened by rain or dew. The
result of the difficulties associated with fertilisation
was a great development of the structure to which the
fertilised ovum gave rise, which produced large numbers
of spores. Gradually this became more and more

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elaborate, and instead of originating nothing beyond
the. spores and certain outgrowths to protect them, it
grew to be self-supporting, by producing cells or areas
of cells which contained the chlorophyll. Before it
succeeded in making chlorophyll for itself it was com-
pelled to derive all its nourishment from the plant which
bore the ovum that gave rise to it. This ovum never
left the archegonium in which it arose. The sporocarp,
as the spore-containing structure is called, originated
accordingly in the cavity of the archegonium and was
able thus to feed on the parent plant.

We find this stage in the evolution of the land plant
represented to-day by the mosses. The plants which
bear the ova and sperms are very small, seldom more
than an inch in height. They have a very slender stem,
bearing a number of delicate leaves, and are anchored
to the soil by a number of rhizoids which spring from
the bottom of the stem (Figs. 37, 40 B). The sperms
are produced in antheridia, which may be found at the
tops of some of the stems among the crowd of leaves
arising there (Fig. 38). The ova are similarly situated
at the tops of other stems ; each is developed singly in
an archegonium, a bottle-shaped body with a long neck
(Fig. 39). The sperm can only reach the archegonium
when the moss plants, which grow thickly together, are
wetted by rain or dew, as it must transport itself by
swimming. When it reaches the archegonium it makes
its way down the neck of the latter and fuses with the
ovum in the swollen basal part. The stem and leaves
of the moss plant are very simple in structure; the
former shows a protective outer layer or epidermis and
an interior mass of delicate thin-walled cells. In the
centre a strand of them is marked off from the rest by
their small size and in some cases by their altered cell
walls ; here we have the first indication of a conducting
system in the land plant. The leaves are flat plates of

EVOLUTION OF THE LAND PLANT,,, ;*qj

cells. When fertilisation has been effected the ovum
becomes clothed with a cell wall and develops to form the
sporocarp. This is a small ovoid body which is
formed at the end of a long stalk-like structure
which grows out of the archegonium. It re-
mains attached in this loose way to the arche-
gonium and so appears to grow out of the

FIG. 38. Section of apex of stem of moss bearing
antheridia.

ordinary moss plant. The sporocarp is rather
complex in its structure (Fig. 40). It is not
all devoted to the formation of spores, but
contains a great deal of nutritive tissue, so
that it is capable of living for a long time. In

,/IG- p- its lower part it develops some chlorophyll-
Moss plant. , . . ,, ,, . r . . r ,

containing cells, so that it can manufacture
its own food. The spores are developed in particular
bands of cells which arise in the interior and which are
usually found in the form of a hollow cylinder surround-

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BOTANY

ing a central core of ordinary cells (Fig. 40 C, s). The
moss sporocarp in most cases is provided with a special
mechanism to cause it to open
when the spores are ripe so that
u the latter may be discharged.

FIG. 40. Funaria. A, young sporo-
carp ; c, capsule or sporogonium. B,
moss plant with attached sporocarp
mature; s, stalk of sporogonium ; /,
capsule; g, leaves of the moss plant.
C, section of a sporogonium ; s, layer
of cells which develop the spores.

,-, -r> When the spore germinates

, it produces a little filamentous
FIG. 39- ,4, section of stem of __£_ __Al_ ___^_._^ ,_„ __^__ x___.,__

moss bearing archegonia ; B, outgro wth which branches freely

open neck of archegonium ; on the moist Soil, and which

resembles very closely a fila-
mentous alga. The moss plants
originate by the development of buds upon this out-
growth, which is known as a protonema.

C, archegonium highly mag-
nified. (After Sachs.)

EVOLUTION OF THE LAND PLANT 103

The altered conditions of their life can now be seen to
lead to a very great change in the life history of plants.
The aquatic forms with their comparatively simple body
and the process of fertilisation depending on the power
of the sperm to swim to the ovum proved exceedingly
unsuitable for life on land, and very soon a great develop-
ment of the sporocarp began, and it gradually assumed
the form of a self-supporting plant — the sporophyte.
From this point upward the tendency of evolution was
to diminish the plant which bore the ova and sperms till
there was hardly anything of it except these repro-
ductive cells, and at the same time to make the sporo-
phyte more and more important, till it became far the
most highly developed, and of infinitely larger size than
what was left to represent the original ancestor.

We find this change well established in the ferns and
the plants which are allied to them. The ova and
sperms are produced on a thin plate-like body of about
half an inch in diameter. This lies upon the soil and is
attached to it by rhizoids. The antheridia and arche-
gonia are borne upon its lower surface and fertilisation
is accomplished by free-swimming sperms. The plant
is known as the prothallus ; it is made up of cells which
are much alike throughout. The sporocarp arising from
it has been replaced by a large plant, showing root, stem,
and leaves. We do not know the stages by which so
large and well-organised a structure has been developed
from the original sporocarp, but it represents the latter
in its place in the life history of the fern. So important
has it become, and so great in comparison with the
prothallus, that it has come to be called the " plant,"
leaving the ovum-bearing plant to be regarded as the
" prothallus of the fern."

We see that with the establishment of terrestrial
habit, the life history of the plants thus became quite
revolutionised. The large tree of the land flora does

104 BOTANY

not correspond with the large seaweed, but with a parti-
cular reproductive structure to which the latter gave rise.
The fern plant differs a good deal from the seed-bear-
ing plants which we have examined in the earlier
chapters. It has- as a rule only underground stems
which are known as rhizomes; each bears very few
leaves at or near its apex. The stem grows horizontally
under the surface of the ground, reacting to gravitation

FIG. 41. Prothallus of fern. X5-

in a way unlike either stem or root of the flower-
ing plant. The leaves emerge from the soil generally
rolled up in the form of a shepherd's crook, in conse-
quence of the great growth of the under surface. They
soon straighten themselves as the growth of the upper
surface becomes vigorous, just as in the case of the leaf
in the bud of the flowering plant. They are then found
to be very much divided, except in a few cases. Roots
are given off from the rhizome in large numbers.

The structure of the fern rhizome differs in detail
from that of either type of flowering plant we have

EVOLUTION OF THE LAND PLANT 105

described. There are the three systems of tissue,
dermatogen, periblem, and plerome, but they are not so
clearly distinguishable. They originate by divisions in
a single large pyramid-shaped cell at the apex — the so-
called apical cell, which cuts off segments of itself by
walls parallel in turn to each of its sides except the
outside one. These segments divide later to make up
the mass of cells at the end, in which the differentiation
into the three regions spoken of takes place.

The plerome does not give rise to a single solid or
hollow cylinder, but to one in which the conducting
strands form a cylindrical network. The strands to
the leaves leave the network at the margins of the
meshes. The strands are composed of wood and bast
as in other cases, but in their arrangement the bast
usually surrounds the wood completely. There is no
cambium and no increase in thickness occurs.

The leaves are something like the leaves of a dicoty-
ledon in structure, but most of the internal tissue re-
sembles the spongy tissue rather than the palissade.
The epidermis contains chloroplasts.

The structure of the root resembles that of a dicoty-
ledon very closely, but there is no provision for any
increase in thickness. The pericycle is several layers of
cells in thickness.

The fern bears its spores in little cases known as
sporangia. In our common ferns these are grouped to-
gether on the under sides of the leaves in little patches called
sori (Fig. 42). Each sporangium contains a number of
spores. Each spore on germination produces a prothallus.

We found it impossible to say how such a plant as
this has come to be formed in the place of the sporocarp
of the mosses. We find almost as great difficulty in
tracing the formation of the flowering plants from
plants having the degree of development of the ferns.

We may well realise that with the gradual increase of

106 BOTANY

complexity of the sporocarp it came to have an inde-
pendent existence, and that when it had achieved this,
it became an erect plant, the conditions we have already
alluded to making this necessary. At some stage or
other in the development a difference arose among the
prothalli arising from the spores, some giving rise to
sperms only while the others only produced ova. Gradu-
ally the change spread to the spores themselves, those
giving rise to sperm-bearing prothalli remaining small,

FIG. 42. Section of sorus of fern showing sporangia
covered by an indusium. X5o. (After Kny.)

those producing ova-bearing prothalli becoming much
larger. The prothalli also changed. Those from the
small spores became in some cases filamentous, and in
all extremely minute, consisting of little more than the
organ giving rise to the sperms. Those from the large
spores only protruded partly from the ruptured spore,
and came to be developed almost entirely inside it, the
spore splitting its coats but slightly. Such prothalli
developed very few archegonia.

These forms are still represented to-day by the
Selaginellas, a group classed among the fern-like plants.

EVOLUTION OF THE LAND PLANT 107

As the spore-bearing plant or sporophyte grew larger
and by its erect position became carried away from the
ground, the separation of the two kinds of prothalli
developed from the spores rendered fertilisation by
means of free-swimming sperms increasingly difficult.
The larger spores,
too, tended to stay
longer and longer nrch
in their sporangia,
so that the time for
fertilisation became
shortened. This led
to the establish-
ment of a new
method of securing
fertilisation which
we find exhibited
by the flowering
plants. The pro-
thalli - bearing ova
became entirely en-
closed in the large
spore, and those
bearing sperms be-
came entirely fila-
mentous. Instead
of the sperm
travelling to the ovum, the small spore itself was brought
by various ways to the immediate neighbourhood of the
large one, so that the prothalli were developed in close
proximity to each other. The filamentous prothallus of the
small spore bored its way into the surroundings of the large
spore and the latter remained altogether in its sporangium.
Fertilisation consequently came about by means of a
tubular outgrowth of the little spore, instead of a free-
swimming sperm. If we compare the two methods, both

FIG. 43. Germination of megaspore of Selagi-
nella, showing prothallus almost entirely
inside the spore, which has opened at the
apex, arch, archegonia; oos, ovum; em,
young embryos at different stages of de-
velopment.

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of which still persist, we find that in the latter all the
structures are exposed freely ; in the former the greater
part are embedded deeply in other parts of the plant.

We shall return to this subject in connection with
the mechanism of the flower in the higher plants.

CHAPTER XIV

REPRODUCTION OF FLOWERING PLANTS —
VEGETATIVE PROPAGATION

THE last requirement of the plant we must consider is
the power of reproducing its species.

There is a good deal of variety in the ways in which
this is possible ; some methods consist in separating
certain parts of the ordinary plant body which after a
time grow into new plants ; in others special reproduc-
tive cells are produced. The first method is often
spoken of as vegetative propagation. The parts which
can be spared for it in different plants are a good deal
modified and have come to be considered as separate
organs. They include modified stems, leaves, or roots.

We find such structures include a part which is cap-
able of growing — some kind of bud — and a store of food
for the nutrition of the shoot which arises from it.

The most easily observed of the modified stems is
known as the tuber. This is a stem or branch which
grows under the ground instead of above the surface.
It consists of a few internodes, which become greatly
swollen and filled with starch and protein. The leaves
are only to be seen with the help of a lens; they
are minute scales, which never develop further. The
buds are in their axils and are generally several in
number, so that a tuber can put out several shoots, each
of which may grow into a new plant. The most familiar
instance of a tuber is afforded by the common potato.

REPRODUCTION 109

Another form of underground stem which may be
included here is the rhizome of such plants as the iris.
It grows partly under the surface of the ground, but its
upper side often protrudes and becomes green. It
never grows vertically into the air, but its terminal bud
sends a shoot upwards which bears the foliage leaves
and flowers. It is swollen and filled with food like the
potato, but it does not become detached as the tuber
does. It is in fact the main stem of the plant and pro-
duces only a single bud, instead of a number.

Two other forms of propagating organ are the bulb
and the corm. The bulbs are very large buds, a rela-
tively small conical stem being covered with a large
number of scaly leaves, the inner ones becoming very
succulent and containing food, chiefly sugar, for the
young plant, into which the growing apex will develop.
The outer leaves remain dry scales and are only pro-
tective to the succulent interior. The onion is a good
example of the bulb.

The corm consists of a few internodes of an under-
ground stem. It is solid but resembles the bulb in
being clothed with dry scaly leaves. There is a bud at
its apex which is like the most internal part of the bulb
of the onion. An example is afforded by the crocus.

Roots which are modified for reproductive purposes
are shown by the dahlia. They swell and store food
after the manner of the tuber, but they do not develop
buds. They give up their contents to a shoot put out
from a portion of the stem of the original plant.

Vegetative propagation is very commonly employed
by gardeners, by means of cuttings. A piece of a young
stem, with a few leaves and buds, cut from the parent
will, when planted, often develop roots from the cut
surface and so establish a new plant. Some plants pro-
pagate themselves naturally in this way ; any detached,
injured portion, when left upon damp soil, will put out

no BOTANY

adventitious roots and so grow into a plant. The power
of putting out adventitious roots is often used by plants
for this purpose. Other familiar examples are afforded
by the runners of the strawberry, the suckers of the
raspberry, the stolons of the gooseberry and other plants.

The runner is a lateral branch which grows along the
surface of the ground and puts out adventitious roots
at its nodes. In the case of the strawberry each runner
usually has two nodes, one terminal and one half way
along its length. Two new plants arise on each in con-
sequence, and when they are established the internodes
connecting them with the parent die.

The stolon is much like the runner ; arising as a lateral
branch above the ground it bends down and on reaching
the soil it puts out adventitious roots and becomes
detached from the parent. The sucker arises from the
underground portion of the stem and growing horizon-
tally for a time ultimately turns upwards and emerges
into the air — becoming then detached as in other cases.

These methods are purely vegetative and do not
involve the production of any form of specialised repro-
ductive cell.

CHAPTER XV

THE INFLORESCENCE AND THE FLOWER

As we traced the development of the reproductive
organs from the seaweeds upwards we found that two
special kinds have become constant, the ova and
sperms on the one hand and the spores on the other.
We also learned that the plant form which produces the
spores has become the conspicuous plant, with its roots,
stems, and leaves ; while those which give rise to the ova
and sperms have dwindled away in dimensions till they
no longer have an existence apart from the spore itself.
We will study first the reproductive processes of the

THE INFLORESCENCE AND THE FLOWER in

form which produces spores — the so-called sporophyte
phase of the plant's life history.

We have pointed out that in the flowering plants two
kinds of spore are produced. The little ones resemble
the spores of the ferns and mosses ; they are small cells
with thick walls, usually spherical in shape. They are
liberated from the sporangia in which they are de-
veloped and germinate after their removal. The large
spores are produced singly in each sporangium and
never escape from it. The sporangium is a bulky struc-
ture and consequently the spore need not develop the
thick wall found on the little spore.

We have now to see where these spores occur and
what is the story of their behaviour.

In most ferns we saw that the sporangia occur in
patches on certain places on the ordinary leaves. In
some, however, they are found on very specialised out-
growths, generally .spoken of as modified leaves. In the
flowering plant the sporangia are usually collected at
the end of special shoots which are known as flowers.
A special system of the branching is set apart for their
production. It may consist of very many branches and
may be very complicated, hence it is usually considered
separately under the name of the inflorescence. Its
ultimate branches are known as flowers, the leaves in
whose axils they are found are termed bracts.

A very common form of the inflorescence is that in
which the main axis continues to increase in length and
bears a succession of flowers as it grows on, so that the
youngest flower is nearest the apex. This corresponds
to the indefinite method of vegetative branching. The
inflorescence is called a raceme. There are many modi-
fications of it on all sides : if the flowers have no stalks
it is called a spike; if they all reach the same level
through the older stalks growing longer than the younger
it is a corymb ; if the axis is so short that all the flowers

U2 BOTANY

appear to start at the same point it is an umbel (Fig. 44) ;
if it is itself branched it becomes a panicle.

A fundamentally different form is that in which the
main stalk is at once terminated by a flower and other
stalks grow out from under it to bear similarly a flower

FIG. 44. Umbellate inflorescence of ivy. (After Marshall Ward.)

at the end of each ; we have here what is called a cyme.
It corresponds to the definite method of branching of
the vegetative parts. As in the case of the raceme
there are many varieties of this kind of inflorescence.

A very noticeable modification of the raceme is the
form in which the apex is not elongated or conical, but
spreads out into a flat receptacle on which large numbers
of small flowers or florets are arranged so that the
youngest are in the centre and the oldest ones round the

THE INFLORESCENCE AND THE FLOWER 113

circumference. This is known as a capitulum. It is
generally surrounded by a number of bracts, which form
what is called an involucre.

In its most primitive form the flower probably con-
sisted of an axis on which the sporangia were borne, and
the latter were of only one kind. In many cases, most
likely arising later, we find this axis bearing two sporan-
gial series of outgrowths, one for the production of each
kind of spore, and this arrangement gradually became
widespread, the large spores being developed in the
series nearest the apex and the small ones in the lower
of the two. As each series stood in a circle round the
axis, all its parts arising at the same node, they received
the name of whorls.

In the more perfect forms of flower which have been
developed as time has gone on (Fig. 45) we find, besides
these two whorls of spore-bearing leaves, two other
whorls growing below them. These two constitute the
perianth of the flower. A leaf lower down still, in the
axil of which the flower arises, is known as a bract.

The perianth of the flower is formed then by two
series or whorls of leaves. The outer ones are green
and often sturdy in their texture, and they protect
the young flower while it is in
the condition of a bud. They
are known as sepals, and the
collection of them forms the
calyx. The inner whorl is
usually made up of highly
coloured leaves which serve to
make the flower conspicuous.
They are called petals, and the FlG' 45' '
collection is termed the corolla.
These whorls frequently have their sepals or petals joined
together for part or even the whole of their length.

The whorls which bear the sporangia are distinguished

H4 BOTANY

from the rest as the sporophylls. They are usually
considered to be modified leaves, as the name indicates.
The outer whorl consists of bodies very unlike leaves in
appearance. Each shows a slender stalk or filament
bearing at its top a swollen head or anther. The whole
club-shaped organ is called a stamen. Each anther
contains a group of four chambers, which are the
sporangia, and inside them are the small spores. They
are commonly spoken of as pollen sacs, containing pollen
grains. These were the old names applied to them
before their true nature was understood, and they are
retained still as a matter of convenience.

Like the sepals and petals the stamens may be joined
together in various ways, or they may be free. They
may grow from the flower stalk, or they may appear to
spring from the calyx or the corolla. These appearances
are due to curious irregularities in the growth of the
parts of the flower.

The sporophylls of the final whorl are known as
carpels, and together they form the pistil. In shape the
carpels resemble a leaf folded on the mid-rib till the
edges meet and become united, so that a cavity is
formed inside them. This cavity is the ovary. It is
more usual to find the carpels united by their edges to
form a large ovary, which is often divided up into several
chambers. The sporangia are found in the interior of
the ovary, attached usually to the edges of the carpels.
They were formerly called ovules, and the name is still
often used (Fig. 46) . The ovule or mega-sporangium is a
fairly substantial structure. As the spore never leaves
it but produces its prothallus in its own interior while
still within it, and as the young embryo plant is developed
on the prothallus inside the spore, it is clearly an organ
of the greatest importance. It consists of a mass of
small cells called the nucellus, and is covered by two
membranes or integuments which also are many-layered

THE INFLORESCENCE AND THE FLOWER 115

and strong. At its upper end the integuments do not
cover it but leave a little aperture, the micropyle. Each
ovule contains a single thin-walled spore, which often
occupies a very large space in its interior. It is often
called the embryo sac,
from the fact that the
embryo is developed in
its interior.

The ovary is only the
basal part of the carpel
or the pistil. There is
always a sticky apex to
it, which is the stigma.
The stigma is usually
placed at the top of an
elongated part of the
pistil, seeming to arise
from the top of the ovary
—this is the style.

The pistil is always the
terminal whorl of the
flower. Some flowers,
however, owing to a
curious mode of growth
of the axis, which be-
comes concave and comes to surround and often to cover
in the carpels, appear to bear their stamens and perianth
above the ovary. The latter is then termed inferior.

Many flowers are not provided with all these parts.
It is not at all uncommon to find that the corolla is not
developed. In such cases it frequently happens that
the calyx is not green but brightly coloured. In many
monocotyledons both calyx and corolla are coloured so
similarly that it is difficult to distinguish between them.
The flowers of some of our forest trees do not possess a
perianth at all, and in the flowers of others only one

Fig. 46. Section of an ovule, mac,
megaspore or embryo -sac; oos,
ovum; pt, pollen tube.

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BOTANY

series of sporophylls is present. Those which bear
stamens only are known as staminate flowers ; those
which possess only carpels are called pistillate. Such
flowers are generally very small and inconspicuous.

Another type of flower altogether is found in the
group of plants called Gymnosperms, which is repre-
sented in this country by the fir trees and their allies.

FIG. 47. A, twig of fir tree bearing a young female cone; B, twig bearing
several male cones; C, ovaliferous scale from A showing two ovules
on the under surface. (After Scott.)

In most of these the sporophylls are arranged in a close
spiral round an axis and form the structures known as
cones. The fir tree itself bears two kinds of these cones
(Fig. 47) each bearing one kind of spore. The smaller
cones are composed of a large number of very small
leaves arranged spirally round the axis of the flower,
covering or overlapping each other very closely. On
the back of each leaf or sporophyll are two sporangia, or
pollen sacs, each containing a large number of micro-
spores or pollen grains. Larger cones are developed in
connection with the production of the megaspores. The

POLLINATION 117

general arrangements of the cones are similar to those
of the smaller ones. There is a central axis round which
the leaves or sporophylls are spirally arranged. Each
sporophyll has on its inner face a flattened outgrowth
which in some cases becomes larger than the sporophyll
itself. On its upper side this so-called ovali/erous scale
bears two sporangia or ovules. There is no closing up
of the sporophyll to form an ovary — hence the name of
the group gymnosperm, meaning naked seed. Each ovule
has much the same structure as that described above, but
there is only one integument and the micropyle is larger.

CHAPTER XVI

POLLINATION AND ITS MECHANISMS — FERTILISATION

THE separation of the spores and the diminution of the
phase of the plant which results from their germination,
consisting of little more than the ova and the sperms,
have made it impossible for fertilisation to take place by
means of free-swimming sperms. To ensure its occur-
rence it has become necessary to secure that the two
spores producing ova and sperms respectively shall be
brought close to each other, so that, when they germinate,
their prothalli, now so rudimentary, shall be able to
meet, in order that the sperm may make its way to the
ovum. This has been effected by the transport of the
pollen grain — the small spore — to that part of the flower
which bears the large spore or embryo sac. This trans-
ference of the pollen is spoken of as pollination. In the
fully organised flowers of Dicotyledons and Monocoty-
ledons the pollen grain is deposited on the stigma — in
the cones of the Gymnosperms it reaches the ovule or
megasporangium itself. The problem of pollination
presents many interesting features which we may now
briefly examine.

n8 BOTANY

As in each perfect flower both spores are produced, the
problem at first sight appears to involve only the transfer-
ence of its pollen grains to its stigma. In many cases this
is all that takes place, but simple as the method is, its
occurrence is the exception rather than the rule, for much
stronger and healthier plants -are yielded when the pollen
from one flower is made to fall upon the stigma of another.
The former simpler process is termed self-pollination; it
was probably the most primitive, but became gradually
superseded by the latter, known as cross-pollination.

The necessity for pollination and the advantages
presented by the crossing have led to the development
of various mechanisms in flowers to bring it about.
Here, perhaps more than in any other case, do we
recognise the difficulties entailed by the stationary
situation of the plants. The small spores though set free
are non-motile and must be carried to the stigma by
some external agency which is not under the control of
either plant. Hence it is that we find perhaps more
adaptations or modifications of structure in connection
with this function than any other.

The transport of the pollen at the outset must have
been brought about by the physical agents of nature,
water in the case of aquatic flowering plants, wind in
that of those of terrestrial habit. In both these cases the
chances of the liberated pollen being floated or blown to
the stigma are not great. Hence the pollen in such
flowers is always produced in large quantities. Aerial
transport is further aided by the production of very
light, dry, relatively smooth pollen, in some cases aided
by mechanical modifications of the wall, expansion
into bladders, etc. The stigmas in plants which are
pollinated in this way are often divided a good deal,
becoming in the case of the grasses quite feathery, so as
to offer as much surface as possible to entangle the
pollen. The surface of the stigma often bears a velvet-

POLLINATION 119

like pile of short hairs, and usually secretes a sticky,
sugary excretion — features which subserve the same end.

This so-called anemophilous pollination is uncertain
and wasteful and has in many cases been superseded by
the intervention of insects. Hence we find developed
the colour, fragrance, and other attractions which
flowers so generally possess. Colour and fragrance cause
conspicuousness, and appeal to insect visitors. The
latter seek, of course, more substantial benefits than
these, regarding the flowers as the seats of supplies of
nutritive substances which they require. While they
rifle the flowers of both pollen and honey, they inad-
vertently serve their turn by transferring the pollen
from the stamens of one flower to the stigma of another.

The relations that have thus come to be so widely
established have probably grown up very gradually and
in an infinite variety of ways. Different insects visit
different flowers and the influence of the particular kind
of insect visitor explains the manner of modification
which the particular flower has undergone. The modi-
fications are almost infinite in variety and we can only
here deal with them in a very general way.

The earliest modifications, which were only slight,
were associated with the discovery by certain lowly
insects that pollen itself formed nutritious food. The
flower was widely open, the perianth leaves spreading
symmetrically round the axis below the stamens, and
the latter opened and let their pollen fall. A slight
change of colour, probably from green to yellow or
white, made the flowers sufficiently conspicuous, and
the visits of the insects followed.

But a further attraction was afforded later by the
formation and storage of honey in the flower. It is
impossible to discuss this subject in detail as it has
exhibited such infinite variety of modification. The
storage of honey led to the development of pocket-like

120 BOTANY

bodies, slippers, or spurs, either on the receptacle or the
perianth leaves. Irregularity of form of the calyx or
corolla was thus caused. Markings on the individual
sepals or petals followed, which directed the attention of
the insect, usually a wasp or bee, to the hidden store.
The honey became hidden in such a way that to reach it
the visitor brought some definite part of its person into
contact with the stamens, and on visiting another
flower touched the stigma with the same region. The
influence of colour and of fragrance were brought to bear
upon the insect visitors with similar results, the whole
mechanism of any particular flower being correlated
with the habit of some appropriate insect.

The mechanisms of some flowers at the present time
are even more complete than this. As cross-pollination
is preferable to self-pollination, the flower is adapted
to some visitor to secure it; but as self-pollination is
better than none at all, if the insect mechanism should
fail to act, the stigma is in some way brought into
contact with the stamens of the same flower.

The apparent ease with which self-pollination of
almost every flower can be brought about has led in
many cases to a peculiar modification to secure the
advantages of crossing. This consists in the maturing
of the stamens and the stigmas of a flower at different
times, so that if self-pollination should take place there
would be no result therefrom. This condition is known
as dichogamy ; flowers whose stamens mature before
their carpels are said to be protandrous ; those in
which the condition is reversed are proterogynous.

It is of course obvious that cross-pollination is the
only possible method in the cases of those plants which
bear pistillate and staminate flowers.

The cones of the fir trees are pollinated by wind.
When the cone is ripe its leaves separate a little from
each other so that the pollen grains, blown in large

POLLINATION 121

numbers, can be carried down into the heart of the cone
to the bases of the scales on which the ovules are seated,
as already described. There is no stigma and the
ovules are exposed; hence the pollen grain is dropped
upon the micropyle of the ovule, down which it is
drawn into a little space just above the body of the
ovule itself and inside its integument. This little
cavity is known as the pollen chamber.

In all cases these mechanisms bring the two spores
very near to one another. In the Gymnosperms they
are separated by only the substance of the upper part of
the ovule, whose spore or embryo sac lies quite near the
upper end. In the other flowering plants the two
spores, the pollen grain and the embryo sac, are separ-
ated by the length of the style, the chamber of the
ovary, and the upper part of the ovule.

The next steps in the life history are the germination
of these two spores. We have already seen that the
result of the germination is the production of little
besides reproductive cells, the vegetative parts of the
prothalli being very small indeed. In the Gymno-
sperm the megaspore or embryo sac becomes filled with
a cellular prothallus — the endosperm — at the upper
end of which several archegonia make their appear-
ance, each containing one ovum. The pollen grain or
microspore puts out a tube which bores its way through
the substance of the upper part of the ovule till it
reaches the embryo sac and comes into contact with its
wall. As it grows it produces two sperms in its interior ;
these are for the most part amorphous pieces of proto-
plasm, though in a few cases each bears a band of cilia.
When the tube reaches the embryo sac the walls at the
point of contact dissolve and the sperms pass through
and each can fuse with an ovum in one of the archegonia.

In the angiospermous plants the process is similar,
but there are characteristic differences. The tubular

122 BOTANY

outgrowth of the pollen grain — the pollen tube — pene-
trates the style, reaches the cavity of the ovary, makes
its way to the micropyle of an ovule -(Fig. 46). During
its progress it develops two sperms, which are quite free
from cilia or motile organ of any kind. The embryo sac
meanwhile develops its prothallus, which consists of a
group of cells at either end and a large nucleus in the
centre. One of the cells at the apical end is the ovum.
The fusion of the walls of the pollen tube and embryo
sac takes place and the two sperms enter the megaspore.
One fuses with the ovum, the other with the large
nucleus in the centre of the sac.

These fusions constitute what is known as fertilisation.

CHAPTER XVII

FORMATION OF THE SEED AND ITS MIGRATION—
THE FRUIT

AFTER a very short interval further development begins.
The ovum in each case clothes itself by a cell wall and
certain complicated divisions take place which we cannot
consider in detail here. They result in all cases in the
formation of an embryo, or young sporophyte, which
remains inside the embryo sac. In the Gymnosperm it
is surrounded from the first by the prothallar tissue;
in the Angiosperm its development is accompanied by
the formation from the fertilised nucleus of the embryo
sac of a similar mass of cells, also known as the endo-
sperm. These changes are accompanied by considerable
growth of the ovule. Its integuments not only increase
in size but become chemically altered, generally dry and
hard. The embryo sac usually grows at the expense of
the substance of the ovule till it has absorbed the whole
of the latter except the integuments. In some cases, as
the bean with which we began our study, the young

THE FRUIT 123

embryo absorbs the contents of the cells of the endo-
sperm and so fills the embryo sac. In others, as in the
castor oil seed, a good deal of endosperm remains ; in
yet others part of the ovule may have escaped absorp-
tion by the growing embryo sac. In all cases there soon
comes a period when the growth and development of all
these parts stops and the resulting structure, now
become the seed, enters on a more or less prolonged
period of absolute rest. This is the period during which
alone the migration of the species is possible, as by
various means the seed is carried from the parent plant.

We called attention at the outset to the fact that the
life of the individual plant has to be spent absolutely in
one spot, that at which it is rooted to the ground. How
disadvantageous this is to the plant and what an endless
series of struggles against the difficulties it involves, has
been one of the principal lines of thought throughout
our study. The final difficulty meets us when we con-
sider the production of offspring. How can they
possibly flourish or even survive when their parent is
hampered in this way ? The solution of the difficulty is
found in the provision which is made for the wider dis-
persal of the reproductive bodies, mainly seeds.

The fact that most plants produce many seeds and
that each seed after a period of rest produces a new
plant makes it imperative that adequate means of dis-
persing seeds shall be found, or clearly the problem of
the maintenance of the species would not be solved.
We must turn, therefore, to consider this matter closely
and to study certain new structures which are immedi-
ately concerned with it.

The migration of seeds is brought about in an almost
infinite variety of ways, each species having its own
mechanism. In most cases it is associated with the
development of a new structure, the fruit.

While the changes are taking place by which the

124 BOTANY

ovule is transformed into the seed, the stimulus to
growth, which the act of fertilisation of the ovum ad-
ministered to the latter, affects also the parts in its
neighbourhood. We have seen the embryo sac enlarg-
ing and the integuments of the ovule developing into the
testa of the seed. The ovary, too, resumes development
and increases in size, often enormously, by a large pro-
duction of succulent cells. When its full dimensions
have been attained the nature of the cells and their
contents changes. In some cases they become woody,
hard, and dry; in others succulent and charged with
sugar and various flavouring matters. These latter
changes make up the process known as ripening. The
enlarged dry or succulent wall is now known as a peri-
carp, and with the seeds which it contains it constitutes
the fruit. The latter is the modified wall of the ovary,
much changed and developed by the processes of growth
which have followed fertilisation. The fruit is thus a
mechanism which is concerned chiefly with the problem
of the dispersal of the seeds ; it plays also an important
protective function during the maturing of the seeds,
though this is not its main purpose.

In some plants the renewed development does not
stop at the ovary or the carpels. Other parts of the
flower are involved, generally the axis or receptacle, as
in the apple, strawberry, etc. In some cases the leaves
of the perianth also become succulent.

In yet other cases the development affects simul-
taneously all the flowers of a closely arranged inflores-
cence. All become succulent and fused together to
form a single fruit, as in the pine-apple and the fig.

There are thus many intricacies of development and
the construction of the fruit is often complicated. Its
purpose is to distribute the seeds over a wide area.

The lines of the development of the fruit are in
almost all cases the same at the outset. The parts

THE FRUIT 125

which ultimately form it grow, and the new material
is at first succulent, being made of ordinary thin-walled
cells. When the full dimensions are attained this
succulent tissue changes and assumes the characters of
the ripe fruit. Two main departures may be noticed —
in the first the succulence becomes more pronounced,
the cells more juicy, and their contents changed by the
deposition of sugar, the development of particular
flavours and fragrance, or of other less attractive
chemical substances. We get thus a class of fruits
which appeal to the animal world and whose fate is
probably to be eaten. The seeds which are developed
inside them are usually furnished with a hard testa or
skin, so that they may escape injury in passing through
the animal's body. Succulent and hard parts thus go
together, though their soft and their resistant parts
have not in all cases the same origin.

In the second departure from the original softness we
find a tendency to hardness and dry ness throughout.
Sometimes the fruit becomes woody — more frequently
dry and papery, or resembling cork in its general pro-
perties. This type of fruit is associated with other
means of dispersal : often endowed with a kind of explo-
sive mechanism, so that rupture of its walls is followed
by a jerking of the seed for some distance; often
furnished with some means of transport, such as hooks,
that may attach themselves to passing animals, or
floats of various kinds that may buoy up the fruit in the
air and enable it to take advantage of currents of wind.

In some cases the seeds themselves are furnished
with one or other of these mechanisms. It is, indeed,
often difficult to distinguish between small fruits and
seeds when the latter are thus endowed. In such cases
they always escape from the fruit before dispersal.

Various methods of classifying fruits have been
adopted, and a somewhat ponderous nomenclature has

126 BOTANY

arisen, which, however, is comparatively unimportant.
The important consideration is the need of the plant;
the various ways in which it is supplied may advan-
tageously occupy our thoughts rather than the duty of
finding a special name for each variety. It has, how-
ever, become the custom to speak of a fruit -which has
been developed from the carpel or carpels only of a
flower as a true fruit. One into whose composition some
other part of the flower enters, usually some part of the
floral axis, is known as a spurious fruit, or pseudocarp.
The distinction is in many cases very difficult to make
and from our point of view is quite unnecessary. Such
fruits as the pine-apple and fig, which are the product of
whole inflorescences and not of single flowers, may be
distinguished as aggregated fruits.

Among the succulent fruits the most prominent is
the berry, which exists in several varieties. It is seen
in its simplest form in the grape, while varieties of it are
illustrated by the gooseberry and the orange. The
hard parts of this mechanism are the walls of the seeds
the berries contain. Another succulent fruit is the
drupe, in which the middle layer of the fruit wall be-
comes pulpy while an inner layer becomes hard and
constitutes the stone. A collection of very small
drupes upon a dry receptacle is met with in the different
kinds of raspberry and blackberry.

Succulent fruits in which the growing axis of the
flower is concerned are met with in two conspicuous
forms. The strawberry has a very succulent convex
axis on which the fruits appear as small hard bodies,
containing, however, the seeds inside their hard coats;
in the apple and its allies the succulent axis has become
concave and has grown up round the carpels and enclosed
them. This fruit is called a pome. The carpels them-
selves are cartilaginous in texture, or in some forms
bony — as in the hawthorn.

THE FRUIT 127

Dry fruits show greater variety. In some cases they
consist of one carpel only, many such fruits arising from
a single flower, as in the buttercup. This single carpel
may remain permanently closed, the seed being set free
only by its decay, or it may open along its front or both
front and back margins. In the latter case many seeds
are generally found inside. In other cases the carpels
while in the flower are joined together in their develop-
ment. When such a pistil is cut across it very generally
shows as many cavities as there are constituent carpels.
Sometimes the walls of the latter do not meet in the
centre, so that there is but one cavity. The seeds are
attached with hardly any exceptions to an outgrowth
of the edge of the carpellary leaf which constitutes a
placenta. There is a good deal of variety in this form
of fruit, as it varies with the number of carpels and
the ways in which they are united. Such fruits are
commonly called capsules. A singular variety of the
capsule is seen in the fruit of the wallflower and its
allies. Two carpels are joined together by their edges
and at the lines of union where a placenta can be seen
a membrane is developed quite across the cavity.

Besides the dry fruits which open and let the seeds
out and those which retain the latter permanently,
another form is met with, consisting of several united
carpels, which separate from each other at maturity,
but each carpel continues to hold its seed. These
fruits are called schizocarps.

A modification of the polycarpillary fruit is seen in the
nut. It has two or three constituent carpels and the
young fruit shows as many cavities as carpels. In
development, however, some or all of the partition walls
become dried up and disappear, so that there is only one
cavity in the adult fruit and this seldom contains more
than one seed. It is associated with a very hard
woody wall. All these modifications of structure show

128 BOTANY

particular adaptations to the mode of dispersal which the
plant has adopted and should be studied mainly from
this point of view.

Small fruits and seeds are blown about by the wind
or carried by birds. Some are furnished with buoyant
accessories, which enable them to remain in the air
for considerable periods. In some cases the fruit splits
open with explosive violence and the seeds are jerked
to some distance. Often small fruits or seeds are
carried long distances embedded in mud, into which
they have fallen and which has subsequently become
attached to the feet of birds or other animals. Larger
fruits become attached by means of hook-like append-
ages to the coats of similar wanderers. Many fruits are
capable of floating long distances in water — are indeed
often furnished with special mechanisms to enable
them to float. Indeed, the means of dispersal are so
numerous and in many cases so intricate that it is im-
possible here to do more than indicate the merest out-
lines of the subject. The mechanisms are easy to study
and every plant one meets affords an example which
will well repay investigation.

THE TEMPLE PRESS, PRINTERS, LETCHWORTH

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