system
Given the following post, respond as if you're commenting on Reddit:
human
Are all plants with thorns closely related? Or are thorns an example of convergent evolution?

Thorns have an obvious advantage for plants that are prone to being eaten, so it seems possible they could have arisen more than once.
gpt
Well, first we have to define the thorn to distinguish it from other sharp plant appendages.

**Thorns are modified branches or stems**, we must contrast this with modified leaves (spines) and other modified protrusions like hairs (prickles).

So, when we use the designation *thorn* we are using a ~~generalization~~ *piece of nomenclature* to classify a modified portion of a plant. This distinction is important for keeping our anatomy consistent. It means that cacti do not have thorns, they have *spines*, each bundle of spines effectively representing the modified form of all the leaves from an ancestral branch (over generalization here, don't think into it too much). Many cacti have this sort of spine arrangement because it was advantageous in that it both protected from predators and reduced respiration by reducing photosynthetic surface area - they didn't need all those leaves, so the selective pressure to reduce their surface area was very strong - that the surface area was reduced to a hardened spine is kind of an evolutionary bonus that proliferated particularly well.

Many plants with thorns are much less specialized than cacti, various trees, shrubs, and herbaceous plants have modified their stems or branches to include sharp protrusions, because this sort of defense is generally advantageous against herbivores. So any plant with the physiological capacity to produce hardened protrusions (which represents a physiological *cost* in the form of calories), and the selective pressure of herbivory, had a possibility of developing a thorn modification to reduce that negative pressure. And so, as you predicted, the modification appears, and disappears, constantly throughout the histories of many distantly related plant varieties. The modification is not too specialized (like the cacti spine arrangement) that developing it would require life altering physiological changes. Thorns contain vascular tissue and are just a projection of existing anatomy, so many plants can take them or leave them, develop them within life-cycle, or have some progeny who will develop them and others who will not. Making their expression quite transient.

edit: **tl;dr - convergence**