The functions of roots are: to act as support for the stem; to absorb and conduct water and nutrients from the soil; and to store food. Root systems consist of a main or primary root, rootlets or secondary roots, and root hairs.

The primary root arises from the embryo. Branches of the primary (tap) root are often fibrous and are called secondary roots . The primary root system of many plants is short-lived and is replaced by a secondary root system. These secondary root systems become the permanent roots of many monocotyledons such as the grasses. Roots coming from bulbs, corms, rhizomes, or tubers are adventitious roots, as are roots developed from aerial stems (stolons and runners) and cuttings from stems or leaves. Rhizophora mangle (red mangrove), Pandanus spp. (screw-pine), and Ficus spp. (banyan) send down adventitious roots which become supports for heavy, horizontal branches.

Root hairs are specialized cell extensions which penetrate into the openings between soil particles. The outside wall of the cell becomes distended to form a tubular outgrowth which makes contact with the soil and absorbs water and dissolved minerals from it. Water and soluble nutrients enter the root hairs, pass into the rootlets, and travel through the main root into the stems and leaves. Root hairs are formed in great numbers near the tips of roots. In most plants they are short-lived. If a plant is transplanted carelessly, it is the loss of many of these small root hairs with their water-absorbing cells that will cause the plant to wilt.

Fibrous roots are adventitious, have no distinguishable primary root and are composed of a number of fine, thread-like roots of the same kind and size originating at the base of the stem. Fibrous roots often spread out near the surface of the soil, rather than penetrating straight down or deep.

Fleshy roots become food reservoirs which retain surplus food during the winter or adverse periods to be used by the plant when it is able to renew its growth. Daucus carota (carrots), Brassica rapa (turnips), and Beta vulgaris (beets) have main or tap roots containing food. Ipomoea batatas (sweet potatoes) and Dahlia spp. (dahlia) have secondary roots transformed into tuberous roots packed with food.

Aerial roots form freely on many land and water plants in a favorable, moist atmosphere. These roots enable climbers such as Philodendron spp. (philodendrons) to attach themselves to a host. The aerial roots of air plants or epiphytes such as some orchids not only attach the plant to its host but also absorb water from the air. Many aerial roots are fleshy or semi-fleshy, functioning as reservoirs for water storage.

Knees (pneumatophores) are developed by Taxodium distichum (bald cypress), when grown in swampy ground. As the water or ground cuts off the air from the roots, these trees develop woody-knees which protrude above the surface to enable the plant to obtain air.

Source: Botany Handbook for Florida, Revised Edition, Kathleen C. Ruppert, January 1999 -- This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.