If we make a thin slice across the stem of a rapidly growing plant,—e.g. geranium, begonia, celery,—mount it in water, and examine it microscopically, it will be found to be made up of numerous cavities or chambers separated by delicate partitions. Often these cavities are of sufficient size to be visible to the naked eye, and examined with a hand lens the section appears like a piece of fine lace, each mesh being one of the chambers visible when more strongly magnified. These chambers are known as “cells,” and of them the whole plant is built up. Fig.1.—A single cell from a hair on the stamen of the common spiderwort (Tradescantia), ×150. pr. protoplasm; w, cell wall; n, nucleus.
Fig.2.—An Amoeba. A cell without a cell wall. n, nucleus; v, vacuoles, ×300. The cell may be regarded as the unit of organic structure, and of cells are built up all of the complicated structures of which the bodies of the highest plants and animals are composed. We shall find that the cells may become very much modified for various purposes, but at first they are almost identical in structure, and essentially the same as the one we have just considered. Fig.3.—Hairs from the leaf stalk of a wild geranium. A, single-celled hair. B and C, hairs consisting of a row of cells. The terminal rounded cell secretes a peculiar scented oil that gives the plant its characteristic odor. B, ×50; C, ×150. Very many of the lower forms of life consist of but a single cell which may occasionally be destitute of a cell wall. Such a form is shown in Figure2. Here we have a mass of protoplasm with a nucleus (n) and cavities (vacuoles, v) filled with cell sap, but no cell wall. The protoplasm is in constant movement, and by extensions of a portion of the mass and contraction of other parts, the whole creeps slowly along. Other naked cells (Fig.12, B; Fig.16, C) Fig.4.—A, cross section. B, longitudinal section of the leaf stalk of wild geranium, showing its cellular structure. Ep. epidermis. h, a hair, ×50. C, a cell from the prothallium (young plant) of a fern, ×150. The contents of the cell contracted by the action of a solution of sugar.
As cells grow, new ones are formed in various ways. If the new cells remain together, cell aggregates, called tissues, are produced, and of these tissues are built up the various organs of the higher plants. The simplest tissues are rows of cells, such as form the hairs covering the surface of the organs of many flowering plants (Fig.3), and are due to a division of the cells in a single direction. If the divisions take place in three planes, masses of cells, such as make up the stems, etc., of the higher plants, result (Fig.4, A, B). |