The term succulent, when applied to plants, refers to those organisms that have very fleshy leaves or stems, regardless of whether they are adapted to dry habitats (as are most true succulents). Specifically, succulent plants are those that are strongly adapted to life in water and/or heat-stressed habitats, and are typically represented by members of certain plant families (see accompanying table). Plants that have evolved in very hot, dry conditions, or those that experience these conditions at certain times of the year, have evolved various structures, habits, and metabolic mechanisms to cope with existence in stressed habitats.
|FLOWERING PLANT FAMILIES CONTAINING SUCCULENTS|
|Family||Common Name||Geographic Distribution*||Number of Species (approximate)||Examples of Succulent Genera|
|Agavaceae||Agave family||North America, Africa||625||Agave, Dasylirion, Nolina, Sanseiveria, Yucca|
|Aloaceae||Aloe family||Africa||440||Aloe, Gasteria, Haworthia|
|Aizoaceae||Ice plant family||Africa||1,300||Carpobrotus, Faucaria, Lithops, Pleiospilos|
|Asclepiadaceae||Milkweed family||Africa||2,000†||Ceropegia, Huernia, Orbea, Piaranthus, Stapelia|
|Cactaceae||Cactus family||North and South America||1,600||Carnegiea, Ferocactus, Mammillaria, Opuntia|
|Crassulaceae||Stonecrop family||Africa, Asia, Europe||1,500||Crassula, Echevaria, Kalanchoe, Sedum|
|Didiereaceae||Didieriea family||Madagascar, Africa||11||Allauadia, Decaryia, Didierea|
|Euphorbiaceae||Euphorbia family||Africa, North America||5,000†||Euphorbia, Jatropha, Monadenium|
|Portulacaceae||Purslane family||Africa, Australia, North and South America||250||Anacampseros, Ceraria, Portulaca|
|* For succulent members of the family.|
|† Approximately 450 species are succulent.|
|† Approximately 750 species are succulent.|
Most succulents are xerophytes, that is, plants that have developed adaptive features for life in dry, often hot, environments. In addition to some shared features with nonsucculent xerophytes, succulent plants have acquired additional specialized features, independently, in several different plant families. The general characteristic of plants that have evolved succulence is the presence of large parenchyma cells in leaves or stems (and occasionally in roots) that serve the purpose of water storage. Furthermore, these plants may also possess one or more of the following adaptations to reduce water loss during periods of heat or drought stress: the presence of epidermal cells with thickened outer walls; increased accumulation of the waxy cuticle layer covering the epidermis; and the evolution of crassulacean acid metabolism (abbreviated CAM; this process delays gaseous exchange through stomata until nighttime, when temperatures are lower and water lost by transpiration is decreased).
see also Cacti; Deserts; Defenses, Physical; Photosynthesis, Carbon Fixation and.
Robert S. Wallace
Mauseth, J. D. Botany: An Introduction to Plant Biology. Boston: Jones and Bartlett, 1998.
Raven, Peter. H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Co., 1999.