Thorns, spines, and prickles
Thorns, spines, and prickles occur in a wide variety of ecologies, and their morphology also varies greatly. They occur as sharpened branches (e.g. in Carissa, Citrus, Crataegus), spiky inflorescences (e.g. in Tylecodon reticulatus), a tiny point at the tip of the leaf (mucronate leaves) (e.g. in Sansevieria), leaves fully converted to spines (e.g. in Opuntia), stipules converted to spines (e.g. in many Acacia), prickles on stems (e.g. of Rosa, Erythrina and Ceiba speciosa), uriticating (i.e. stinging) hairs, bristles, and finely barbed spines called glochids. Some thorns are hollow and act as myrmecodomatia, others (e.g. in Crataegus monogyna) bear leaves. Thorns of some species are branched (e.g. in Crataegus crus-galli, Carissa macrocarpa).
It has been proposed that thorny structures may first have evolved as a defense mechanism in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage. However, the suggestion was unsupported by any argument to discount the likelihood that spiny defences might have been developed as means of defence in say, resource-rich environments where herbivory might have been more intense than in the hypothesized sandy environments.
Not all functions of spines or glochids are limited to defence from physical attack by herbivores and other animals. In some cases, spines have been shown to shade or insulate the plants that grow them (e.g. saguaro cactus spines shade the apical meristem in summer and in members of the Opuntioideae glochids insulate the apical meristem in winter).
| image sources: 1, 2