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§ 1. Structure and Growth of the Elementary Tissues

186. If a very thin slice of any part of a plant be placed under a microscope of high magnifying power, it will be found to be made up of variously shaped and arranged ultimate parts, forming a sort of honeycombed structure. These ultimate parts are called cells, and form by their combination the elementary tissues of which the entire plant is composed.

187. A cell in its simplest state is a closed membranous sac, formed of a substance permeable by fluids, though destitute of visible pores. Each cell is a distinct individual, separately formed and separately acting, though cohering with the cells with which it is in contact, and partaking of the common life and action of the tissue of which it forms a part. The membranes separating or enclosing the cells are also called their walls.

188. Botanists usually distinguish the following tissues :—

(1) Cellular tissue, or parenchyma, consists usually of thin-walled cells, more or less round in form, or with their length not much exceeding their breadth, and not tapering at the ends. All the soft parts of the leaves, the pith of stems, the pulp of fruits, and young growing parts, are formed of it. It is the first tissue produced, and continues to be formed while growth continues, and when it ceases to be active the plant dies.

(2) Woody tissue, or prosenchyma, differs in having its cells considerable longer than broad, usually tapering at each end into points and overlapping each other. The cells are commonly thick walled; the tissue is firm, tenacious and elastic, and constitutes the principal part of wood, of the inner bark, and of the nerves and veins of leaves, forming, in short, the framework of the plant.

(3) Vascular tissue, or the vessels or ducts of plants, so called, from the mistaken notion that there functions are analogous to the vessels (veins and arteries) of animals. A vessel in plants consists of a vertical row of cells, which have their transverse partition walls obliterated, so as to form a continuous tube. All phænogamous plants, as well as ferns and a few other cryptogamous plants, have vessels, and are therefore called vascular plants; so the majority of cryptogams having only cellular tissue are termed cellular plants. Vessels have their sides very variously marked; some, called spiral vessels, have a spiral fibre coiled up their inside, which unrolls when the vessel is broken; others are marked with longitudinal slits, cross bars, minute dots or pits, or with transverse rings. The size of vessels is also very variable in different plants; in some they are of considerable size and visible to the naked eye in cross sections of the stem, in others they are almost absent of can only be traced under a strong magnifier.

189. Various modifications of the above tissues are distinguished by vegetable anatomists under names which need not be enumerated here are not being in general practical use. Air-vessels, cysts, turpentine vessels, oil-reservoirs, etc., are either cavities left between the cells, or large cells filled with peculiar secretions.

190. When tissues are one formed, they increase, not by the general enlargement of the whole of the cells already formed, but by cell-division, that is, by the division of young and vitally active cells, and the enlargement of their portions. In the formation of the embryo, the first cell of the new plant is formed not be division, but around a segregate portion of the contents of a previously existing cell, the embryo-sac. This is termed free cell-formation, in contradistinction to cell-division.

191. A young and vitally active cell consists of the outer wall, formed of a more or less transparent substance called cellulose, permeable by fluids, and of ternary chemical composition (carbon, hydrogen, and oxygen); and of the cell-contents, usually viscid or mucilaginous, consisting of protoplasm, a substance of quaternary chemical composition (carbon, hydrogen, oxygen and nitrogen), which fills and important part in cell-division and growth. Within the cell (either in the centre or excentrical) is usually a minute, soft, subgelatinous body called the nucleus, whose functions appear to be intimately connected with the first formation of the new cell. As this cell increases in size and its walls in thickness and protoplasm and watery cell-sap become absorbed or dried up, the firm cellulose wall alone remaining as a permanent fabric, either empty of filled with various organized substances produced or secreted within it.

192. The principal organized contents of cells are

sap, the first product of the digestion of the food of plants; it contains the elements of vegetable growth in a dissolved condition.

sugar, of which there are two kinds, called cane-sugar and grape-sugar. It usually exists dissolved in the sap. It is found abundantly in growing parts, in fruits, and in germinating seeds.

dextrine, or vegetable mucilage, a gummy substance, between mucilage and starch.

starch or fecula, one of the most universal and conspicuous of cell-contents, and often so abundant in farinaceous roots and seeds to fill the cell-cavity. It consists of minute grains called starch-granules, which vary in size and are marked with more or less conspicuous concentric lines. The chemical composition of starch is the same as that of cellulose; it is unaffected by cold water, but forms a jelly with boiling water, and turns blue when tested by iodine.

chlorophyll, very minute granules, containing nitrogren, and coloured green under the action of sunlight. These granules are most abundant in the layers of cells immediately below the surface or epidermis of leaves and young bark. The green colouring matter is soluble in alcohol, and may thus be removed from the granules.

chromule, a name given to a similar colouring-matter when green.

wax, oils, camphor, and resinous matter are common in cells or in cavities in the tissues between the cells, also various mineral substances, either in an amorphous state or as microscopic crystals, when they are called Raphides.

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