Cactus spines have several purposes, one of which is to defend the plant against predators.A single thin spine doesn't cast much shade, but when there are enough of them, they can offer a sizable amount of sun protection.By reflecting sunlight, white and other pale-coloured spines reduce solar damage.By controlling the airflow around the plant, spines on cacti are an intriguing way to thrive in arid settings.By preserving moisture for the plant, spines help cactus survive in the desert. When it rains, fogs, mists or dews, the moisture in the air condenses on the cactus' spines and drips to the ground, where its weak roots take it in.