A cactus (pl.: cacti, cactuses, or less commonly, cactus)is a member of the plant family Cactaceae (/kækˈteɪsi.i:, -ˌaɪ/),[a] a family of the order Caryophyllales comprising about 127 genera with some 1,750 known species.[6] 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.[7] Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north, with the exception of Rhipsalis baccifera, which is also found in Africa and Sri Lanka. Cacti are adapted to live in very dry environments, including the Atacama Desert, one of the driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, 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 true leaves, cacti's enlarged stems carry out photosynthesis.

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 with a number of ribs which corresponds to a Fibonacci number (2, 3, 5, 8, 13, 21, 34 etc.). This allows them to expand and contract easily for quick water absorption after rain, followed by retention over 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),[9] and the smallest is Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity.[10] A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 760 liters (200 U.S. gal) of water during a rainstorm.[11] A few species differ significantly in appearance from most of the family. At least superficially, plants of the genera Leuenbergeria, Rhodocactus and 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. Leuenbergeria 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) – are also spiny stem succulents and because of this are sometimes incorrectly assumed to be cacti, although they are not.]Morphology

Ferocactus species, a cactoid, in its native Arizona habitat
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.


Aberrant form of Lagenosocereus
The remaining cacti fall into only two groups: three tree-like genera, Leuenbergeria, Pereskia and Rhodocactus (all formerly placed in Pereskia), and the much smaller Maihuenia. These two groups are rather different from other cacti, which means any description of cacti as a whole must frequently make exceptions for them. Species of the first three genera superficially resemble other tropical forest trees, in that 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 succulent but non-photosynthetic stems and prominent succulent leaves.
Growth habit
Cacti show a wide variety of growth habits, which are difficult to divide into clear, simple categories.

Arborescent cacti
Cacti can be tree-like (arborescent), meaning they typically have a single more-or-less woody trunk topped by several to many branches. In the genera Leuenbergeria, Pereskia and Rhodocactus, the branches are covered with leaves, so the species of these genera may not be recognized as cacti. In most other cacti, the branches are more typically cactus-like, bare of leaves and bark and covered with spines, as in Pachycereus pringlei or the larger opuntias. Some cacti may become tree-sized but without branches, such as larger specimens of Echinocactus platyacanthus. Cacti may also be described as shrubby, with several stems coming from the ground or from branches very low down, such as in Stenocereus thurberi.[14]

Columnar cacti
Smaller cacti may be described as columnar. They consist of erect, cylinder-shaped stems, which may or may not branch, without a very clear division into trunk and branches. The boundary between columnar forms and tree-like or shrubby forms is difficult to define. Smaller and younger specimens of Cephalocereus senilis, for example, are columnar, whereas older and larger specimens may become tree-like. In some cases, the "columns" may be horizontal rather than vertical. Thus, Stenocereus eruca can be described as columnar even though it has stems growing along the ground, rooting at intervals.[14]

Globular cacti
Cacti whose stems are even smaller may be described as globular (or globose). They consist of shorter, more ball-shaped stems than columnar cacti. Globular cacti may be solitary, such as Ferocactus latispinus, or their stems may form clusters that can create large mounds. All or some stems in a cluster may share a common root.[14]