Spines on a cactus are functional. Of course, they have the function of protection but the protection extends to shading the main part of the plant from the sun. Most cactus, but not all, live in arid climates. The ability to store water and maintain life in drought conditions is a major part of their survival. But some cactus have carried their production of spines to an extreme. Here is a comment on the function of spines for both protection and shade from James D. Mauseth, Section of Integrative Biology, The University of Texas, Austin.
Many cacti are protected from full sunlight by a dense covering of spines. Rather surprisingly, perhaps as many as half of all cactus species are adapted to dark rainforests (Epiphyllum, Rhipsalis) or semi-shady shrublands/grasslands (Echinopsis, Mammillaria, Notocactus) or cool, wet, cloudy highlands (Austrocylindropuntia, Oroya, Soehrensia) (Habitats are described in Cactus Odyssey). Such plants would be killed quickly by exposure to full sunlight in Phoenix, Arizona. Species in such cool or semi-shady habitats often have either just a few spines or only very short ones. However, cacti from intensely sunny, hot deserts often must have a complete covering of spines. In many cases, the spines are so strong and painful that they obviously offer both protection and shade, but in many species the spines are so soft that a hungry, thirsty animal would chew through them with no trouble. The benefit of such spines definitely appears to be blocking sunlight and thus preventing the plant from over heating, the chlorophyll from being bleached and the plant's DNA from being damaged. For example, in Mammillaria plumosa, spine epidermis cells project outward as long trichomes, giving the spine a feathery appearance. In other species, the spines are flat, thin and papery, being too flexible to deter animals, but broad enough to shade the plant (as well as to camouflage the cacti among the grasses with which it grows).
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