In hot, sunny, arid weather plants are faced with the problem of maintaining water reserves. If the stomata close, water is conserved, but in turn this means that photosynthesis eventually slows as CO2 levels decrease (CO2 can't enter the leaf if the stomata are closed.). Certain groups of plants have evolved alternate methods for conserving water while maintaining CO2 in the leaf.

C4 Plants:

C4 pathway is a photosynthetic pathway in whichcarbon dioxide is fixed twice in two types of cells. The Carbon dioxide accumulates in the leaf and leps counter photorespiration. The first compund formed when CO2 is added is a 4-carbon compound.

CAM Plants

CAM plants, including most cacti, conserve water in a different manner than other plants. They only open their stomata at night, when they add carbon dioxide to an intermediate molecule. The next day, the stomata close and the CAM then use the build up of carbon dioxide containing molecules to finish photosynthesis.