An obvious adaptation against predation is plant armature, i.e., sharp projections that threaten or injure the visitor. Very tiny animals can move around such defenses, so sharp projections are mainly there to impale, and thereby discourage visits by, larger animals, particularly birds, herbivorous lizards, and mammals, or to discourage foraging by animals with chewing mouth parts. Armature may be absent on shoots and reproductive structures at times when large animals are invited to be pollinators and then form later on reproductive structures to enable dispersal of fruits.
It is virtually impossible to determine in most modern cases what animals were involved in natural selection of the structures, because armature in many groups is ancient, and therefore related to types of vertebrates that may no longer be extant. Plants with armature can serve as refuges for nesting animals, which nest among the spines and emergences, relatively safe from their predators.
Most often, a sharply pointed tough or woody structure is called a spine if it is a modified plant organ,. Plant armature is properly termed an emergence or prickle if the structure is instead a superficial development on a surface of a plant organ. A spine generally contains some vascular tissue, whereas an emergence generally does not. Referring to projections from the surface of a plant organ, an emergence has epidermal and subepidermal or periderm tissue, whereas a trichome (hair) would include mostly cells of epidermal origin only. Many spines and emergences contain only dead cells at maturity, but that is not always the case.
Any hard projection (either a spine or emergence) has cells with cell walls fortified especially with lignin. The cells making the structure hard often belong to a type of cell called sclerenchyma, and most commonly a spine has lignified fibers or, in some cases, lignified wood fibers. Less commonly the term "thorn" is used, and there is confusion and controversy sometimes as to which term would be most proper. Spines and emergences are straight-tipped, curved, or hooked, and they may also have cellular barbs or backward-pointing prominences at the tip or along the edges.
Emergences merge into softer multicellular trichomes. The most obnoxious of the multicellular trichomes used to discourage animal feeding are stinging (irritating) hairs, which, when brushed, stab the skin and release toxins (e.g., stinging nettles, stinging euphorbs).
There are a few cases where spination is so incredibly dense that the living portion of the shoot is shaded by the spines. In such instances, spines may serve an added function of modifying the plant body temperature, as in certain cacti, where a spine canopy keeps a shoot apex a degree or two warmer on a cold night or reflects some infrared radiation during the day, cooling it slightly, but it is a widely held misbelief by many biologists and amateurs that spines create thick boundary layers and thereby reduce transpiration.
Plant morphologist want a term to be explicit in indicating nature of origin and location: