1. (in botany) The part of a vascular plant that grows beneath the soil surface in response to gravity and water. It anchors the plant in the soil and absorbs water and mineral salts. Unlike the stem, it never produces leaves, buds, or flowers and usually does not contain chlorophyll. The radicle (embryonic root) may give rise either to a tap root system with a single main tap root from which lateral roots develop, or a fibrous root system, with many roots of equal size. Adventitious roots arise from the stem—whether above ground or underground—and these are common in monocots such as grasses and cereals, replacing the primary root as the plant grows. The apical meristem at the root tip gives rise to a protective sheath, the root cap, and to the primary tissues of the root. The vascular tissues usually form a central core (see illustration). This distinguishes roots from stems, in which the vascular tissue often forms a ring. A short distance behind the root tip root hairs develop from the epidermis and greatly increase the surface area for absorption of water and minerals (see rhizosphere). Beyond this, lateral roots develop.

Roots may be modified in various ways. Some are swollen with food to survive the winter, as in the carrot. Certain plants, such as orchids, have absorptive aerial roots; others, such as ivy, have short clasping roots for climbing. The roots of leguminous plants, such as beans and peas, contain root nodules, which have an important role in nitrogen fixation. Other modifications include prop roots, stilt roots, and buttress roots, which support the plant; and pneumatophores. The roots of most plants form associations with fungi called mycorrhizae, which greatly enhance the plant’s ability to absorb nutrients from the soil and to communicate with neighbouring plants.

2. (in dentistry) The portion of a tooth that is not covered with enamel and is embedded in a socket in the jawbone. Incisors, canines, and premolars have single roots; molars normally have several roots.

3. (in anatomy) The point of origin of a nerve in the central nervous system. There are two roots for every spinal nerve (see dorsal root; ventral root).

Section through the tip of a plant root