The Role of Roots in Plant Growth

Root (botany), organ of higher plants, usually subterranean and having several functions, including the absorption and conduction of water and dissolved minerals, food storage, and anchorage of the plant in the soil. The root is distinguished from the stem by its structure, by the manner in which it is formed, and by the lack of such appendages as buds and leaves. The first root of the plant, known as the radicle, elongates during germination of the seed and forms the primary root. Roots that branch from the primary root are called secondary roots. In many plants the primary root is known as a taproot because it is much larger than secondary roots and penetrates deeper into the soil. Beets and carrots are examples of plants with very large taproots. Some plants having taproots cannot be transplanted easily, for breaking the taproot may result in the loss of most of the root system and cause the death of the plant.

Roots arising from the stem are known as adventitious roots. Such roots may be seen near the base of a corn stem. Adventitious roots formed high up on a stem are termed aerial roots or prop roots. Such roots aid in supporting the stem, as in the banyan, the mangrove, and certain orchids.

COMPOSITION

The root is composed of three types of tissue: the epidermis, or surface layer; the ground tissue, or cortex; and the vascular core, situated at the center of the root. Certain cells of the epidermis are modified for an absorptive function. Long, tubelike projections, called root hairs, grow from these cells into the absorptive surface of the root and anchor the root to soil particles. Water absorbed by root hairs is transferred across the cortex, the region of water and food storage, and into the vascular core, which carries it up into the stem. Organization of the vascular core in a root is markedly different from that in a stem. In the stem the vascular tissues xylem and phloem are grouped together in vascular bundles. In the root a central core of xylem has radial bands that extend outward toward the cortex, and between these bands are strands of phloem. In aerial roots the xylem core, which is usually solid in subterranean roots, often has a central zone of pith.

GROWTH

Under normal conditions the growth of roots is influenced chiefly by gravity and by the presence of water. Roots tend to grow downward into soil, unless water is more readily available at the surface. In addition to the primary growth in length occurring at the apex of the root, a secondary growth occurs that adds xylem, or wood, to the inside of the root and phloem toward the outside. Phloem produced in this manner becomes involved in the formation of bark, which covers old roots as well as old stems. Old roots often are virtually identical therefore with old stems.

Because in many plants roots can be formed from a cut end of a stem, cuttings may be used for plant propagation. Some plants, such as the willow or geranium, root quite easily, whereas others, such as the conifer, rarely root without special treatment. Root formation can be stimulated on cuttings of many plants by the application of the so-called root hormones, substances found naturally in the plant when new roots are formed. Most commercial preparations of root hormones contain indoleacetic acid, one of the most common root-stimulating substances. Occasionally roots may be formed from leaves, as in the African violet, which may be propagated by rooting the cut end of a leaf base in water. In some plants roots may give rise to shoots. For example, the stems that are formed at various distances from the base of a Lombardy poplar arise from roots.

USE

Roots of many plants are edible and contain considerable quantities of food materials, particularly starch. Root crops important in agriculture include the sweet potato, beet, turnip, carrot, parsnip, and cassava. The wild forms of these plants have much smaller roots than the cultivated forms because continued development by agricultural peoples has improved the size, texture, food value, and flavor of the roots in cultivated varieties.

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