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Cactus / Succulents
This nOde last updated March 10th, 2019 and is permanently morphing...
A cactus (plural: cacti,
cactuses, or cactus) is a member of the plant family Cactaceae, a
family comprising about 127 genera with some 1750 known species of the
order Caryophyllales. The word "cactus" derives, through Latin, from
the Ancient Greek κάκτος, kaktos, a name originally used by
Theophrastus for a spiny plant whose identity is not certain. Cacti
occur in a wide range of shapes and sizes. Most cacti live in habitats
subject to at least some drought. Many live in extremely dry
environments, even being found in the Atacama Desert, one of the
driest places on earth. Cacti show many adaptations to conserve water. Almost all cacti are
succulents, meaning they have thickened, fleshy parts adapted to store
water. Unlike many other succulents, the stem is
the only part of most cacti where this vital process
takes place. Most species of cacti have lost true leaves, retaining
only spines, which are highly modified leaves. As well as defending
spines help prevent water loss by reducing air flow close
to the cactus and providing some shade. In the absence of leaves,
enlarged stems carry out photosynthesis. Cacti are native to the
Americas, ranging from Patagonia in the south to parts of western
Canada in the north—except for Rhipsalis baccifera, which also grows
in Africa and Sri Lanka.
Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled. Many cacti have short growing seasons and long dormancies, and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system that quickly absorbs any water reaching the ground surface. Cactus stems are often ribbed or fluted, which allows them to expand and contract easily for quick water absorption after rain, followed by long drought periods. Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis. Transpiration, during which carbon dioxide enters the plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night. The plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in photosynthesis. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced.
Many smaller cacti have globe-shaped stems, combining the highest possible volume for water storage, with the lowest possible surface area for water loss from transpiration. The tallest free-standing cactus is Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 200 U.S. gallons (760 l; 170 imp gal) of water during a rainstorm. A few species differ significantly in appearance from most of the family. At least superficially, plants of the genus Pereskia resemble other trees and shrubs growing around them. They have persistent leaves, and when older, bark-covered stems. Their areoles identify them as cacti, and in spite of their appearance, they, too, have many adaptations for water conservation. Pereskia is considered close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers and epiphytes (plants that grow on trees). Their stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines, such as the well-known Christmas cactus or Thanksgiving cactus (in the genus Schlumbergera).
Cacti have a variety of uses: many species are used as ornamental plants, others are grown for fodder or forage, and others for food (particularly their fruit). Cochineal is the product of an insect that lives on some cacti.
Many succulent plants in both the Old and New World – such as some Euphorbiaceae (euphorbias) – bear a striking resemblance to cacti, and may incorrectly be called "cactus" in common usage.
A number of species of cacti have been shown to contain psychoactive agents, chemical compounds that can cause changes in mood, perception and cognition through their effects on the brain. Two species have a long history of use by the indigenous peoples of the Americas: peyote, Lophophora williamsii, in North America, and the San Pedro cactus, Echinopsis pachanoi, in South America. Both contain mescaline.
L. williamsii is native to northern Mexico and southern Texas. Individual stems are about 2–6 cm (0.8–2.4 in) high with a diameter of 4–11 cm (1.6–4.3 in), and may be found in clumps up to 1 m (3 ft) wide. A large part of the stem is usually below ground. Mescaline is concentrated in the photosynthetic portion of the stem above ground. The center of the stem, which contains the growing point (the apical meristem), is sunken. Experienced collectors of peyote remove a thin slice from the top of the plant, leaving the growing point intact, thus allowing the plant to regenerate. Evidence indicates peyote was in use more than 5,500 years ago; dried peyote buttons presumed to be from a site on the Rio Grande, Texas, were radiocarbon dated to around 3780–3660 BC. Peyote is perceived as a means of accessing the spirit world. Attempts by the Roman Catholic church to suppress its use after the Spanish conquest were largely unsuccessful, and by the middle of the 20th century, peyote was more widely used than ever by indigenous peoples as far north as Canada. It is now used formally by the Native American Church.
Echinopsis pachanoi is native to Ecuador and Peru. It is very different in appearance from L. williamsii. It has tall stems, up to 6 m (20 ft) high, with a diameter of 6–15 cm (2.4–5.9 in), which branch from the base, giving the whole plant a shrubby or tree-like appearance. Archaeological evidence of the use of this cactus appears to date back to 2,000–2,300 years ago, with carvings and ceramic objects showing columnar cacti. Although church authorities under the Spanish attempted to suppress its use, this failed, as shown by the Christian element in the common name "San Pedro cactus"—Saint Peter cactus. Anderson attributes the name to the belief that just as St Peter holds the keys to heaven, the effects of the cactus allow users "to reach heaven while still on earth." It continues to be used for its psychoactive effects, both for spiritual and for healing purposes, often combined with other psychoactive agents, such as Datura ferox and tobacco. Several other species of Echinopsis, including E. peruviana, also contain mescaline.
In botany, succulent plants, also known as succulents, are plants that have some parts that are more than normally thickened and fleshy, usually to retain water in arid climates or soil conditions. The word "succulent" comes from the Latin word sucus, meaning juice, or sap. Succulent plants may store water in various structures, such as leaves and stems. Some definitions also include roots, thus geophytes that survive unfavorable periods by dying back to underground storage organs may be regarded as succulents. In horticultural use, the term "succulent" is sometimes used in a way which excludes plants that botanists would regard as succulents, such as cacti. Succulents are often grown as ornamental plants because of their striking and unusual appearance.
Many plant families have multiple succulents found within them (over 25 plant families). In some families, such as Aizoaceae, Cactaceae, and Crassulaceae, most species are succulents. The habitats of these water preserving plants are often in areas with high temperatures and low rainfall. Succulents have the ability to thrive on limited water sources, such as mist and dew, which makes them equipped to survive in an ecosystem which contains scarce water sources.