A 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 now 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.
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.
Cacti inhabit diverse regions, from coastal plains to high mountain areas. With one exception, they are native to the Americas, where their range extends from Patagonia to British Columbia and Alberta in western Canada. A number of centers of diversity exist. For cacti adapted to drought, the three main centers are Mexico and the southwestern United States; the southwestern Andes, where they are found in Peru, Bolivia, Chile and Argentina; and eastern Brazil, away from the Amazon Basin. Tree-living epiphytic and climbing cacti necessarily have different centers of diversity, as they require moister environments. They are mainly found in the coastal mountains and Atlantic forests of southeastern Brazil; in Bolivia, which is the center of diversity for the subfamily Rhipsalideae; and in forested regions of Central America, where the climbing Hylocereeae are most diverse.
Rhipsalis baccifera is the exception; it is native to both the Americas and the Old World, where it is found in tropical Africa, Madagascar, and Sri Lanka. One theory is it was spread by being carried as seeds in the digestive tracts of migratory birds; the seeds of Rhipsalis are adapted for bird distribution. Old World populations are polyploid, and regarded as distinct subspecies, supporting the idea that the spread was not recent. The alternative theory is the species initially crossed the Atlantic on European ships trading between South America and Africa, after which birds may have spread it more widely.
Many other species have become naturalized outside the Americas after having been introduced by people, especially in Australia, Hawaii, and the Mediterranean region. In Australia, species of Opuntia, particularly Opuntia stricta, were introduced in the 19th century for use as natural agricultural fences and in an attempt to establish a cochineal industry. They rapidly became a major weed problem, but are now controlled by biological agents, particularly the moth Cactoblastis cactorum. The weed potential of Opuntia species in Australia continues however, leading to all opuntioid cacti except O. ficus-indica being declared Weeds of National Significance by the Australian Weeds Committee in April 2012.
The plant now known as Opuntia ficus-indica, or the Indian fig cactus, has long been an important source of food. The original species is thought to have come from central Mexico, although this is now obscure because the indigenous people of southern North America developed and distributed a range of horticultural varieties (cultivars), including forms of the species and hybrids with other opuntias. Both the fruit and pads are eaten, the former often under the Spanish name tuna, the latter under the name nopal. Cultivated forms are often significantly less spiny or even spineless. The nopal industry in Mexico was said to be worth US$150 million in 2007. The Indian fig cactus was probably already present in the Caribbean when the Spanish arrived, and was soon after brought to Europe. It spread rapidly in the Mediterranean area, both naturally and by being introduced—so much so, early botanists assumed it was native to the area. Outside the Americas, the Indian fig cactus is an important commercial crop in Sicily, Algeria and other North African countries. Fruits of other opuntias are also eaten, generally under the same name, tuna. Flower buds, particularly of Cylindropuntia species, are also consumed.
Almost any fleshy cactus fruit is edible. The word pitaya or pitahaya (usually considered to have been taken into Spanish from Haitian creole) can be applied to a range of "scaly fruit", particularly those of columnar cacti. The fruit of the saguaro (Carnegiea gigantea) has long been important to the indigenous peoples of northwestern Mexico and the southwestern United States, including the Sonoran Desert. It can be preserved by boiling to produce syrup and by drying. The syrup can also be fermented to produce an alcoholic drink. Fruits of Stenocereus species have also been important food sources in similar parts of North America; Stenocereus queretaroensis is cultivated for its fruit. In more tropical southern areas, the climber Hylocereus undatus provides pitahaya orejona, now widely grown in Asia under the name dragon fruit. Other cacti providing edible fruit include species of Echinocereus, Ferocactus, Mammillaria, Myrtillocactus, Pachycereus, Peniocereus and Selenicereus. The bodies of cacti other than opuntias are less often eaten, although Anderson reported that Neowerdermannia vorwerkii is prepared and eaten like potatoes in upland Bolivia.