What distinguishes a plant as a succulent can be difficult to determine, and not all plant biologists agree on what plants are and are not succulents. According to Mark Dimmitt, Director of Natural History at the Arizona-Sonora Desert Museum, succulent plants are able to store water and survive for short periods of time without a water supply. Succulent plants include cacti, familiar houseplants like jade and aloe plants and some species of orchids.
Urs Eggli's "Illustrated Handbook of Succulent Plants" defines the plants primarily by their ability, through special adaptations, to survive in arid or semi-arid environments where they receive little to no water. Other plant biologists include plants that survive well in salty soils in the succulent category. Still others consider plants that sprout during dry seasons--such as some plants grown from bulbs--as succulents, because they undergo growth during dry spells even if they do not live in arid environments.
Succulents are found in areas with little to no regular rainfall. For example, cacti--the best-known example of succulents--live in the desert, where months may pass without rain. Succulents do not necessarily live in hot environments, however, and succulent plants may grow in cold climates as well. Because of the adaptations that prevent them from losing water, succulents may also suffer from receiving too much water, as water collects inside the plant's tissues, is unable to be released and drowns the plant.
Leaf and Stem
Several traits make succulents able to survive in arid and semi-arid environments. First, the stems and leaves of succulents are covered with a waxy, water-resistant coating that prevents water from evaporating from the plant. Similarly, succulents have fewer stoma than other plants. Stoma are pores in the leaves through which gases are exchanged and, in the process, water is lost as well. Fewer stoma mean that less water evaporates from succulents.
Succulents have also evolved shapes that allow maximum storage capacity with minimal surface area. Round shapes serve this purpose best, so succulents tend to be fleshy and round. Inside the plant bodies, special cells store water for the plant to use during dry periods, and the more volume the plant can hold, the longer it can survive without water. Likewise, small surface areas reduce the amount of water that is lost through evaporation by the sun.
Cacti and Succulents
According to the Cactus and Succulent Society of American, people commonly and mistakenly equate succulents with cacti. However, while cacti are a type of succulent, not all succulents are cacti. Cacti are New World plants that typically grow in deserts and have needles or spines that protect their flesh from being consumed by thirsty animals. They are succulents because they share the above traits with succulents, allowing them to store water in order to survive during arid periods.