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many of the herbaceous plants, are exogenous. In herbaceous stems, the medullary rays are generally large, and the more solid part of the stem consists of vascular bundles, arranged around the pith, and these bundles, when cut across, appear somewhat wedge-shaped, becoming broader outwards. The arrangement is beautifully regular. In trees and shrubs, the medullary rays generally become very fine as the wood hardens, so that they do not much affect its solidity.

The pith consists of cellular tissue.-The wood consists of vascular tissue, and chiefly of long vessels, closed at the ends, which, as they become old, are gradually filled up with solid matter. The wood nearest the pith is harder than the younger wood near the bark, and in some trees is also very different in colour. The matured wood is called heart-wood or duramen;1 and the soft wood, which forms the outer part of the stem, is called sap-wood or alburnum.2-Next to the wood, on the outside, is a layer of soft cells, called the cambium layer, formed in a mucilaginous fluid, called cambium,3 which may be best observed in young shoots, and in spring when vegetation is most active. From the cambium layer, the new layer of wood is formed, and the necessary increase is also made to the bark.—The bark, in young stems or branches, consists of mere cellular tissue, but vessels are afterwards formed in the inner portion of it, which often become long and strong fibres. The valuable fibres of flax and hemp belong to the inner bark of these plants; and bast mats are made of the inner bark of the linden or lime tree. There is an Indian tree of the bark of which sacks are made by merely stripping the bark from the wood, which is done by beating it till it comes off readily, as boys make 'plane-tree' whistles. The inner bark is also called the bast. Its Latin name is liber, which is also the Latin for 'a book,' this bark having been used for writing on before the use of paper or parchment was known. The outer part of the bark is entirely cellular, but consists of two layers, easily to be distinguished. The outermost of these is that which, in the cork-tree, increases to a great thickness, affording us the substance known as cork. The bark itself is surrounded by an integument, called the epidermis, the use of which seems merely to be the protection of the whole: in old stems it is thrown off, although it is always to be found in young and tender shoots.

Endogenous Stems.-Endogenous stems have no pith and no bark. The outer part of the stem may indeed be somewhat different from the rest, and have the appearance of a bark, but it cannot be separated from the wood like the bark of an exogenous tree. As an endogenous plant grows, the stem increases in thickness, not by the addition of layers on the

1 From Latin durus, hard. 2 From Latin albus, white.

3 Low Latin, 'nutriment,' from Latin cambio, to change.

outside, but by the formation of new bundles of vessels in the centre,

which push out those previously formed, as long as they are soft enough; but when they are thoroughly hardened, this no longer takes place, and the stem increases in length without further increase of thickness.

[graphic][graphic]

Fig. 75.-Transverse and Vertical Sections of
Endogenous Stem.

Thus palms are, in gene-
ral, of almost uniform thickness from the root to the crown of leaves, and
the outer part of the stem is often very hard, whilst the central part is soft
and spongy, and amongst its cells there is often a great deposit of starch,
which, in the sago palms, is so abundant as to be of economical import-
ance. Palms are almost the only endogenous trees, and no endogenous tree is
found anywhere but in the warmer parts of the globe. Endogenous plants,
however, abound in all countries, and amongst them are all the grasses
and all the plants which have bulbs. Endogenous stems are very often
unbranched, producing no buds but the terminal bud. The great terminal
bud of a palm resembles a cabbage in size, and also in quality, so that
palms are often cut down for the sake of it. It is called the palm cabbage;
and some kinds of palm, in which it is particularly good for the table,
receive the name of cabbage palms. The rapid growth of many endogenous
stems ruptures the central cells, and thus a hollow stem is produced,
as in grasses.

Acrogenous Stems.-Acrogenous stems resemble endogenous stems in having neither pith nor separable bark, but differ from them in their mode of growth, all the bundles of vessels, or fibres, being developed at once, so that the stem increases only by additions at the summit. The acrogenous stem has no branches, but produces a crown of leaves; so that

[graphic]

a tree-fern, which is the

only kind of acrogenous Fig. 76.-Transverse Section

tree, has a general resem

of Acrogenous Stem.

Fig. 77. Tree-fern.

blance to a palm. In the ferns of Britain and other temperate countries, the stem creeps along the ground as a root-stock.

Leaves. Leaves are most important organs of plants. They expose a great surface to the air, imbibe what the plant requires for its nourishment, and give out what is no longer useful to it. They are of very great importance in the system of nature, the gases which the leaves give out being generally those most needful for the support of animal life, so that the multitude of leaves on the face of the earth keeps the air fit for the use of animals, whereas it would otherwise become such that animals could no longer breathe it and live.

Leaves are curiously folded or rolled up in the bud before they are developed. In some plants, they are folded by the midrib, the two halves of the leaf lying together; in others, they are folded in a fan-like manner; in some, they have their edges rolled inwards, in others, outwards; and in some, they are rolled together in a single coil. The different modes in which the leaf is formed in the bud are characteristic of different plants. The cherry and the plum are trees nearly allied, but the leaves of the cherry are folded together in the bud, while those of the plum are rolled up.

The expanded part of a leaf generally faces the sun, the influence of light being necessary as well as that of air. This expanded part is called the blade of the leaf. The leaf, however, is often supported by a stalk, called the leaf-stalk or petiole, which in trees and shrubs is often woody, and in some, particularly in palms, becomes thick and hard like a branch. Leaves which have no leaf-stalk are called sessile,2 because they seem to sit upon the stem or branch. In many plants, the leaves which spring from the crown of the rcot differ very much from those which are produced higher on the stem. Thus, the common harebell has root-leaves nearly round, whilst those of its stem are very narrow. The leaf-stalk divides into branches, which form the ribs or veins of the leaf, and give it the necessary strength. In endogenous plants, the veins of the leaf are in general nearly parallel, running unbranched throughout the whole length of the leaf, as may be seen in grasses; in exogenous plants they break into branches, which spread in various directions, and are again and again branched, so as to form a kind of network, as may be seen in the leaf of the elm or the primrose. This difference in the leaves characterises these two classes of plants as much as that in the structure of the stem. The leaves of acrogenous plants have generally forked veins.

In some plants, the leaf-stalk is broad, and the blade of the leaf at its extremity is very small, or scarcely exists, as in many species of Acacia. In this case the leaf-stalk serves the purposes which are ordinarily served

1 'The little foot,' from Latin pes, pedis, the foot.

2 From Latin sessilis, sitting, from sedeo, sessum, to sit.

by the blade of the leaf. Some leaves are thick and fleshy. The forms of leaves are very various, and many are divided into lobes or segments. A leaf much divided into lobes may yet consist altogether of one piece; and a leaf which consists of one piece, whether lobed or not, is called a simple leaf. Many leaves, however, are compound—that is, they are made of a number of pieces, the leaf-stalk branching by joints, and sometimes branching again and again, each branch or branchlet bearing a separate blade, called a leaflet. Such leaves are called pinnated, when the leaflets are arranged on opposite sides of the stalk, as in the ash or the laburnum. A leaf may be bipinnate—that is, twice pinnate—or the leaf-stalk may be still more subdivided. In some compound leaves, the leaf-stalk divides into three branches, as in the clover or trefoil; in others it sends off a number of leaflets in a radiating manner from its extremity, as in the horse-chestnut.

Stomata. In the epidermis of leaves there are minute openings, called stomata, a Greek word signifying mouths. These are also found in the epidermis of young shoots, and of other green parts of plants. They serve for the admission of air as the plant requires it, and for the exhalation of what it gives off into the air; and leaves have therefore sometimes been called the lungs of plants. The stomata are very small, and can only be discerned by the aid of a microscope. There are sometimes 160,000 or more in a square inch of surface; and in those plants which have fewest and largest stomata, there are about 200 in a square inch.

Circulation of Sap.—Plants have no organ resembling the heart of an animal, and the circulation of sap which takes place in them is of a very different nature from the circulation of blood. The sap imbibed by the roots ascends through the vessels of the stem, and passes from vessel to vessel by osmotic action. In plants wholly formed of cellular tissue, the sap seems to proceed in any direction from one cell to another, till the whole substance of the plant is permeated; in vascular plants, particular parts of the plant appear to be chiefly concerned in this process, which, however, is very imperfectly understood. It is known that the

sap circulates most abundantly through the youngest layers of wood, whilst in the old and thoroughly hardened layers it almost entirely ceases, and these may therefore almost be regarded as having ceased to live, and as useful to the plant only by giving strength to its stem. The sap reaches every part of the plant, being conveyed through the finest leaf-stalks and flower-stalks, and penetrating the most delicate parts of the flowers. In the leaves and other green parts, it is modified by the action of the air and light, and afterwards descends again to the root, which it is generally believed to do through vessels in the bark, giving off supplies of nourishment

1 From Latin pinna, a wing.

to every part of the plant as it descends; for the nourishment of the organs of a plant is supposed to be derived more from the sap that has been elaborated in the leaves than from the ascending sap in its original state. The rise of the sap in spring is one of the unexplained wonders of nature; it cannot be ascribed to the mere increase of heat, for it begins in many plants during the very coldest weather of winter. They have had their period of rest, and their time of activity comes again. Plants need rest as well as animals. A plant continually forced to grow, by artificial application of heat and moisture, soon ceases to live; and gardeners well know the necessity of allowing rest to hothouse plants. The plants of tropical countries often have their periods of rest and activity determined by the wet and dry seasons, which are to them as summer and winter.

Reproductive Organs of Plants.

The reproductive organs of plants are very different in the lower and the higher kinds. Of some of the lowest kinds we know nothing more than that they increase by the addition of one cell to another, and are propagated by the separation and diffusion of these cells; although it is probable that even these plants produce seeds or spores, by which they are multiplied. Their very minute size, however, makes it difficult to investigate the processes of their life. In the highest kinds of plants, the reproductive organs are the flowers, and the fruit produced by them.

Flowers-Inflorescence. The arrangement of the flowers upon a stem or branch is called the inflorescence1 of a plant. In some plants, each flower arises from the root on a separate stalk; in others, it arises in like manner from the stem or from a branch. In some, many flowers are produced on a single stalk, and the upper part of a stem or branch often becomes itself a flower-stalk, its leaves becoming modified into bracts,2 which are placed under each flower or each division of the flower-stalk. Bracts sometimes resemble the ordinary leaves of the plant, but are very often much smaller and less divided, and are sometimes membraneous and dry, sometimes of colours very different from the leaves. In some plants, the bracts are large, and in the bud enclose a stalk which produces numerous flowers. The bract in this case is called a spathe,3 and the flower-stalk a spadix.3 An example is seen in the wake-robin, and palms have this form of inflorescence. In some palms, the spathe is very large, thick, and leathery, so that it is often made into a sack or bag; and there are palms which have a spadix twenty feet long, and bearing more than 200,000 flowers.

1 From Latin in, upon, and floresco, to flower.

2 From Latin bractea, a thin plate of metal, gold-leaf.

3 From Latin spatha, a broad leaf.

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