Cacti stay cool by having ribs that provide shade and enhance heat radiation.
"The same applies to the intricate structural designs of cacti, which are exposed to a great deal of heat pressure in the desert. Their heat-reflecting capacity is low, since their surface is greatly reduced so as to cut down on evaporation. Nature has solved the problem by equipping many cacti with cooling ribs. These shade the cactus's surface against the scorching sun and simultaneously improve heat radiation. The alternating planes of light and shade of the vertical cooling ribs of the torch thistle produce rising and falling air currents, which improve heat radiation. And when the sun reaches its highest position, it hits the torch thistle from above, where it presents its smallest surface. A botanist discovered that torch thistles perish of burns when they are placed horizontally in the sun." (Tributsch 1984: 136)
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