When you think of a cactus, one of the first things that comes to mind are the spines. And indeed, most cactus do bear spines. But why?
The most obvious reason is protection from predators. Most spines are sharp and animals are deterred from eating or otherwise damaging the plant by its painful protrusions. (Some animals do make it through the prickly barrier, though: nothing is perfect!)
Spines also help the plant collect moisture. In very dry deserts where it rarely rains, there can still be dew in the morning and a spiny outside on which dew can condense can help cactus gather a considerable amount of moisture.
Spines also offer shade from the sun’s strongest rays. If it seems preposterous that such thin objects could offer much shade, look at again at how dense some cactus spines can be.
At the same time they keep off sunlight, spines can also be keeping the stems cooler and reducing loss of precious moisture to evaporation.
Finally, spines can help some species get around. Chollas or jumping cacti (several species of Cylindropuntia) have spines that readily hook onto passing animals (including people). Their stems then break off and travel with the animal until it manages to dislodge the painful voyager. The segment, now far from its original home, can then fall on the ground, root and start a new colony.
Many other succulent plants – and even non-succulent ones! – also bear spines and use them for many of same reasons as cactus.
You’ve got to hand it Mother Nature: if there’s a way to get something done, she’ll find it!