Learn more about Cactus
- For other meanings, see Cactus (disambiguation).
- This article is about the plant family. For the genus Cactus, see Mammillaria, Melocactus, and Opuntia.
Cacti are distinctive and unusual plants which have adapted to extreme arid environments, showing a wide range of anatomical and physiological features which conserve water. Their stems have expanded into green succulent structures containing the chlorophyll necessary for life and growth, while the leaves have become the spines for which cacti are so well known.
 Distribution and evolution
Cacti are almost exclusively New World plants. This means that they are native only in North America, South America, and the West Indies. There is however one exception, Rhipsalis baccifera; this species has a pantropical distribution, occurring in the Old World in tropical Africa, Madagascar and Sri Lanka as well as in tropical America. This plant is thought to be a relatively recent colonist in the Old World (within the last few thousand years), probably carried as seeds in the digestive tracts of migratory birds. Many other cacti have become naturalized to similar environments in other parts of the world after being introduced by people.
Cacti are believed to have evolved in the last 30 to 40 million years. Long ago, the Americas were joined to the other continents, but separated due to continental drift. Unique species in the New World must have developed after the continents had moved apart. Significant distance between the continents was only achieved in around the last 50 million years. This may explain why cacti are so rare in Africa; the continents had already separated when cacti evolved. Many succulent plants in both the Old and New World bear a striking resemblance to cacti, and are often called "cactus" in common usage. This is, however, due to parallel evolution; none of these are closely related to the Cactaceae.
Prickly pears (genus Opuntia) were imported into Australia in the 19th century for use as a natural agricultural fence and to establish a cochineal dye industry, but quickly became a widespread weed. This invasive species is inedible for local herbivores and has rendered 40,000 km² of farming land unproductive.
 Adaptations to dry environment
Like other succulents, cacti are well-adapted to life with little precipitation. The leaves have evolved into spines, which in addition to allowing less water to evaporate through transpiration than regular leaves, defend the cactus against water-seeking animals. The spines grow from specialized structures called areole. Photosynthesis is carried out by enlarged stems, which also store water. Unlike many other succulents, the stem is the only part of a true cactus where this takes place. Very few members of the family have leaves, and when present these are usually rudimentary and soon fall off; they are typically awl-shaped and only 1-3 mm long. Two genera, Pereskia and Pereskiopsis, do however retain large, non-succulent leaves 5-25 cm long, and also non-succulent stems. Pereskia has now been determined to be the ancestral genus from which all other cacti evolved.<ref>http://www.eurekalert.org/pub_releases/2006-05/uocp-hdc051206.php</ref> The roots are often extensive and close to the surface of the ground, an adaptation to infrequent rains. Cacti use the Crassulacean acid metabolism process.
Cacti come in a wide range of shapes and sizes. Saguaro cacti may exceed 13 meters in height, while many other species barely leave the ground. Cactus flowers are large, and like the spines and branches arise from areoles. Many cactus species are night-blooming, as they are pollinated by nocturnal insects or small animals, principally moths and bats. Cacti's sizes range from small and round to pole-like and tall.
Cacti, cultivated by people worldwide, are a familiar sight as potplants, houseplants or in ornamental gardens in warmer climates. They often form part of xerophilic (dry) gardens in arid regions, or raised rockeries. Some countries, such as Australia, have water restrictions in many cities, so drought-resistant plants are increasing in popularity. Numerous species have entered widespread cultivation, including members of Echinopsis, Mammillaria and Cereus among others. Some, such as the Golden Barrel Cactus, Echinocactus grusonii, are prominent in garden design. In some places where cacti are common, they may be adapted into a cactus fence.
As well as garden plants, many cacti have important commercial uses; some cacti bear edible fruit, such as the prickly pear and Hylocereus, which produces Dragon fruit or Pitaya. Opuntia are also used as host plants for cochineal bugs in the cochineal dye industry in Central America.
The Peyote, Lophophora williamsii, is a well-known psychoactive agent used by native Americans in the Southwest of the United States of America. Some species of Echinopsis (previously Trichocereus) also have psychoactive properties.
The word cactus is ultimately derived from Greek Κακτος kaktos, used in classical Greek for a species of spiny thistle, possibly the cardoon, and used as a generic name, Cactus, by Linnaeus in 1753 (now rejected in favor of Mammillaria). There is some dispute as to the proper plural form of the word; as a Greek loan into English, the correct plural in English would be "cactuses". However, as a word in Botanical Latin (as distinct from Classical Latin), "cactus" would follow standard Latin rules for pluralization and become "cacti", which has become the prevalent usage in English.
 Selected important genera
- For a full list see Taxonomy of the Cactaceae
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- Anderson, E. F. (2001). The Cactus Family. Timber Press ISBN 0-88192-498-9 - Comprehensive and lavishly illustrated.
- Benson, L. (1981). The Cacti of Arizona. University of Arizona Press ISBN 0-8165-0509-8 - Thorough treatment of the Arizona, U.S.A., species
- Kiesling, R., Mauseth, J. D., & Ostolaza, C. N. (2002). A Cactus Odyssey. Timber Press ISBN 0-88192-526-8
 External links
bg:Кактус bn:ক্যাক্টাস bo:ཀླུ་ཥིང་ ca:Cactàcia cs:Kaktusovité da:Kaktus-familien de:Kakteengewächse es:Cactaceae fr:Cactaceae gl:Cacto hr:Kaktusi id:Kaktus it:Cactus he:קקטוס lt:Kaktusiniai nl:Cactus ja:サボテン no:Kaktus pl:Kaktusowate pt:Cactaceae ru:Кактус simple:Cactus fi:Kaktuskasvit sv:Kaktusar vi:Họ Xương rồng zh:仙人掌