Some plants bear aërial roots, that may _propagate the plant_ or may _act as braces_. They are often called =prop-roots=. The roots of Indian corn are familiar (Fig. 38). Many ficus trees, as the banyan of India, send out roots from their branches; when these roots reach the ground they take hold and become great trunks, thus spreading the top of the parent tree over large areas. The mangrove tree of the tropics grows along seashores and sends down roots from the overhanging branches (and from the fruits) into the shallow water, and thereby gradually marches into the sea. The tangled mass behind catches the drift, and soil is formed. [Illustration: FIG. 38.—INDIAN CORN, showing the brace roots at _oo_.] =Adventitious Roots.=—Sometimes roots grow from the stem or other unusual places as the result of some accident to the plant, being located without known method or law. They are called =adventitious= (chance) =roots=. Cuttings of the stems of roses, figs, geraniums, and other plants, when planted, send out adventitious roots and form new plants. The ordinary roots, or soil roots, are of course not classed as adventitious roots. The adventitious roots arise on occasion, and not as a normal or regular course in the growth of the plant. =No two roots are alike=; that is, they vary among themselves as stems and leaves do. _Each kind of plant has its own form or habit of root_ (Fig. 39). Carefully wash away the soil from the roots of any two related plants, as oats and wheat, and note the differences in size, depth, direction, mode of branching, number of fibrils, colour, and other features. The character of the root system often governs the treatment that the farmer should give the soil in which the plant or crop grows. [Illustration: FIG. 39.—ROOTS OF BARLEY AT _A_ AND CORN AT _B_. Carefully trace the differences.] Roots differ not only in their form and habit, but also in colour of tissue, character of bark or rind, and other features. It is excellent practice to _try to identify different plants by means of their roots_. Let each pupil bring to school two plants with the roots very carefully dug up, as cotton, corn, potato, bean, wheat, rye, timothy, pumpkin, clover, sweet pea, raspberry, strawberry, or other common plants. =Root Systems of Weeds.=—Some weeds are pestiferous because they seed abundantly, and others because their underground parts run deep or far and are persistent. Make out the root systems in the six worst weeds in your locality. CHAPTER VIII THE ROOT.—FUNCTION AND STRUCTURE =The function of roots is twofold=,—to provide _support or anchorage_ for the plant, and to _collect and convey food_ materials. The first function is considered in Chapter VII; we may now give attention in more detail to the second. [Illustration: FIG. 40.—WHEAT GROWING UNDER DIFFERENT SOIL TREATMENTS. Soil deficient in nitrogen; commercial nitrogen applied to pot 3 (on right). ]