I noticed while watering my cacti a few days ago that one effect of all those spines is to trap and hold water droplets. I can understand the advantage this would give plants in temperate conditions -- that eventually the drops will fall off onto the ground, adding a little more moisture to the roots. But I can't help but think that in a cactus' natural habitat, this would be at best pointless. In hot and dry conditions, small water drops would evaporate within minutes. While clinging to the cactus, they could potentially concentrate the sun like a magnifying glass, so that the cactus would be burnt. Can anyone enlighten me?
Mostly, they're to discourage animals from eating them. But there are some cacti where the spines (and even more, the hairs on cacti like Cephalocereus senilis) act as water-gatherers, trapping mist, fog and night-time dew, and dribbling it onto the roots. This is most important in the coastal Atacama Desert, where it never rains but they do get regular sea fog (some nice footage of it in action on the Planet Earth TV series, showing desert cacti covered in rainforest epiphytes). The droplets are too small to have any burning effect; the idea that they do is an old gardener's urban legend.
My understanding was that the spines also provide shade for the plant albeit not much shade but I guess a little is better than none at all. Anne
I think they also can create an air layer that buffers the plant against the loss of heat and moisture, but mostly it protects them from herbivores.
I'd agree with Eric add that really dense spines, like the ones on the Jumping Chollas (Opuntia spp. i forget) for example, can also act as a physical sunblock to prevent overheating of the plants, much the same way that the dense hairs on the leaves of highland frailejones do. I've seen some pretty succesful cacti growing in the paramo here, which is extreme as far as solar radiation goes. These are, to a cactus, furred with dense white spines. And yes, the spininess does seem to keep the llamas from eating them.
That the spines are white would reflect light, and so help in protecting the cactus from burning, of course.
Now in the genus Cylindropuntia. This genus adds another adaptation for the spines - as a dispersal mechanism. The pads break off very easily, with the barb-tipped spines helping the pads to stick to passing animals. The animals then knock the pads off some distance away from the parent plant, where they can take root and grow into a new plant.
When I was on a trip to Arizona someone tried to kid me that cholla pads really could detect people coming and jump out on them from a distance . . . . . . but he was the one who ended up with a pad on his leg, and I'm certain that he never went within two metres of the plant :-)
Ah, for the arboreal epiphytic species (like Hylocerus and Stenocerus) the spines on the treeward side help to anchor the cactus to its host. Interestingly, when Hylocerus fruits are ripe the spines fall off of them to encourage beasties to chow down and spread the seeds. Good indicator if you're growing them for your own consumption!
