ROOT, that plant part which normally penetrates the soil, absorbs water and chemicals in solution, for nutritive purposes, forces it through the plant and also acts as an anchor and support. Roots, however, in modified forms, may draw nutriment from other vegeta tion or from the air, and may even serve as support and aid to climbing-plants. Low forms of vegetation and certain submerged aquatics able to absorb water through most of their tissues have no roots, or only enough to act as anchors.
The primary form of root is that which secures the germinating seed to the ground and strikes downward into the soil, sending off branches which rebranch until the ground oc cupied by the fibrous reticulation is thoroughly exhausted of food. In many cases, however, this single central tap-root, so-called, is miss ing, and numerous horizontal and lateral roots take its place. The growth of the root-system is commensurate with that of the plant above ground. In epiphytes the roots either adhere to the site of the plant, to hold it in place, or depend freely in the air, drawing nourishment therefrom.
Plants are called annuals, biennials or per ennials, according to the length of time the root lives, but the differences are not always ab solute, and are often changed by cultivation. Perennials, such as quack grass and peony, of ten have thickened, tuber-like roots, in which nour ishment is stored during the growing periods, to be drawn upon when the plant flowers. Bi ennials such as beets and carrots show similar development.
Each root is a naked organ, incapable of bearing leaves, which has a blunt aoex shielded by a root-ucapp more or less distinct, of old and firm cells. The growing cells of the root lie directly behind this cap, and still farther back is a zone of delicate tubular, one-celled root-hairs, which are devoted to the absorption of liquid foods from the soil and are constantly dying off and being renewed on the side next the root-cap. The growing tip with cap and root-hairs continually advances through the soil, while the older portions become inactive so far as absorption is concerned, thicken, envelop themselves in cork and increase their means of conducting water from the region of the root-hairs to the rest of the plant.
Roots may be produced from stem parts (easiest at the nodes) or even leaves as in Bryophyllum and Begonia. They are then called adventitious. Familiar examples of adven titious roots are the aerial roots of banyan and mangrove, which start from the branches and eventually penetrate the ground. These roots not only assist in feeding the trees, but also help to support their gigantic heads. Vines, such as the poison-ivy, have masses of aerial rootlets which spring from and fasten the stem firmly to its support.
The haustoria of certain parasitic plants, such as dodder (Cuscuta), are forms of re duced roots. They are special organs of ab sorption which develop from the tissues of the parasite and penetrate the body of the host plant either by means of solvent ferments or the pressure resulting from their own growth or both combined. Once there, they draw sus tenance from their host and often dispense with assimilative organs of their own.
Other parasites have disc-like haustoria upon their root-systems, which fasten upon the roots of other plants. Although appearing above ground like self-supporting herbs with green foliage, vigorously assimilative, these plants do not reach full development without this clandes tine connection. The young roots of sapro phytes, or plants living upon the decaying re mains of vegetation, seem to require the co operation of fungi in obtaining food.
The symbiotic connection of bacteria and leguminous plants is well known. The bacteria penetrate through the root-hairs of the Legumi into the cortex of the root and there raise tubercular growths. They draw upon the carbohydrates developed by the host. but being capable of fixing free nitrogen, they exchange this element in elaborated form to the host plant. By plowing the legumes under the ground they decay, returning the nitrogen to the soil in a form available for the next crop. This process, called green manuring, is good treatment, especially for exhausted, non-nitroge nous soils.