1911 Encyclopædia Britannica/Root

ROOT (late O.E. rōt, adopted from Scand., cf. Norw. and Swed. rot, Dan. rod; the true O.E. word was wyrt, plant, represented in Ger. Wurz or Wurzel; the ultimate root is the same in both words, and is seen in Lat. radix), the underground part of a plant. This is the popular meaning of the word. In its botanical use the term is more restricted (see below). The various other meanings have all developed from this, its primary, significance. Of these the principal are: the source or origin of a condition, state, quality, &c.; the base or embedded part of a structure of the body, such as a nail, tooth, the hair, &c.; in mathematics, a number, quantity or dimension which produces a given expression when multiplied by itself a requisite number of times; and in philology an ultimate element of language, incapable of further analysis. A particular extension of the primary meaning is that which applies the word generally to a class of plants, such as the turnip or carrot, whose root is fleshy, and edible either by man or domestic animals.

The embryo of a typical plant, for instance a pea plant (fig. 1), has an ascending axis which will grow into the shoot, and a descending axis or radicle which will grow into the root. Fig. 1.—The Dicotyledonous Embryo of the Pea laid open. c, c, the two fleshy cotyledons, or seed-lobes, which remain under ground when the plant sprouts; r, the radicular extremity of the axis which develops into the root; t, the axis bearing the young stalk and leaves g, which lie in a depression of the cotyledons f. When the seed germinates, the radicle is the first to appear; it grows downwards, and its primary function is to act as a holdfast for the plant; its most important function, however, is the absorption of water and dissolved nutrient substances from the soil, and it also frequently serves for storage of food-stuffs. 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 protects 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. It is by means of the root-hairs especially that the root is brought into close relation with the soil particles and absorbs the nutrient materials in solution in the water which surrounds these 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 developed 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 (endogenous) they develop in longitudinal rows and have to break through the overlying tissue of the parent root (fig. 2). True forking of the root (dichotomy) occurs in the Lycopodiaceae (the shoots of which also branch dichotomously), but is unknown in the higher plants.

From Vines's Student's Botany, by permission.

Fig. 2.—Lateral Roots n arising endogenously from the pericycle of the Tap-Root of Vicia Faba (longitudinal section. f, axial cylinder (stele); r, cortex of main root; h, root-cap of lateral root.

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 adventitious roots. They may also arise from leaves under similar conditions, as, for instance, from begonia leaves when planted in soil.

From Green's Vegetable Physiology, by permission.

Fig. 3 a and b.

Root-hair in contact with particles of soil (highly magnified).

Ultimate root-branches, showing position of root-hairs.

Fig. 4.—Fibrous Root of a Grass. Numerous fibrils coming off from one point.		From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
		Fig. 5.—Root-Tubers of Dahlia variabilis. s, the lower portions of the cut stems.

The forms of roots depend on their shape and mode of branching. 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 (fig. 4); when the fibrils are thick and succulent, the root is fasciculated, as in Ranunculus Ficaria, Asphodelus luteus, and Oenanthe crocata; when some of the fibrils are developed in the form of tubercles, the root is tubercular, as in dahlia (fig. 5); when the fibrils enlarge in certain parts only, the root is nodulose, as in Spiraea Filipendula, or moniliform, as in Pelargonium triste, or annulated, as in Ipecacuanha. Some of these so-called roots are formed of a stem and root combined, as in Orchis (fig. 6), 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, Fig. 6.—Base of plant of Orchis, showing tubercules or tuberous roots. 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 Banyan (Ficus indica, fig. 7), 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. In tree-ferns they form a dense coating around, and completely concealing, the stem; such is also the case in some Dracaenas and palms. In Epiphytes, or plants growing in the air, attached to the trunks of trees, such as orchids of warm climates, the aerial roots produced do not reach the soil; they continue always aerial and greenish, and they possess stomata. Delicate hairs are often seen on these epiphytal roots, as well as a peculiar spongy investment formed by the cells of the epidermis which have lost their succulent contents and are now filled with air. This layer is called the velamen, and serves to condense the moisture contained in the air, on which the plant is dependent for its water-supply. The aerial 'roots of the ivy are not the nutritive roots of the plant, but are only intended for mechanical support. The climbing roots of many orchids, aroids and epiphytic ferns branch and form places of lodgment for humus into which absorbent branches of the climbing roots penetrate. Some leafless epiphytic orchids, such as species of Angraecum, depend entirely upon their aerial roots for nourishment; the roots, which are green, perform the functions both of leaves and roots. A respiratory or aerating function is performed by roots of certain mangroves growing in swampy soil or water and sending vertical roots up into the air which are provided with aerating passages by which the root system below can communicate with true outside air.

Fig. 7.—Ficus indica, the Banyan tree, sending out numerous aerial roots, which reach the soil, and prop the branches.

Parasitic plants, as the mistletoe (Viscum), broom-rape (Orobanche) and Rafflesia, send root-like processes into the substance of the plants whence they derive nourishment. In the dodder (Cuscuta), the tissue around the root swells into a kind of sucker (haustorium), which is applied flat upon the other plant, and ultimately becomes concave, so as to attach the plant by a vacuum. From the bottom of the sucker the root protrudes, and penetrates the tissue of the host plant. Leaf-buds are sometimes formed on roots, as in plum, cherry and other fruit trees; the common elm affords an excellent example, the young shoots which grow up in the neighbourhood of a tree arising from the roots beneath the soil. In some plants no roots are formed at all; thus in the orchid Corallorhiza, known as coral-root, a stem-structure, the shortly branched underground rhizome, performs all the functions of a true root which is absent. In aquatic plants the root acts merely as a holdfast or is altogether absent as in Salvinia, Utricularia and others.