_Rootlet and root-hair differ._ The rootlet is a _compact cellular structure. The root-hair is a delicate tubular cell_ (Fig. 45), _within which is contained living matter (protoplasm); and the protoplasmic lining membrane of the wall governs the entrance of water and substances in solution._ Being long and tube-like, these root-hairs are especially adapted for taking in the largest quantity of solutions; and they are the principal means by which plant-food is absorbed from the soil, although the surfaces of the rootlets themselves do their part. Water plants do not produce an abundant system of root-hairs, and such plants depend largely on their rootlets. [Illustration: 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.