Proper Temperature for Root Action.—The root must be warm in order to perform its functions. Should the soil of fields or greenhouses be much colder than the air, the plant suffers. When in a warm atmosphere, or in a dry atmosphere, plants need to absorb much water from the soil, and the roots must be warm if the root-hairs are to supply the water as rapidly as it is needed. If the roots are chilled, the plant may wilt or die.
Fig. 44.—Root-hairs of the Radish.
Roots need Air.—Corn on land that has been flooded by heavy rains loses its green colour 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.
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.
Fig. 45.—Cross-section of Root, enlarged, showing root-hairs.
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.
