=Roots need Air.=--Corn on land that has been flooded by heavy rains loses its green color and turns yellow. _Besides diluting plant-food, the water drives the air from the soil, and this suffocation of the roots is very soon apparent in the general ill health of the plant._ Stirring or tilling the soil aërates it. Water plants and bog plants have adapted themselves to their particular conditions. They get their air either by special surface roots, or from the water through stems and leaves. =Rootlets.=--_Roots divide into the thinnest and finest fibrils: there are roots and there are rootlets._ The smallest rootlets are so slender and delicate that they break off even when the plant is very carefully lifted from the soil. [Illustration: FIG. 44.--ROOT-HAIRS OF THE RADISH.] _The rootlets, or fine divisions, are clothed with the_ =root-hairs= (Figs. 44, 45, 46). _These root-hairs attach to the soil particles, and a great amount of soil is thus brought into actual contact with the plant._ These are very _delicate prolonged surface cells of the roots_. They are borne for a short distance just back of the tip of the root. _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. 45.--CROSS-SECTION OF ROOT, enlarged, showing root-hairs.] [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 mold-like or gossamer-like covering. _Root-hairs soon die_: they do not grow into roots. New ones form as the root grows.