Fig. 46.—Root-hair, much enlarged, in contact with the soil particles (s). Air-spaces at a; water-films on the particles, as at w.
The root-hairs are very small, often invisible. They, with the young roots, are usually broken off when the plant is pulled up. They are best seen when seeds are germinated between layers of dark blotting paper or flannel. On the young roots they will be seen as a mould-like or gossamer-like covering. Root-hairs soon die: they do not grow into roots. New ones form as the root grows.
Osmosis.—The water with its nourishment goes through the thin walls of the root-hairs and rootlets by the process of osmosis. If there are two liquids of different density on the inside and outside of an organic (either vegetable or animal) membrane, the liquids tend to mix through the membrane. The law of osmosis is that the most rapid flow is toward the denser solution. The protoplasmic lining of the cell wall is such a membrane. The soil water being a weaker solution than the sap in the roots, the flow is into the root. A strong fertilizer sometimes causes a plant to wither, or “burns it.” Explain.
Structure of Roots.—The root that grows from the lower end of the caulicle is the first or primary root. Secondary roots branch from the primary root. Branches of secondary roots are sometimes called tertiary roots. Do the secondary roots grow from the cortex, or from the central cylinder of the primary root? Trim or peel the cortex from a root and its branches and determine whether the branches still hold to the central cylinder of the main root.
Internal Structure of Roots.—A section of a root shows that it consists of a central cylinder (see Fig. [45]) surrounded by a layer. This layer is called the cortex. The outer layer of cells in the cortex is called the epidermis, and some of the cells of the epidermis are prolonged and form the delicate root-hairs. The cortex resembles the bark of the stem in its nature. The central cylinder contains many tube-like canals, or “vessels” that convey water and food (Fig. [45]). Cut a sweet potato across (also a radish and a turnip) and distinguish the central cylinder, cortex, and epidermis. Notice the hard cap on the tip of roots. Roots differ from stems in having no real pith.
Fig. 47.—Growing Point of Root of Indian Corn.
d, d, cells which will form the epidermis; p, p, cells that will form bark; e, e, endodermis; pl, cells which will form the axis cylinder; i, initial group of cells, or growing point proper; c, root-cap.
Microscopic Structure of Roots.—Near the end of any young root or shoot the cells are found to differ from one another more or less, according to the distance from the point. This differentiation takes place in the region just back of the growing point. To study growing points, use the hypocotyl of Indian corn which has grown about one-half inch. Make a longitudinal section. Note these points (Fig. [47]): (a) the tapering root-cap beyond the growing point; (b) the blunt end of the root proper and the rectangular shape of the cells found there; (c) the group of cells in the middle of the first layers beneath the root-cap,—this group is the growing point; (d) study the slight differences in the tissues a short distance back of the growing point. There are four regions: the central cylinder, made up of several rows of cells in the centre (pl); the endodermis, (e) composed of a single layer on each side which separates the central cylinder from the bark; the cortex, or inner bark, (e) of several layers outside the endodermis; and the epidermis, or outer layer of bark on the outer edges (d). Make a drawing of the section. If a series of the cross-sections of the hypocotyl should be made and studied by the pupil beginning near the growing point and going upward, it would be found that these four tissues become more distinctly marked, for at the tip the tissues have not yet assumed their characteristic form. The central cylinder contains the ducts and vessels which convey the sap.
