Fig. 2.—An Amœba. 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 [Figure 2]. 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) are provided with delicate thread-like processes of protoplasm called “cilia” (sing. cilium), which are in active vibration, and propel the cell through the water.
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.