Ferocactus cylindraceus (California barrel cactus) ( )

A means by which desert plants influence the energy balance of their leaves comes with leaf orientation. Many desert leaves have a favored direction of placement, often meaning that they are oriented vertically. Such a position allows the leaves to maximize the amount of direct solar radiation they receive in the morning and afternoon hours when temperatures are cooler, and to minimize the direct solar radiation they receive at midday. By orientating the leaf vertically, chlorophyll and stomata are effectively hidden on both upper and lower leaf surfaces. This reduces solar irradiance during peak, mid-day exposure and maximizes it when the light angle is less direct. Directionality is seen in many cactus species, with the barrel cactus serving as a good example. The succulent stems of these cacti lean toward the south, maximizing solar radiation on the growing tip and reproductive area of the cactus, and minimizing radiation on the sides of the stem. "California Desert Plants, Philip W. Rundel, Michael J. Gustafson, Michael E. Kauffmann."