Structure of a Root Hair.--A single root hair examined under a compound microscope will be found to be a long, round structure, almost colorless in appearance. The wall, which is very flexible and thin, is made up of cellulose, a substance somewhat like wood in chemical composition, through which fluids may easily pass. Clinging close to the cell wall is the protoplasm of the cell. The interior of the root hair is more or less filled with a fluid called _cell sap_. Forming a part of the living protoplasm of the root hair, sometimes in the hairlike prolongation and sometimes in that part of the cell which forms the epidermis, is found a _nucleus_. The protoplasm and nucleus are alive; the cell wall formed by the living matter in the cell is dead. _The root hair is a living plant cell_ with a wall so delicate that water and mineral substances from the soil can pass through it into the interior of the root. [Illustration: Diagram of a root hair; _CS_, cell sap; _CW_, cell wall; _P_, protoplasm; _N_, nucleus; _S_, particles of soil.] How the Root absorbs Water.--The process by which the root hair takes up soil water can better be understood if we make an artificial root hair large enough to be easily seen. An egg with part of the outer shell removed so as to expose the soft skinlike membrane underneath is an example. Better, an artificial root hair may be _made_ in the following way. Pour some soft celloidin into a test tube; carefully revolve the test tube so that an even film of celloidin dries on the inside. This membrane is removed, filled with white of egg, and tied over the end of a rubber cork in which a glass tube has previously been inserted. When placed in water, it gives a very accurate picture of the root hair at work. After a short time water begins to rise in the tube, having passed through the film of celloidin. If grape sugar, salt, or some other substance which will dissolve in water were placed in the water outside the artificial root hair, it could soon be proved by test to pass through the wall and into the liquid inside. Osmosis.--To explain this process we must remember that gases and liquids of different densities, when separated by a membrane, tend to flow toward each other and mingle, the greater flow always being in the direction of the denser medium. _The process by which two gases or fluids, separated by a membrane, tend to pass through the membrane and mingle with each other, is called osmosis._ The method by which the root hairs take up soil water is exactly the same process. It is by osmosis. The white of the egg is the best possible substitute for living matter; the celloidin membrane separating the egg from the water is much like the delicate membrane-like wall which separates the protoplasm of the root hair from the water in the soil surrounding it. The fluid in the root hair is denser than the soil water; hence the greater flow is toward the interior of the root hair.[10]