<P> Stinging trichome in Urtica dioica . </P> <P> Spinescent is a term describing plants that bear any sharp structures that deter herbivory . It also can refer to the state of tending to be or become spiny in some sense or degree, as in: "...the division of the African acacias on the basis of spinescent stipules versus non-spinescent stipules ..." </P> <P> There are also spines derived from roots, like the ones on the trunk of the "Root Spine Palms" (Cryosophila spp .). The trunk roots of Cryosophila guagara grow downwards to a length of 6--12 cm, then stop growing and transform into a spine . The anatomy of crown roots on this species (roots among the bases of the living fronds) also alters during their life . They initially grow upwards and then turn down and finally they, too, become spinous . Lateral roots on these two types of roots, as well as those on the stilt roots on this species, also become spinous . Some authors believe that some of these short spiny laterals have a ventilating function so they are' pneumorhizae' . Short spiny laterals that may have a ventilating funcion may also be found on roots of Iriartea exorrhiza . There are also spines that function as pneumorhizae on the palm Euterpe oleracea . In Cryosophila nana (formerly Acanthorhiza aculeata) there are spine roots or root spines, some authors may prefer "root spines" if the length of the root is less than 10x the thickness, and "spine roots" if the length is more than 10x the thickness . Adventitious spiny roots have also been described on the trunks of dicotyledonous trees from tropical Africa (e.g. Euphorbiaceae, as in Macaranga barteri, Bridelia micrantha and B. pubescens; Ixonanthaceae, Sterculiaceae), and may also be found protecting perennating organs such as tubers and corms (e.g. Dioscorea prehensilis - Dioscoreaceae - and Moraea spp . - Iridaceae - respectively). Short root spines cover the tuberous base of the epiphytic ant - plant Myrmecodia tuberosa (Rubiaceae), these probably give protection to ants which inhabit chambers within the tuber as they wander over the plant's surface . (Jackson 1986 and references therein). In many respects the pattern of spine formation is similar to that which occurs in the development of thorns from lateral shoots . (Jackson 1986 and references therein). </P> <P> It has been proposed that thorny structures may have first evolved as a defense mechanism in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage . However, this suggestion was unsupported by any argument to discount the likelihood that spiny defenses might have been developed as a means of defense in resource - rich environments, where herbivory might have been more intense than in the hypothesized sandy environments . </P>

Thorns on plants is a feature of trees in