ROOT, in popular use, the part of a plant which is normally below the surface of the earth. Botanically its application is more restricted. The embryo of a typical plant, for instance a pea plant, has an ascending axis which grows into the shoot, and a descend ing axis or radicle which grows into the root. When the seed germinates, the radicle is the first to appear ; it grows downwards, and its primary function is to act as a hold fast for the plant ; its most important func tion, however, is the absorption of water and dissolved nutrient substances from the soil, and it also frequently serves for storage of foodstuffs. The root is distinguished from underground shoots by not bearing leaves and by having its apex (growing point) protected by a cap (root-cap), which can be clearly seen by making a median vertical section through the root-tip; the cap pro tects it in its passage through the soil. The root also generally bears root-hairs, slender unicellular outgrowths of the outer layer, borne in the region a little behind the root tip. The root-hairs serve to increase the surface of the root for absorption of water and mineral substances; they also are of service in bringing the root into intimate relationship with the soil particles; the older root-hairs are continually dying off, so that they are borne only on a small part of the area behind the apex. Branches of the root, which repeat the form and structure of the main root, are devel oped in regular succession from above downwards (acropetal), and owing to the fact that they originate in a definite position in the interior of the root (generally opposite the xylem masses) they develop in longitudinal rows and have to break through the overlying tissue of the parent root. True forking of the root (dichotomy) occurs in the Lycopodiaceae (the shoots of which also branch dichotomously), but is unknown in the higher plants.
Roots which originate elsewhere than as acropetal outgrowths of a main root are known as adventitious, and may arise on any part of a plant. They are especially numerous on underground stems, such as the under side of rhizomes, and also develop from stem nodes under favourable conditions, such as moisture and absence of light; a young shoot or a cutting placed in moist soil quickly forms adventi tious roots. They may also
arise from leaves under similar conditions, as, for instance, from begonia leaves when planted in soil.
The forms of roots depend on their shape and mode of branch ing. When the central axis goes deep into the ground in a tapering manner without dividing, a tap root is produced. This kind of root is sometimes shortened, and becomes swollen by storage of food-stuffs, forming the conical root of carrot, or the fusiform or spindle-shaped root of radish, or the napiform root of turnip. In ordinary forest trees the first root protruded continues to elongate and forms a long primary root-axis, whence secondary axes come off. In primary plants, especially monocotyledons, the primary axis soon dies and the secondary axes take its place. When the descending axis is very short, and at once divides into thin, nearly equal fibrils, the root is called fibrous, as in many grasses; when the fibrils are thick and succulent, the root is fasciculated, as in Ranunculus Ficaria, and Oenailthe crocata; when some of the fibrils are developed in the form of tubercules, the root is tuber culate, as in dahlia. Some of these so-called roots are formed of a stem and root combined, as in Orchis, where the tuber consists of a fleshy swollen root bearing at the apex a stem bud. As in the case of the stem, growth in length occurs only for a short distance behind the apex, but in long-lived roots increase in diameter occurs continually in a similar manner to growth in thickness in the stem.
Roots are usually underground and colourless, but in some cases where they arise from the stem they pass for some distance through the air before reaching the soil. Such roots are called aerial. They are well seen in the screw-pine (Pandanus), the Ban yan (Ficus indica), and many other species of Ficus, where they assist in supporting the stem and branches. In the mangrove they often form the entire support of the stem, which has decayed at its lower part.