Thorns

Thorns, or SPINES, are hard, pointed, woody structures. They are metamorphosed leaves or parts of leaves, as is shown by their position on plants; for lateral buds occur in their axils—i.e. in the angle made by them with the internodes (parts of stem) above their insertions, which is one of the characteristics of leaves. In the Barbary the compound spines are entire metamorphosed leaves; in Robinia pseudacacia the spines are metamorphosed stipules, parts of the leaf-sheaths; in many species of Caragana and Astragalus they are pointed persistent petioles (leaf-stalks). The thorns of the Blackthorn and Hawthorn are pointed branches; in the Furze (whin) they are both pointed leaves and stems. They are to be distinguished from Prickles (q.v.), which are merely pointed projections of the epidermis, and are therefore to be compared with hairs and surface-glands. Spines, and prickles also, tend to disappear from plants under cultivation—i.e. from plants that are growing luxuriantly; and conversely, they tend to increase in plants growing in unfavourable conditions. These and other facts have led to the theory that spines in general are an expression of the 'ebbing vitality of a species,' and not, as is usually thought, the result of a process of natural selection which has caused them to be evolved as protections against mammals. But this theory has been severely criticised by Mr A. R. Wallace in his Darwinism, who argues that spinous plants are vigorous and have an extensive range; and, further, that they are very rare or entirely absent from those districts, oceanic islands for instance, where there are few or no mammals, which is what the theory of their selection as protective organs would lead one to expect; and genera which have many spinous species in other countries have no such representatives in oceanic islands.