(pili radicales), outgrowths of the cells of the surface tissue (epiblem) of the absorption zone of a root.

Root hairs contain the parietal layer of protoplasm, the nucleus, and a large vacuole of the epiblemic cells. Their thin mucous walls, readily penetrated by water, stick to clumps of soil, releasing various substances into the soil that convert poorly soluble compounds into forms that are readily assimilable by the plant. Other of these substances aid the development of microflora. The root hairs also serve to support the growing tip of the root.

Root hairs are short-lived, usually dying off in 15–20 days. Their length varies in different plants from 0.06 to 10 mm. On the other hand, the total length, surface, and number of root hairs in a single plant can reach significant quantities (for example, the overall length in a wheat plant is about 20 km). With increased moisture in the soil and poorer aeration, the formation of root hairs slows, and they fail to form at all in very dry soil. Many aquatic and mycotrophic plants (for example, pines and beech) lack root hairs.