Historical reference: Do not rely on this information. It is very old. About this collection
Thorn, a term preferably confined to the rigid woody sharp-pointed modifications of leaf-structures, similar stem-structures being termed spines, and mere hypodermal emergences which resemble them, prickles. In the teazles, thistles, holly, and Mahonia we have surface or marginal thorns on the blade of the leaf; in Robinia and Acacia the stipules are thorns; in cactuses and some barberries the entire leaf is reduced to a thorn. Some writers use the terms thorn and spine in a manner the converse of the above.