SUCCULENT

any plant with fleshy, thick tissues adapted to water storage. Some succulents (e.g., cacti) store water only in the stem and have no leaves or very small leaves, whereas others (e.g., agaves) store water mainly in the leaves. Most succulents have deep or broad root systems and are native to either deserts or regions that have a semiarid season. Stomata are small mouthlike structures on the surface of plant leaves and stems that allow for the uptake of carbon dioxide from the environment and the loss of water and oxygen to the environment. In succulent plants the stomata behave opposite what is normal; that is, they are closed during the day and open at night. As a result, the loss of water (transpiration) during the hot, dry daytime hours is minimized. However, carbon dioxide (CO2) uptake occurs in the dark. Succulent plants, therefore, exhibit a modified form of CO2 fixation and photosynthesis called crassulacean acid metabolism. In crassulacean acid metabolism, CO2 is fixed into an organic acid, malic acid, and is stored in cellular vacuoles until the energy from sunlight is available for photosynthesis.