In crassulacean acid metabolism plants, characteristics such as a limited amount of leaves, a thick plant cuticle, and gas exchange pits, stomata, sunken into the plant are present. These are known as xeropythic features. At night, stomata open to allow the absorption of CO2 (later kept as a 4-carbon acid). During the day, the stomata open and release the CO2 from the Calvin cycle. This prevents water loss.