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A cactus (plural: ''cacti'', ''cactuses'', or ''cactus'')〔 is a member of the plant family Cactaceae within the order Caryophyllales. The word "cactus" derives, through Latin, from the Ancient Greek , ''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 against herbivores, 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 absorb 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 ,〔 and the smallest is ''Blossfeldia liliputiana'', only about in diameter at maturity.〔 A fully grown saguaro (''Carnegiea gigantea'') is said to be able to absorb as much as 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. ==Morphology== The 1,500 to 1,800 species of cacti mostly fall into one of two groups of "core cacti": opuntias (subfamily Opuntioideae) and "cactoids" (subfamily Cactoideae). Most members of these two groups are easily recognizable as cacti. They have fleshy succulent stems that are major organs of photosynthesis. They have absent, small, or transient leaves. They have flowers with ovaries that lie below the sepals and petals, often deeply sunken into a fleshy receptacle (the part of the stem from which the flower parts grow). All cacti have areoles—highly specialized short shoots with extremely short internodes that produce spines, normal shoots, and flowers.〔 The remaining cacti fall into only two genera, ''Pereskia'' and ''Maihuenia'', and are rather different,〔 which means any description of cacti as a whole must frequently make exceptions for them. ''Pereskia'' species superficially resemble other tropical forest trees. When mature, they have woody stems that may be covered with bark and long-lasting leaves that provide the main means of photosynthesis. Their flowers may have superior ovaries (i.e., above the points of attachment of the sepals and petals), and areoles that produce further leaves. The two species of ''Maihuenia'' have small, globe-shaped bodies with prominent leaves at the top.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cactus」の詳細全文を読む スポンサード リンク
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