Desert Flora and Water-Conserving Adaptations

Desert plants face a single overriding constraint, water scarcity, and among the most distinctive physiological adaptations to it is Crassulacean Acid Metabolism, or CAM photosynthesis, named for the Crassulaceae family in which it was first studied and now documented in cacti, agaves, and many other succulents. Unlike most plants, which open their stomata during the day to admit carbon dioxide while inevitably losing water vapor to the hot, dry air, CAM plants open their stomata only at night, when temperatures are lower and humidity higher, taking in carbon dioxide and storing it as malic acid within their cells; during daylight hours their stomata remain sealed while the stored acid is broken down internally to supply carbon dioxide for photosynthesis, decoupling gas exchange from the sun's heat and dramatically reducing water loss compared with ordinary daytime-open stomata.