Brushing by a succulent with a spiny trunk this week, I noticed that an interpretive sign said the spines were actually modified stems. That seemed wrong, and it turned out (at least to me) the claim was faulty. In my way of defining words, any sharp plant part that is called a spine has to have been formed in the same way that leaves (and stipules) form. Therefore, cactus "spines" are rightly called spines - because they grow from the same tissues and in the same position as leaves would form, if the cacti made normal leaves. If a pointed plant part is formed by a stem tip that hardens into a sharp spike, then to me this should be called a thorn. Stems grow differently from leaves, and actually form leaves as temporary appendages, thus stems that grow to become hardened, pointed spikes are very different in origin from spines - though the resulting damage might be the same.
The most amusing fallout of adopting precise, origin-based definitions for pointed plant parts is that the sharp, recurved things we call rose thorns are not thorns at all, because they are not formed by stem tips or side branches. Moreover, they do not originate in the same manner or position as leaves, which means they are not spines either. To me, and most botanists, the sharpnesses on roses should be called prickles. That is because prickles are made by a plant in the same way that bark forms. Prickles grow from the outer layers of stem tissue, and can be snapped off without tearing into veins or other tissues that form inside stems and leaves. They begin as tiny flecks of new bark in the outer stem layers, each fleck pushing the tip outward from a widening base. If you check out a rose stem, you can see that the prickles form all around and along the stem, not at all in the predictable pattern you expect from leaves and side branches. Other plants, like Chorisia speciosa and Caesalpinia cacalaco, make bark in the shape of prickles; as the stems grow older and broader they continue to generate more prickles that sometimes merge into more solid patches of bark.
Back to the succulent I brushed against, a Pachypodium. The spines form along the trunk, but they form in a very regular and predictable pattern that has nothing to do with bark. They can always be found in pairs, a ready clue to the fact that they are the sharp and hardened evidence of stipules, which are bits of foliage that form at the base of leaves in some kinds of plants. Stipules are considered to be leaf tissue because they originate as part of the leaf, which means that any stipules that become pointed and hard would also be considered spines. We see these paired "stipular spines" in many other plants, most notably the Crown of Thorns Euphorbia.
Oddly, because they are scattered along the stem, the similarity to rose thorns cannot be ignored. But the difference is real and significant, when you consider origin.
So why the spines, in this case? And why prickles or thorns in others? We can see obvious ways that succulents in drylands and deserts might benefit from the pure mechanical protection of dagger-like parts. But we know very little about the life histories and ecologies of individual kinds of spiny succulents, so we are left with the generalization that spines, thorns, and prickles give desert plants an upperhand in conserving some of the hard-earned water they hoard agains even harder times.
And why should we care to discriminate between a spine, a thorn, and a prickle? To me these words have meaning, metadata, that helps tell the story of how plants grow and develop. It is a brush with comparative morphology, when the word homology reminds us that studying the common origin of differently appearing organs allows a richer appreciation of adaptation and evolution.