CAM plants close their stomata during the day, when evaporation rates are highest, to conserve water. Most CAM plants are native to arid regions, like deserts.
CAM means Crassulacean acid metabolism; the name comes from the succulent family Crassulaceae. The stomata, or leaf pores, of most plants remain open throughout the day and night to take in carbon dioxide, resulting in a loss of 97 percent of the plant's water. Desert plants cannot tolerate the same level of water loss and can only open their stomata at night. This decreases the amount of carbon dioxide that CAM plants pull from the atmosphere. To cope with this, CAM plants convert the carbon dioxide they take in during the night into malic acid and store it in vacuoles; later, during the day, the acid breaks down again into carbon dioxide and the light reactions of photosynthesis begin.
In some ways, CAM photosynthesis is more efficient than other forms of photosynthesis because it concentrates the amount of carbon available at the beginning of the Calvin cycle. It is, however, slower at sugar production overall, meaning that CAM plants tend to grow more slowly than other plants. Because CAM plants have so much energy investment in their mass, they often produce defensive spines and chemicals for protection.