=Aërial Roots.=--Although roots usually grow underground, _there are some that naturally grow above ground_. These usually occur on climbing plants, the roots becoming _supports_ or fulfilling the office of tendrils. These aërial roots _usually turn away from the light_, and therefore enter the crevices and dark places of the wall or tree over which the plant climbs. The trumpet creeper (Fig. 36), true or English ivy, and poison ivy climb by means of roots. [Illustration: FIG. 36.--AËRIAL ROOTS OF TRUMPET CREEPER OR TECOMA.] [Illustration: FIG. 37.--AËRIAL ROOTS OF AN ORCHID.] In some plants all the roots are aërial; that is, _the plant grows above ground_, and the roots gather food from the air. Such plants usually grow on trees. They are known as _epiphytes_ or _air-plants_. The most familiar examples are some of the tropical orchids, which are grown in glass-houses (Fig. 37). Rootlike organs of dodder and other parasites are discussed in a future chapter. 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 muscadine grape of the Southern states often sends down roots from its stems. 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, color, 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.]