A cactus is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1,750 known species of the order Caryophyllales. 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. 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.
In 1984, the International Organization for Succulent Plant Study set up a working party, now called the International Cactaceae Systematics Group, to produce a consensus classification of the cactus family, down to the level of genus. Their classification has been used as the basis for systems published since the mid-1990s. Treatments in the 21st century have generally divided the family into around 125–130 genera and 1,400–1,500 species, which are then arranged in a number of tribes and subfamilies. However, subsequent molecular phylogenetic studies have shown that a very high proportion of the higher taxa are not monophyletic, i.e. they do not contain all of the descendants of a common ancestor. As of August 2023, the internal classification of the family Cactaceae remained uncertain and subject to change. A classification incorporating many of the insights from the molecular studies was produced by Nyffeler and Eggli in 2010.
Armatocereus is a genus of mostly tree-like cacti from South America. These species have a conspicuous constriction at the end of the annual growth. The flowers are mostly white, with a more or less spiny ovary. The fruits are mostly spiny.
The term ceroid cactus is used to describe any of the species of cacti with very elongated bodies, including columnar growth cacti and epiphytic cacti. The name is from the Latin cēreus, wax taper, referring to the stiff, upright form of the columnar species. Some species of ceroid cacti were known as torch cactus or torch-thistle, supposedly due to their use as torches by Native Americans in the past.
Disocactus is a genus of epiphytic cacti in the tribe Hylocereeae found in Central America, the Caribbean and northern South America. It should not be confused with Discocactus, which is a different genus.
Aporocactus is a genus of cacti in the tribe Hylocereeae native to Mexico. It used to be classified as a subgenus in Disocactus, but according to molecular evidence, it should be excluded from Disocactus and treated as a separate genus.
Hylocereus is a former genus of epiphytic cacti, often referred to as night-blooming cactus. Several species previously placed in the genus have large edible fruits, which are known as pitayas, pitahayas or dragonfruits. In 2017, a molecular phylogenetic study confirmed an earlier finding that the genus Hylocereus was nested within Selenicereus, so all the species of Hylocereus were transferred to Selenicereus.
Pseudorhipsalis is genus of cacti. This genus is often included in Disocactus. It is epiphytic, many branched, and elongated with flattened, serrated cladodes. In its early life, it stands erect, but soon becomes prostrate. It produces numerous flowers.
Deamia is a genus of cacti. Its species are native from south Mexico through Central America to Nicaragua. Its species have been placed in Selenicereus and Strophocactus.
Strophocactus is a genus of cacti in the subfamily Cactoideae. Its status and circumscription remain somewhat uncertain, with the genus containing one to three species. Molecular phylogenetic data suggest that it consists of three species, including two formerly comprising the genus Pseudoacanthocereus. With this circumscription, the species have different growth habits, but share similarities in their flowers, which are white and open at night.
Morangaya is a monotypic genus of ribbed, usually small to medium-sized, cylindrical shaped cacti, that is native to north western Mexico. The only species is Morangaya pensilis. It is found in the mountains and rocky hills.
Leptocereus is a genus of cacti native to the Greater Antilles. It has been placed in the tribe Leptocereeae or in a broadly defined Echinocereeae.
The Cactoideae are the largest subfamily of the cactus family, Cactaceae. Around 80% of cactus species belong to this subfamily. As of August 2018, the internal classification of the family Cactaceae remained uncertain and subject to change. A classification incorporating many of the insights from the molecular studies was produced by Nyffeler and Eggli in 2010. Various revisions have been published since, e.g. to the tribe Hylocereeae and the tribe Echinocereeae. Classifications remained uncertain as of March 2019.
Cacteae is a tribe of plants of the family Cactaceae found mainly in North America especially Mexico. As of August 2018, the internal classification of the family Cactaceae remained uncertain and subject to change. A classification incorporating many of the insights from the molecular studies was produced by Nyffeler and Eggli in 2010.
Aporocactus flagelliformis, the rattail cactus, is a species of flowering plant in the cactus family Cactaceae, and is the most cultivated species in the genus Aporocactus. Due to its ease of cultivation and attractive floral displays, it is often grown as an ornamental potted plant.
Aporocactus martianus is a species of cactus found in Oaxaca, Mexico.
The Hylocereeae are a tribe of cacti. Most are found in the tropical forests of Central and northern South America, and are climbers or epiphytes, unlike most cacti. The tribe includes between six and eight genera in different circumscriptions. The plants known as "epiphyllum hybrids" or "epiphyllums", widely grown for their flowers, are hybrids of species within this tribe, particularly Disocactus, Pseudorhipsalis and Selenicereus, less often Epiphyllum, in spite of the common name.
Cereeae is a tribe of cacti belonging to the subfamily Cactoideae containing about 50 genera, divided among three subtribes Cereinae, Rebutiinae and Trichocereinae.
Kimnachia is a monotypic genus of cacti. Its only species is Kimnachia ramulosa, synonym Pseudorhipsalis ramulosa, which is native from southern Mexico to northern South America and also found in Jamaica.
Epiphyllum chrysocardium is an epiphytic cactus native to Mexico, commonly called fern leaf cactus, golden heart epiphyllum, shark tooth cactus and shark fin cactus.
