Spines, prickles and thorns are common terms bandied about when referring to sharp objects protruding from plants. There is a distinction to be made between these three terms. Spines denote a structure that is evolutionarily derived from a leaf or part of a leaf (cactuses have spines). Thorns circumscribe modified shoot systems (honey locust trees have thorns). Prickles make up the rest of these plant defensive structures as outgrowths of the skin (epidermis) and underlying tissues (roses are a great example).
Today, thorns are on the agenda, with three distantly related species of plants whose ancestors have independently converged on modified shoot systems to stab opponents. Gleditsia tricanthos (honey locust, 1237-79*C, top), from the well-armed collection of honey locust trees on top of Peters Hill. Branching is a good indicator that these structures are modified shoot systems, in contrast to spines and prickles. Almost identical in appearance, but independently evolved, are the branching thorns of Crataegus crus-galli (a wonderful specimen of cockspur hawthorn from 1903, 5029*A, bottom left).
Thorns need not be branched, and Cudrania tricuspidata (silkworm thorn, 1354-73*C, from the mulberry family) demonstrates that point perfectly. How do I know this is a modified shoot (thorn) when it is unbranched and could be a prickle or a spine? The key is that it is always found in the axil of a leaf or leaf scar, exactly where branches arise in most plants. But beware of pairs of unbranched thorn-like structures associated with the bases of leaves or leaf scars. If found in pairs, they are almost certainly modifications of the leaf base (hence, spines) – a topic for part 3.