Page:EB1911 - Volume 23.djvu/748

Rh Hamilton, 1896; Yale, 1900; Columbia, 1904; New York University, 1904; Williams, 1905; Princeton, 1906; University of Buenos Aires, 1906; University of San Marcos, of Lima, 1906; and Harvard, 1907.  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. 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.

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.

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