Thorns, spines, and prickles

In plant morphology, thorns, spines, and prickles, and in general spinose structures (sometimes called spinose teeth or spinose apical processes), are hard, rigid extensions or modifications of leaves, roots, stems, or buds with sharp, stiff ends, and generally serve the same function: physically deterring animals from eating the plant material. In common language, the terms are used more or less interchangeably, but in botanical terms, thorns are derived shoots (so that they may or may not be branched, they may or may not have leaves, and they may or may not arise from a bud), spines are derived from leaves (either the entire leaf or some part of the leaf that has vascular bundles inside, like the petiole or a stipule), and prickles are derived from epidermis tissue (so that they can be found anywhere on the plant and do not have vascular bundles inside). Leaf margins may also have teeth, and if those teeth are sharp, they are called spinose teeth on a spinose leaf margin (some authors consider them a kind of spine). On a leaf apex, if there is an apical process (generally an extension of the midvein), and if it is especially sharp, stiff, and spine-like, it may be referred to as spinose or as a pungent apical process (again, some authors call them a kind of spine). When the leaf epidermis is covered with very long, stiff trichomes (more correctly called bristles in this case; for some authors a kind of prickle), it may be referred to as a hispid vestiture; if the trichomes are stinging trichomes, it may be called a urent vestiture. There can be found also spines or spinose structures derived from roots. The predominant function of thorns, spines, and prickles is deterring herbivory in a mechanical form. For this reason, they are classified as physical or mechanical defenses, as opposed to chemical defenses. Not all functions of spines or glochids are limited to defense from physical attacks by herbivores and other animals. In some cases, spines have been shown to shade or insulate the plants that grow them, thereby protecting them from extreme temperatures.