A cactus is a member of the plant family Cactaceae, within the order Caryophyllales. The word cactus is derived through Latin from the Ancient Greek κάκτος (kaktos), a name originally used for a spiny plant whose identity is not certain. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north. There is only one exception, Rhipsalis baccifera, which is also found in Africa and Sri Lanka.
Most cacti live in habitats which are subject to at least some degree of 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. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines reduce air flow close to the cactus and provide some shade, both of which help to prevent water loss. 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 multi-petaled.
In the absence of leaves, enlarged stems carry out photosynthesis. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Cactus stems also store water, often being ribbed or fluted which allows them to expand and contract easily. Cacti come in a wide range of shapes and sizes. 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. The smaller cacti usually have globe-shaped stems, combining the highest possible volume with the lowest possible surface area. Many cacti have a short growing season and a long dormancy and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 200 US gallons (760 l; 170 imp gal) of water during a rainstorm.
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.
A few species are significantly different in appearance from most of the family. At least superficially, plants of the genus Pereskia resemble the other trees and shrubs which grow 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 to be close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers and epiphytes (plants which grow on trees). Their stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines, like 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, others for food (particularly their fruit). Cochineal is the product of an insect that lives on some cacti.