Leuenbergeria is considered close to the ancestral species from which all cacti evolved. Their areoles identify them as cacti, and in spite of their appearance, they, too, have many adaptations for water conservation. They have persistent leaves, and when older, bark-covered stems. At least superficially, plants of the genera Leuenbergeria, Rhodocactus and Pereskia resemble other trees and shrubs growing around them. A few species differ significantly in appearance from most of the family. gallons (760 l 170 imp gal) of water during a rainstorm. A fully grown saguaro ( Carnegiea gigantea) is said to be able to absorb as much as 200 U.S. 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. 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. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced. The plant stores the carbon dioxide it takes in as malic acid, retaining it until daylight returns, and only then using it in 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. Like other succulent plants, most cacti employ a special mechanism called ' crassulacean acid metabolism' (CAM) as part of photosynthesis. Cactus stems are often ribbed or fluted, which allows them to expand and contract easily for quick water absorption after rain, followed by retention over long drought periods. 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. As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled. Areoles are an identifying feature of cacti. 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.Ĭactus spines are produced from specialized structures called areoles, a kind of highly reduced branch.

In the absence of true leaves, cacti's enlarged stems carry out photosynthesis. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. For example, almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Because of this, cacti show many adaptations to conserve water. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on Earth. Although some species live in quite humid environments, most cacti live in habitats subject to at least some drought. Cacti occur in a wide range of shapes and sizes. The word cactus derives, through Latin, from the Ancient Greek word κάκτος ( káktos), a name originally used by Theophrastus for a spiny plant whose identity is now not certain. Many species of cactus have long, sharp spines, like this Opuntia.Ī cactus (plural cacti, cactuses, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales.

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