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)

How Life Learned to Live: Adaptation in NatureJanuary 1, 1983
Helmut Tributsch

“Presumably as a result of the turbulence and air flow patterns created by the ribbing, hc [the heat convection coeffcient], expressed on a unit surface area basis was 67% greater than for a smooth cylinder of the same outside diameter under the turbulence intensity appropriate to field conditions (12). Since the ribbed surface area for this barrel cactus was 54% greater than that of the circumscribing polygonal surface, the total convective loss per level would be just over 2.5-fold higher than for a smooth cylinder.” (Lewis and Nobel 1997:615)

“The simulated rib elimination reduced both the convection and the latent heat terms [of an energy budget model] by reducing the actual plant surface area to that of the circumscribing polygonal surface. Most of the rather small increase in surface temperature at night in the absence of ribs…was due to the accompanying decrease in latent heat loss, while the 1 to 2 C rise during the day was due to the decrease in heat convection from the stem. The importance of ribbing for daytime convective cooling is also seen by comparing the spineless plants…where rib removal increased the average daily surface temperature by 0.8 C for the summer day.” (Lewis and Nobel 1997:615)

Journal article
Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodesPLANT PHYSIOLOGYOctober 27, 2016
D. A. Lewis, P. S. Nobel

“Ribs and tubercles are another type of surface modification that can affect surface temperature. Based on previous data (Lewis and Nobel 1977), the convection coefficient for Ferocactus without spines is 2.8x higher than expected for a smooth cylinder of the same diameter under field conditions (Nobel 1974). For a spineless Mammillaria, the convection coefficient was ~2.6x higher than for a smooth cylinder. Thus, surface irregularities indeed favor convective heat loss and tend to minimize stem-to-air temperature differences, although the effect is lessened somewhat by the presence of spines (Hadley 1972). Ribs of Ferocactus wizlizenii and Carnegiea can be about 30% closer together on the southwest compared to the northeast side (Walter 1931); this could enhance the convective cooling at the region subjected to the highest stem surface temperatures by providing more area for heat exchange.” (Nobel 1973:993)

Journal article
Surface Temperatures of Cacti--Influences of Environmental and Morphological FactorsEcologySeptember 5, 2006
Park S. Nobel

Cactus FamilyCactaceaeFamily