BIOLOGY. PART I. BOTANY.
PLANTS AND ANIMALS—STRUCTURE OF PLANTS—FLOWERING PLANTS—THE STEM—THE
LEAVES—FORMS OF LEAVES.
BIOLOGY is derived from the Greek word _Bios_, “life,” and _logein_,
“to speak,” and constitutes the science of ORGANIC LIFE. This science
is divided into two branches: BOTANY, relating to the life of plants;
ZOOLOGY, to the animals.
Plants, then, are living things, and as we proceed we shall find them
born, or “germinating,” growing up as young plants, maturing as adults,
and finally dying, and their particles resolving into their elements.
There is more than one application of the text, “Man is but as a flower
of the field.”
In the GEOLOGICAL section we noticed the progressive stages of the
vegetable creation, and if we turn back to those pages wherein the
various epochs of the earth’s formation are enumerated, we shall see
how plant-life developed. Thus we find in the Cambrian the first traces
of vegetable life in the weeds of primeval seas. The Silurian strata
and the Devonian furnish us with many fossils of marine algæ, and if
we examine the succeeding periods we shall find a progressive increase
and development; pines and tree-ferns in the sandstone, and most of the
plants (by which term we include all varieties) were different from
those at present existing in the earth.
[Illustration: Fig. 735.—Branch of the oak.]
We spoke of climate lately, and referred to the vegetation having an
influence upon it. The same is true of the effect of climate upon
vegetation. The conditions of plant-life depend upon climate, as it
partly depends upon plant-life. But of all the necessary conditions
the first created thing is the most necessary—light. Without light the
plant is nothing.
Plants have many points of similarity with animals. They live, they
possess organs, their compositions contain similar substances, such
as carbon and albumen, and close chemical analyses have found the
existence of the elements oxygen, nitrogen, hydrogen, and carbon in
animals and plants. Therefore water must play a conspicuous part in
all. Professor Huxley puts this question in his usual clear fashion. He
says:—
[Illustration: Fig. 736.—The pine.]
“It is a very remarkable fact that not only are such substances as
albumen, gluten, fibrin, and syntonin known exclusively as products of
animal and vegetable bodies, but that every animal and every plant at
all periods of its existence contains one or other of them, though in
other respects the composition of living bodies may vary indefinitely.
Thus some plants contain neither starch nor cellulose, though these
substances are found in some animals; while many animals contain no
horny matter and no gelatine-yielding substance. So that the matter
which appears to be the _essential_ foundation of both the animal and
the plant, is the _proteid_ united with water, though it is probable
that in all animals and plants these are associated with more or
less fatty and amyloid (starchy and saccharine) substances, and with
very small quantities of certain mineral bodies, of which the most
important appear to be phosphorus, iron, lime, and potash. Thus there
is a substance composed of water, _plus_ proteids, _plus_ fat, _plus_
amyloids, _plus_ mineral matters, which are found in all animals and
plants. When these are alive this substance is termed PROTOPLASM.”
[Illustration: Fig. 737.—The fir.]
We have taken the liberty to extract the above paragraph, as it
expresses in a few words, and very clearly, the common origin of plants
and animals. We will now consider the conditions of plant life. Heat,
light, and moisture are the principal necessaries, with of course air
and certain earthy matter. Some plants, like some few animals, live
in darkness, such as truffles and fungi, as do cave-fish and bats.
But this is the exception, and the sense in which plants (or animals)
can exist without light is a very restricted one, and only to be
sustained at the expense of the plant material, which must originally
have been derived by the action of light. Light, therefore, is the
great “producer.” It gives life to plants upon which animals feed, and
therefore light is in one sense the beginning of all things. We can now
understand why light must have been created first.
Many interesting experiments can be made to observe the effect of
darkness and different coloured light (transmitted through coloured
glasses) upon plants; and it will be observed that although the leaves
may not develop the natural green tint, the flowers will exhibit their
usual colour. One effect of light upon plants is to make them green.
We all admire the beautiful green of the spring leaves, and the
freshness of the colours of the trees and grass. But if we pluck up
a plant its root is not green. Why then is the cleaned root not as
green as the upper portion?—Because of the absence of light. There
is a substance called _Chlorophyl_ which, _when acted upon by light,
becomes green_. This is contained in plants, and when the daylight
falls upon it the substance turns green. So, as we said above, plants
are not green when kept in the dark. Celery is a common instance. Heat,
of course, has much to do with the activity or vitality of plants, and
the range extends from just above freezing point to 122° Fahr. We find
tiny plants blossoming in Alpine regions close to the snow, and others
in full life in the tropics, protected from the fierce rays by scaly
coverings and huge leaves. In the northern regions buds appear as soon
as the surface warmth is felt, and even when no heat can yet penetrate
to the roots. Thus we see that Nature fits the animal and the plant to
the localities in which they live, and they exist interdependently.
Some can defy cold, others flourish in drought; some love moisture,
others live in great heat encased in prickly armour.
[Illustration: Fig. 738.—Branch of elm.]
With this introduction to biology we may now pass on to speak of the
seeds and germination of plants, which we divide into the flowering and
non-flowering species. We suppose that the appearance of various organs
of plants are familiar to our readers, and the _root_, the _stem_, the
_leaves_, and the _flower_ itself, as well as the _seeds_, are well
known, and their uses understood generally.
Now if we compare a mineral—say a crystal of quartz—with a plant, we
find the crystal uniform, consisting of small particles of quartz
throughout, and it appears an aggregation from outside of these
particles in a particular form. It cannot grow from within. But a plant
can; and it is very different in structure and appearance. It receives
nourishment from outside also, but it _assimilates_ the materials,
which are not the same as those we meet with in the plant itself. The
mineral, on the contrary, is essentially the same throughout; it can
only grow by aggregation of atoms like itself. A plant, therefore, like
an animal, must have organs within it, and must be capable of change
in itself; it has powers of reproduction, and in some few instances of
locomotion; it can eat flies, and assimilate them as an animal does.
A plant, therefore, is an organized body without external voluntary
movement; and hereby it is essentially distinct from an _animal_, with
which, in organization, it is closely connected. The simplest form
of the animal as of the plant, is that of a minute vesicle or cell,
containing a fluid in which are some granular substances. At this stage
it could not be distinguished from the simplest plant, if it had not
the faculty of voluntary movement—the power of changing its place. The
animal has a locomotive power. Sometimes, indeed, it is a very limited
sphere to which it is confined; yet it may change its place for another
more conducive to the exigencies of its being.
It is sufficient for the present to have given the most general
characteristics by which plants are distinguished from the other
objects that, with them, compose the great kingdom of Nature. A precise
and clear apprehension of their varied forms and wonderful phenomena
can only be obtained by a careful analysis of the nature and structure
of the subjects of the vegetable kingdom.
The _cell_ is the fundamental or elementary organ of plants, and the
knowledge of its metamorphoses and functions constitutes the foundation
of botany. We must therefore first consider the simple organs of plants.
STRUCTURE OF PLANTS.
It will be necessary for the reader to gain some little knowledge of
the tissues and cells of plants before he proceeds to examine the
organs of development, and a microscopic examination will soon disclose
the few simple tissues which are termed cells and vessels. These exist
in all plants of whatever nature. Plants are aggregations of cells,
“every one of which has its little particle of _protoplasm_ enclosed
by a casing of the substance called _cellulose_, a non-nitrogenous
substance nearly allied in chemical composition to starch.”[35] The
tissues are “cellular” and “vascular” respectively.
The cells have an outer _sac_ or covering which is transparent, and
this cover is the _cellulose_ above mentioned. It contains (1) the
_protoplasm_, a kind of jelly-like substance (which holds the proteine
or basis of life); (2) water or cell-sap; (3) the nucleus; and (4)
chlorophyl. This protoplasm is apparent in both plants and animals.
The cells containing these various substances—in which we find oxygen,
hydrogen, nitrogen, sulphur, and carbon, with phosphorus perhaps—are
divided to form new cells, and so on with most astonishing rapidity,
amounting in some instances to millions in a day, and a case of this
nature will readily be recognized in the mushroom.
[Illustration: A B C
Fig. 739.—Plant cells.]
_Cellular tissue_ is composed of these cells, and _vascular tissue_
is composed of vessels or tubes like coiled springs, which are cells
without divisions or partitions. These tissues will be referred to
farther on as dotted ducts or tubes.
[Illustration: Fig. 740—Form of cells.]
In most of the spongy parts of plants, as in the pulp of fruits
and pith of elder, the cells preserve the globular or oval shape
represented in fig. 739 A. But the cells, in consequence of that mutual
pressure, more frequently assume the form of a polygon (fig. 739 B),
the section of which is generally hexagonal. The cellular tissue may
generally be compared to the bubbles produced by blowing through a
straw or tobacco-pipe into soap and water; or it may be illustrated by
placing balls of moist clay together, and then pressing them more or
less strongly. In this manner every individual ball assumes a polygonal
shape corresponding to the form of the cells represented in fig. 739
C, and which disposition is, in many plants, preserved with the utmost
regularity. Such cells as are, with tolerable equality, extended in
all directions, are named _parenchyma_, and of these are composed
the tuberous parts of plants, as the potato, dahlia-roots, etc., and
especially the soft, spongy parts of the pith, bark, leaves, etc. We
frequently, however, meet with cells which are extended longitudinally,
and pointed at both extremities, as in fig. 740. The sections of these
cells, which are compactly arranged, have the appearance of a hexagon.
They are termed _woody cells_, or _woody tissue_ (prosenchyma), and
constitute the chief portions of the more solid parts of plants, as
the ligneous parts of trees, shrubs, etc. Very long, flexible cells,
as those which constitute the fibres of flax and hemp, are called
_bast-cells_, and appear under the microscope as round threads of
uniform thickness, whereas the fibres of cotton wool, which rarely
exceed one or two inches in length, when magnified, present the
appearance of flattish bands with somewhat rounded margins. By these
marks, the union of flax and cotton in the same web or piece of cloth
may be detected.
Occasionally the cells assume very abnormal shapes, as the stellate or
star-formed cells. These are described as _irregular cells_.
As every plant, whether small or great, is only an aggregate of a great
number of cells, so, also, the life of a plant is nothing else but the
sum of the activities of all the cells of which it is composed. The
special province of the cells is to receive from the soil or atmosphere
the water necessary for the various vegetative purposes, together with
the nutritious materials dissolved in the watery and aerial fluids, and
to circulate them through the whole body of the plant. The circulation
within a plant is not carried on through the agency of tubular
channels, but only by the passage of sap in all directions from one
cell to another.
[Illustration: Fig. 741.—Vascular tissue.]
Since the cells have no openings, it is somewhat difficult to
understand in what manner the fluid can enter into the plant from
without, and by what means it can inwardly pass from cell to cell.
This phenomenon, however, is dependent on the peculiar quality both
of vegetable and animal membranes and fibres—viz., that they are
permeable by many fluids, without being dissolved by them. Experiments
show that this permeative action is carried on in accordance with
definite laws. When two fluids of _unequal densities_—as, for example,
an aqueous solution of sugar and mere water—are separated from
each other by a diaphragm of pig’s bladder, we perceive a constant
tendency on both sides to restore the equilibrium in the density of
the two fluids. A portion of the water penetrates the bladder, mixing
with the solution, and a portion of the latter finds its way to the
former by the same medium. In this experiment one important fact is
to be observed—viz., that the lighter fluid always passes through
the separating medium more rapidly to the denser than _vice versâ_;
consequently, in this experiment more of the water passes through the
bladder to the saccharine solution than of the latter to the water.
This permeative capability of the tissue of vegetables and animals is
called _endosmose_.
The cells both circulate the sap and alter its condition, so we find
differing substances in the same plant. The cell as described creates
new cells, and the force with which the sap rises is rather greater
than the pressure of the atmosphere.
The vascular, or fibrous tissues, are illustrated in the margin (fig.
741). They usually contain air. Some plants have no vascular tissue,
and are termed cellular plants—such as mushrooms, fungi, mosses, and
seaweeds. Many contain both tissues, and these are the more highly
developed kinds.
[Illustration: Fig. 742.—Cells of epidermis (leaf).]
Sometimes we find a milky juice in plants. This is called _latex_; and
caoutchouc is always present in it. This juice is contained in tiny
tubular vessels, which have their origin in the new cellular tissue of
the lactiferous plants.
The tissue of the cuticle, or _epidermis_, which externally covers all
parts of the plant while they remain green, is of a peculiar nature,
and demands special consideration. It is formed of flat tubular cells,
very much compressed, and in close contact, with the exception of
some parts where the _stomata_, or mouths, are placed. In fig. 742
a section of the leaf is represented, the large transparent empty
cells of the epidermis, and above these the parenchymatous cells
of the leaf filled with greenish-coloured granules. In four places
(fig. 743) stomata (_s s s s_) are seen, which have their openings
surrounded by parenchymatous cells disposed in semilunar forms. Under
each stoma (mouth) there is a hollow space which is connected with the
intercellular passages of the leaf. These stomata, represented in fig.
743, are so numerous on the under side of the leaf, that hundreds have
been counted in the space of a square line. Through these minute organs
an intimate connection exists between the interior of the plant and the
external air.
[Illustration: Fig. 743.—Stomata.]
The epidermal cells not unfrequently exhibit very abnormal formations.
When much extended in length they appear as _hairs_ which are
frequently branched, and in many plants they contain an irritating sap
(in the nettle, for example). Bristles, prickles, glands, warts, and
especially the substance which forms the well-known cork, are all due
to the metamorphosis of this exterior integument.
FLOWERING PLANTS.
[Illustration: Fig. 744.—Water lily.]
[Illustration: Fig. 745.—Transformation of petals into stamens in white
water lily.]
Flowering plants have certain distinct features which cannot be
mistaken, for they grow well above ground, and can easily be examined.
There are a hundred-thousand different species of flowering plants, and
a visitor to Kew can study them there. Any child can tell a flower when
he sees it, but a _flowering plant_ is no more restricted in BOTANY to
actual bright blossoming plants, than the term _rock_ in GEOLOGY means
a mass of stone only. Flowering plants may be either very gorgeous or
very simple; and so long as _they contain a reproductive apparatus_
they are flowering plants. The rose is a flowering plant, but the oak
is equally one. The beech tree and the primrose are classed under the
same heading.
Flowering plants must possess _stamens_ and _pistils_, which bring
forth _seeds_ which contain an embryo, and the germination of seeds
can be easily perceived by any one who will take the trouble to soak
them (say “scarlet runners”) in warm water, and keep them warm in moist
flannel. The process may then be examined at leisure.
We need hardly insist, after what we have said, upon the necessity for
some air and light, or remind the reader that he must not keep the
seeds in a close, dark place, though light is not so necessary at first
as air. The embryo connects the “cotyledons” or halves of the seed,
and this develops into a tiny rootlet or “radicle,” and upwards into
the stem, the commencement of which is known in botany as a “plumule.”
The rootlet seeks nourishment from the ground. The albumen secreted in
the cotyledons feeds the embryo, until (in some cases) it is exhausted
and they die away. In other cases they grow up and obtain food for
the young plant in the air. Some plants have (like wheat) only one
seed-leaf, or cotyledon; and these kinds are called monocotyledons,
or _endogens_, in which the growth is upright. The others are called
dicotyledons, or _exogens_.
So far now, perhaps, you may understand that the outer covering of the
seed is called the _testa_; the opening which may be perceived in the
ordinary bean near the dark spot is the _micropyle_, or little gate;
that the halves of the covering are termed _cotyledons_, or cups, and
that the embryo sprouts upwards and downwards, the upper part of the
stem being the plumule, and the lower portion the radicle. Even if the
seed be put micropyle upwards into the ground, or between layers of
flannel, to germinate, you will find that the radicle will always curve
_downwards_.
[Illustration: Fig. 746.—Pistils of violet.]
[Illustration:
Fig. 747.—Tetradynamous stamens.]
[Illustration: Fig. 748.—Polyadelphous stamens.]
[Illustration:
Fig. 749.—Pistil of primrose.]
[Illustration: Fig. 750.—Diadelphous stamens.]
[Illustration: Fig. 751.—Fibrous root.]
The root then being displayed, it pushes its way into the ground to
seek for nourishment, and when the proper moisture has been admitted to
the seedling, which has been reposing in the cotyledons all the time,
it sprouts up rapidly. The root and its fibrous extremities have been
pushing and insinuating themselves into and through the ground, and by
small knobs or suckers known as _spongioles_, the rootlets or fibrous
parts of the root pick up sustenance for the plant, and it is then
carried by tubes to the root, and so on throughout the plant, and with
air ducts serve to keep the plant alive.
The stem emanates from the _plumule_, and in a short time little knots
develop upon it, which are the incipient leaves. The knots are divided
into nodes and internodes, because they appear on different sides of
the stem and intermediate, so as to alternate with each other, and are
really buds. The issues unite also into leaf-stalks or _petioles_,
and extend into the leaf-frame or skeleton as we see it when the leaf
has decayed. So thus we have an upward and a descending growth, which
respectively constitute the stem and root of a flowering plant.
[Illustration: Fig. 752.—Tuberous (fasciculated) root.]
Some trees have roots growing from the stem, as in the banyan tree,
and roots can produce stems as well as the latter can form roots. The
uses of roots are so well understood that we need not particularize
them. In many trees we find what are termed _lenticellæ_, like holes
in the bark. These fissures will put forth roots under favourable
circumstances. These stem roots are called adventitious, and by taking
“cuttings” from plants we make good use of them for propagation.
[Illustration: Fig. 753.—Banyan tree.]
But there are underground stems as well as those which flourish and
climb above it. “Bulbs” and “tubers” are common instances of these
underground stems, or “rhizoma,” which are horizontal. The ordinary
stems are termed “aerial” stems to distinguish them from the earthy
and subterranean. The aerial roots of ivy are only used for support,
and are not its proper roots, though some parasitic plants strike into
the trees and are nourished by them.
THE STEM.
The stem is that portion of the plant-axis which grows upwards or above
ground, and may be, as we have just read, subterranean. As the great
function of the root is to procure sustenance for the plant, the stem
assists in carrying the nourishment through the branches and leaves. We
shall find two forms of stem—the underground, or root-stock, and the
stem proper. There are in these two former several varieties as under:—
1. The BULB, which is a short globular stem surrounded by thick leaves,
and producing buds—as, for example, the onion.
2. The TUBER, similar to the foregoing in shape, having no leaves,
however; the potato is an instance.
3. The RHIZOME (root-stock), like a root only producing buds, which
roots do not. The iris will serve as an example.
The varieties of the stem-proper are:—
(1) _Filiform_, or thread-like, simple, or branched, as in mosses.
(2) The _culno_, a thin, hollow, and frequently-jointed stem.
(3) The _palm_ or simple stem, seen in tree-ferns and palms. It is
marked by the scars of dropped leaves.
(4) The _stalk_, very common, of a green hue, and its life is limited
to a twelve-month as a rule. The so-called “stem” of the hyacinth
is not a stem, it is a stalk, or flower-stalk, pushed forth for a
temporary purpose.
(5) The _ligneous_ stem is the perfected kind, and an example will be
apparent in every tree.
[Illustration:
Fig. 754.—Transverse section of exogenous wood, showing the growth of
nine years.]
[Illustration: Fig. 755.—Section (magnified portion) of the small cut
_a_.]
[Illustration: Fig. 756.—Section of an endogenous stem.]
The duration of the stem of a plant is usually the same as of the
plant—so we have annuals, biennials, and perennials. The substance of
the stem determines its character, so we may have it solid, or soft,
hollow, tubular, flexible, rigid, or a tough stem. There are fibrous,
herbaceous, and juicy stems. They may be directed uprightly, straight,
procumbently, arched, or creeping, above, or underground, climbing,
clinging, floating, or twining.
There are many plants with little or no stem deserving the name, as in
the onion; and we must all remember when studying botany that it is not
the _place_ where a portion of a plant may be found that constitutes
it a root or a stem. The form and structure should be studied, and its
purpose in creation. So stems may be underground and roots above it.
The root and stem, briefly treated of in the foregoing paragraphs, have
certain points of resemblance, inasmuch as both consist of a main or
trunk line, so to speak, from which branches diverge as “rootlets” and
“twigs”; and how beautiful the latter are any one can see in a good
photograph of a wintry landscape. But stems have nodes and internodes,
and roots have not, and this is the great and apparent difference.
The covering of plant-stems is varied, and many instances of such
clothing will occur. We have woolly stems and hairy stems, which
develop into thorny ones—for thorns are only strong hairs. Spines
and stings and prickles defend the stems, and keep rude hands from
meddling. We will now cut the stem and see what it is composed of,
and how it looks inside. We have only to cut it across and again
perpendicularly to find out a great deal about the interior structure
of the stems of branching plants (exogens).
The elder, from which the whistle of our boyish days is fashioned with
a penknife, will serve any lad for an illustration. Inside we find what
is called “pith,” which is _cellular tissue_. Round this is fibre, and
outside is a skin, or the plant-cuticle. We may remark that the tissues
of flowering plants are characteristic of the monocotyledonous and
the dicotyledonous plants. Of the former we append an illustration,—a
section of palm-stem,—and we find bundles of vascular tissue dispersed
apparently at random amongst the cellular tissue of the parenchyma,
or cellular tissue. These stems do not grow by the increase of
the existing vascular tissue, but by their new production at the
circumference, and so they grow in both directions, laterally and
uprightly. These plants belong to the ENDOGENS, and if Indian corn be
grown we shall have full opportunity to study the formation. In cutting
a fern stem we are familiar with the “oak” pattern of the matter it
contains. We have few specimens of endogens in England.
The dicotyledonous stems are common to our trees and most plants, and
may therefore be considered with advantage. The stem consists of the
vascular tissue called “pith,” and we give an illustration of the cells
magnified very considerably. The arrow indicates the outward direction
(fig. 757).
We here perceive the vascular bundle proper surrounded by a very
large-celled tissue, _aa′bef_. The almost square cells, _aa′_ , form
the epidermis on which follows the less dense cellular tissue of the
bark. The latter surrounds a half-moon-shaped bundle of bast-cells,
_c_, which are separated in the direction towards the interior, by
a layer of cambium, _dd′d″_, from the bundles of vascular tissue,
consisting of vessels and longitudinal cells. The latter tissues may be
distinguished in the transverse section by the thicker walls, _gg_, and
by their greater breadth, _hh_. It is further to be remarked that the
cambium transparent tissue, _dd″_, appears on both sides of the bundles
of vascular tissue, and extends to the next bundle, and thus presents
an uninterrupted circle throughout the entire circumference of the stem.
[Illustration: Fig. 757.—Dicotyledonous stem.]
[Illustration: Fig. 758.—Stem one year old.]
On examining the section of a one-year-old dicotyledonous stem,
magnified six times, as in fig. 758, we perceive several parts clearly
distinguishable from each other, corresponding with the arrangement of
the bundles of vascular tissue.
Enclosed by the epidermis, _a_, is a large-celled tissue, _b_ _f_ and
_m_, in which a number of vascular bundles form a circle. In each of
these we notice that the outer portion, consisting of bast-shell, _c_,
is separated by the cambium, _d_, from the inner woody portion, _e_.
The cambium forms a closed circle which penetrates through all the
vascular bundles.
In the course of the further development of the stem, the parts, _a_
_b_ _c_, constitute the _bark_, the vascular bundles, _e_, the _wood_,
and the cellular tissue, _f_, its pith. The tissues, _m_, penetrating
between the vascular bundles, are called the _medullary rays_. The
cambium is to be regarded as the most important part, since it is the
source of new bundles of vascular tissue which year by year increase
the circumference of the stem.
[Illustration: Fig. 759.—Stems three and five years old.]
The growth of a dicotyledonous stem is continued by the formation of a
new circle of vascular bundles on the circumference of the stem in the
second year. Each new bundle, as has already been shown, is produced in
the cambium, and consequently is deposited between the wood and inner
bark.
Thus every year a new layer is deposited between the previous formation
and the bark; and a section will exhibit these concentric rings of
wood obviously distinct from each other; and as one year is requisite
for the formation of a single layer of wood, these depositions are
named _annual layers_ or _rings_. In fig. 759 we have a representation
of a stem three years old, and one of five years of age.
The number of rings in the stem do not invariably agree with the number
of years the tree has been growing, but it may be accepted as a rule.
The stem is the medium of communication between the roots and leaves
at first; but after a year this important duty is deputed to the
cambium layers of new woody tissue, etc., and as time goes on the
living power has accumulated immediately under the bark. So although
the tree be quite hollow it will live. The interior has been closed up
by deposition of wood and has decayed; but the life functions being
relegated to the bark, the old tree lives on. If we remove the rind all
round a tree it will die.
THE LEAVES.
When in the spring the young leaves appear upon the trees, and as
summer advances they become fully developed, we are all grateful for
the beautifully varied tints of green, and for the shade we can so
fully enjoy. The study of leaves is a most interesting and instructive
one, and nobody should omit to examine them. Their forms are infinite,
or, at any rate, countless; their size as varied as their forms.
Many attributes of the leaf will occur to every reader, and we will
briefly describe these essential organs of plants. Air and light are
necessary to the development of leaves, and their principal use is to
present a surface to the food material which the plant absorbs. They
breathe, as it were, and absorb the carbonic acid from the atmosphere.
These functions are called “assimilation,” “transpiration,” and
“respiration,” which we will detail by-and-by.
[Illustration: Fig. 760.—Compound leaf.]
[Illustration: Fig. 761.—Simple leaf.]
Leaves are distinguished according to position and duration. Some
leaves have very simple forms, others are compound, so to speak. Some
are plain and rounded, others are toothed, like the holly. The skeleton
of a leaf is a very interesting study, and it will show the beautiful
structure of these common objects. The delicate lines of the green leaf
are “veins,” or sap-vessels, which convey the necessary nourishment.
The leaves are called the “embryonic” (seed-like), the radicle, or
root-leaf, the stalk-leaf, and the stipules, which grow at the base of
stem-leaves, and the floral leaves, which bear the flowers or fruit
buds. Leaves which are developed at the end of a chief axis are termed
_blossoms_. Of course it must be understood that all the different
kinds of leaves do not occur upon the same plant. The leaf may be
accepted to mean the stem-leaf.
Leaves are folded up in various ways, and the manner in which this
is accomplished is termed the _vernation_ of the plant. The leaves
of endogens and exogens differ in their veining. The former veins
do not touch; there is none of that beautiful interlacing which we
find in the exogenous leaves. In the former the veins rise from base
to apex, curving as they advance, as in the well-known lily of the
valley. This “nervous system” of the leaf is its “venation,” and the
veins distributed in the blade or _lamina_ of the leaf are twofold,—as
remarked,—ascending in curves, or diverging from a central nerve
called the “mid-rib.” These lateral nerves are either parallel or
“reticulate”—that is, net-like.
[Illustration: Fig. 762.—Net-veined leaf.]
We will now examine the forms of leaves which are regulated by the
divergence and extension of the divisions of the mid-rib. Thus we get
an orbicular, or _peltate_ leaf; _palmate_, _digitate_, and _pedate_
forms also occur, as may be seen in the illustrations, pages 672 and
673, where all the varied shapes can be studied. The leaf consists of a
_petiole_ or stalk, and the _lamina_ or blade. The petiole is composed
of bundles of vascular tissue; the lamina is formed by their extension,
the interstices being filled with cellular tissue. So we perceive that
the leaves and stems are composed of similar materials. To defend
the tissue a skin, or epidermis, is placed upon the surface of the
leaf, and this _epidermis_ is full of breathing holes, or _pores_,
called _stomates_ (compare page 664). There are also cells filled with
chlorophyl, which gives the leaf its green tint.
[Illustration: Fig. 763.—Linden tree.]
[Illustration: Fig. 764.—Stomates, highly magnified.]
The _petiole_ may be absent in a leaf, and when it is the leaves are
termed _sessile_, or sitting leaves. These leaves sometimes coil round
the stem, and are called “amplexicaul,” or stalk embracers. These are
simple leaves. Compound leaves are composed of several blades or
laminæ on a stalk, and are seldom sessile. SIMPLE LEAVES are almost
innumerable in form and variety. Leaves may be _equal_ or _unequal_,
_acicular_ or _linear_, _rounded_ or _oval_, _cordate_ or _obcordate_,
_reniform_ or _sagittate_, _perfoliate_ or _connate_, _crisp_,
_whorled_ or _truncate_, _retuse_, _acuminate_ or _mucronate_. The
margins of simple leaves again are entire, deft, notched, crenated,
crenulated, sinuous, or dentated. They are pinnatifid, multifid, or
lobed, according to the divisions of the leaf.
[Illustration: LEAVES.
I. DEPENDING ON FORM.
Lanceolate (Privet).
Ovate (Fuchsia fulgens).
Oblong (Primrose).
Cordate (White Bryony).
Palmate (vine).
II. DEPENDING ON MARGIN AND ARRANGEMENT.
Serrate (Rose).
Biserrate (Elm).
Crenate (Betony).
Entire (Lilac).
Digitate (Lupin).
Pinnate (Vetch).
Bipinnate (Acacia).
Pinnatifid (Crepis).
Ternate (Clover).
Biternate (Columbine).
III. DEPENDING ON POINT.
Obtuse (Dock).
Mucronate (Holly).
Retuce (Snowball).
Emarginate (Bladder Senna).]
Compound leaves are also divided into classes. The _pinnate_, as
the rose-leaf, the clover trefoil. There are “doubly pinnate,” the
digitate, as in the horse chestnut. Compound and simple leaves can be
readily distinguished by inspection, for the former are “articulated”
to the stalk and can be separated, but the simple leaves will be torn,
for they are confluent throughout.
Leaves are evergreen or deciduous, accordingly as they retain or shed
themselves. The ordinary leaf is deciduous; the fir and the yew and the
imported laurel are evergreens. We have very few of these as natives of
England, the ivy, yew, and fir being the three most common. Sometimes
a plant peculiar to Killarney, and known as the arbutus, is included
in the list. But the Scotch fir and the yew are distinctly native
evergreens.
The detailed characteristics of leaves must be passed over until we
come to the fly-catching leaves—such as Venus’ fly-trap, the droseras,
and nepenthes, which appear to catch and devour insects for food. The
Venus’ fly-trap may be examined, and we shall find the leaves covered
with tiny and very sensitive hairs. Often a fly happens to alight
upon the leaf, which is extended in a most innocent manner (_see_
illustration). As soon as the fly settles the leaves close, and the
digits lock tightly together, thus preventing the escape of the prey.
The droseras, sarracenias, and nepenthes also kill their food. The
sarracenias form curious cups, into which insects are enticed in search
of fluid, and then, as in the case of the house-haunting cockroach,
they cannot get out again. The nepenthes have a cup and lid for
insect-catching, and within the cup a liquid is secreted.
We will close this portion of our subject with a quotation from a
recent article upon botany referring to leaf arrangement. The writer
says:—
[Illustration: Fig. 765.—Leaf of Dionæ.]
[Illustration: Fig 767.—Leaf of Nepenthes.]
[Illustration: Fig. 766.—Sarracenia.]
“Efforts have been made to determine the laws to which these various
modes of leaf-arrangement may be referable. The result is found in the
doctrine of ‘Phyllotaxy,’ as it is called, the fundamental principle
of the whole being that Nature, in the disposition of the leaves upon
the stem, works upon precisely the same idea as that which is set forth
so distinctly and elegantly in the common pine-cone; and, on a minor
scale, in the beautiful cone of the female hop; not to mention the
quasi-cones of many species of tropical palm, such as the Sagus and the
Mauritia; nor to mention either, the very delicate repetition of the
whole series in the florets of the Rudbeckia and the ripening fruits
of Chaucer’s daisy. In every one of the flower and fruit arrangements
mentioned, the idea is the spiral,—the same sweet old fashion which we
have had in the twining stems of the convolvulus, the woodbine, and
the scarlet bean; which comes out again in many a sea-shell, and in
human ringlets; and this idea, according to ‘Phyllotaxy,’ governs the
position of the leaves. Following alternate leaves up the stem, their
sequence is clearly spiral. Through the non-development of internodes,
they are brought closer and closer together; and even when the entire
mass of foliage is concentrated and condensed into the rosulate
form, as in the houseleek and the Echeverias, the spiral prototype
is still distinguishable. The whole matter has been reduced to one
of arithmetical exactitude; and for those who love calculations and
“fractions,” the determination of the spirals, their continuity and
intermixture, supplies abundance of curious entertainment. All three
modes of leaf-arrangement are found in certain herbaceous plants,
none disclosing this particular kind of playfulness more plainly than
the common pyramidal Loosestrife, _Lysimachia vulgaris_, and the
purple Lythrum of the waterside, in each of which very handsome wild
flowers, alternate, opposite, and whorled leaves may be found in near
neighbourhood. Alternate and opposite leaves are also met with, side
by side, in various species of Myrtaceæ; and imperfectly, upon young
shoots of the common ash-tree. The rule is, nevertheless, that there
shall be uniformity, and in many of the largest natural orders the
rule is never broken. In the Rosaceæ the stem leaves are invariably
alternate; in the Gentianaceæ they are invariably opposite.”
[Illustration: Fig. 768.—Branch of horse chestnut.]
FOOTNOTES:
[35] Dr. Carpenter.