Thorns, spines, and prickles

In some cases, spines have been shown to shade or insulate the plants that grow them, thereby protecting them from extreme temperatures.

Agrawal et al. (2000) found that spines seem to have little effect on specialist pollinators, on which many plants rely in order to reproduce.

Some cacti have also glochids (or glochidia, singular glochidium) – a particular kind of spine of different origin, which are smaller and deciduous with numerous retrose barbs along its length (as found in areoles of Opuntia).

[citation needed] A study published for peer review to the journal Science concluded that plants with these types of prickles have been identified as sharing a common gene family.

Trichomes, in particular, are distinct from thorns, spines, and prickles in that they are much smaller (often microscopic) outgrowths of epidermal tissue, and they are less rigid and more hair-like in appearance; they typically consist of just a few cells of the outermost layer of epidermis, whereas prickles may include cortex tissue.

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..."[11] There are also spines derived from roots, like the ones on the trunk of the "Root Spine Palms" (Cryosophila spp.).

[5] 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.

In many respects, the pattern of spine formation is similar to that which occurs in the development of thorns from lateral shoots.

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.

Examples include hawthorn hedges in Europe, agaves or ocotillos in the Americas and in other countries where they have been introduced, Osage orange in the prairie states of the US, and Sansevieria in Africa.