The usually underground part of the plant axis, specialized for anchorage, absorption, and sometimes food storage. It may usually be distinguished from a stem by the absence of chlorophyll and buds. (Exceptions are the aerial roots of certain epiphytes, e.g. Taeniophyllum, that can develop chlorophyll when illuminated. Adventitious buds may form on roots and give rise to suckers.) The root system may include an obvious main root derived from the radicle (see
taproot ) or may be fibrous due to repeated branching of the radicle. Alternatively it may include or consist solely of adventitious roots arising from the base of the stem. The vascular tissues of the root normally form a solid central stele, which is better able to resist the tensions and pressures exerted on a root than would be a hollow stelar cylinder, typical of most stems. The absorptive properties of the root are enhanced by the formation of *
root hairs behind the tip. Beyond these the root branches to form lateral roots. Unlike stem branches, these do not arise superficially but develop from the outer tissues of the stele and grow through the root cortex. Lateral root formation is thus said to be endogenous. Roots are usually positively hydrotropic and geotropic. A root cap (see
calyptra ) at the root tip protects the root as it grows down through the soil. Numerous modifications of roots exist (see
aerial root ,
climbing root ,
contractile root ,
pneumatophore ,
prop root ). Symbiotic associations between plant roots and various fungi, bacteria, and blue-green algae are common (see
mycorrhiza ,
root nodule ).
