This is a mechanism to conserve water by the desert plants.
Desert p?lants undergo a different type of photosynthesis, known as crassulacean acid metabolism, or CAM. The cells of these plants contain enzymes that are able to metabolize carbon dioxide at night, so the plant can close the stomata during daylight hours and open them ?a?t? ?n?i?g?h?t?,? when temperatures are cooler and evaporation is reduced. Carbon dioxide is stored in organic acids at night and released within the plant during the day so the normal photosynthetic pathways can synthesize sugars using sunlight?.