Cactus
The first cacti are thought to have been only slightly succulent shrubs or small trees whose leaves carried out photosynthesis. They lived in tropical areas that experienced periodic drought. If Leuenbergeria is a good model of these early cacti, then they had already evolved strategies to conserve water. These strategies included being able to respond rapidly to periods of rain and keeping transpiration low by using water very efficiently during photosynthesis. The latter was achieved by tightly controlling the opening of stomata. Like Pereskia species today, early ancestors may have been able to switch from the normal C3 mechanism to CAM cycling. In CAM cycling, when the stomata are closed, carbon dioxide produced by respiration is stored for later use in photosynthesis.
No known fossils of cacti exist to throw light on their evolutionary history. However, the geographical distribution of cacti offers some evidence. Except for a relatively recent spread of Rhipsalis baccifera to parts of the Old World, cacti are plants of South America and mainly southern regions of North America. This suggests the family must have evolved after the ancient continent of Gondwana split into South America and Africa. That split occurred during the Early Cretaceous, around 140 million years ago. Precisely when after this split cacti evolved is less clear. Older sources suggest an early origin around 90 to 66 million years ago, during the Late Cretaceous. More recent molecular studies suggest a much younger origin, perhaps in very Late Eocene to early Oligocene periods, around 35 to 30 million years ago.
Based on the phylogeny of the cacti, the earliest diverging group Leuenbergeria may have originated in Central America and northern South America. The caulocacti, those with more-or-less succulent stems, evolved later in the southern part of South America and then moved northwards. Core cacti, those with strongly succulent stems, are estimated to have evolved around 25 million years ago. A possible stimulus to their evolution may have been uplifting in the central Andes, some 25 to 20 million years ago. This was associated with increasing and varying aridity. However, the current species diversity of cacti is thought to have arisen only in the last 10 to 5 million years from the late Miocene into the Pliocene.
A cactus is a member of the plant family Cactaceae, a family of the order Caryophyllales comprising about 127 genera with some 1,750 known species. The word cactus derives through Latin from the Ancient Greek word kaktos, a name originally used by Theophrastus for a spiny plant whose identity is now not certain. In 1737, Carl Linnaeus placed the cacti he knew into two genera, Cactus and Pereskia. When he published Species Plantarum in 1753, he relegated them all to one genus, Cactus.
By the early 20th century, botanists came to feel Linnaeus's name Cactus had become so confused as to its meaning that it should not be used as a genus name. The 1905 Vienna botanical congress rejected the name Cactus and instead declared Mammillaria was the type genus of the family Cactaceae. It did, however, conserve the name Cactaceae, leading to the unusual situation in which the family Cactaceae no longer contains the genus after which it was named. Later botanists, such as Philip Miller in 1754, divided cacti into several genera. Antoine Laurent de Jussieu placed these in his newly created family Cactaceae in 1789.
Detailed treatments published in the 21st century have divided the family into around 125 to 130 genera and 1,400 to 1,500 species. These are then arranged into a number of tribes and subfamilies. The International Cactaceae Systematics Group recognized four subfamilies. The largest of these was divided into nine tribes. Subfamily Pereskioideae contained only the genus Pereskia. Plants are trees or shrubs with leaves; their stems are smoothly round in cross section rather than being ribbed or having tubercles. Two systems may be used in photosynthesis, both the normal C3 mechanism and crassulean acid metabolism. Molecular phylogenetic studies showed that when broadly circumscribed, Pereskia was not monophyletic and has been split into three genera: Leuenbergeria, Rhodocactus and a narrowly circumscribed Pereskia.
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 the deserts of North America, the southwestern Andes found in Peru, Bolivia, Chile and Argentina, and the Caatinga of eastern Brazil.
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, they grow 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. 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.
Many other species have become naturalized outside the Americas after having been introduced by people. This includes Australia, Hawaii, and the Mediterranean region. In Australia, species of Opuntia were introduced in the 19th century for use as natural agricultural fences. They rapidly became a major weed problem but are now controlled by biological agents particularly the moth Cactoblastis cactorum. The Arabian Peninsula has a wide variety of ever-increasing introduced cactus populations. Some of these are cultivated, some are escapes from cultivation, and some are invasives that are presumed to be ornamental escapes.
There is still controversy as to the precise dates when humans first entered those areas of the New World where cacti are commonly found. An archaeological site in Chile has been dated to around 15,000 years ago suggesting cacti would have been encountered before then. Early evidence of the use of cacti includes cave paintings in the Serra da Capivara in Brazil. Seeds found in ancient middens in Mexico and Peru have dates estimated at 12,000 to 9,000 years ago. Hunter-gatherers likely collected cactus fruits in the wild and brought them back to their camps.
Opuntias were used for a variety of purposes by the Aztecs whose empire lasted from the 14th to the 16th century. Their capital from the 15th century was Tenochtitlan. One explanation for the origin of the name is that it includes the Nahuatl word nōchtli referring to the fruit of an opuntia. The coat of arms of Mexico shows an eagle perched on a cactus while holding a snake. This image is at the center of the myth of the founding of Tenochtitlan. The Aztecs symbolically linked the ripe red fruits of an opuntia to human hearts. Just as the fruit quenches thirst so offering human hearts to the sun god ensured the sun would keep moving.
A number of species of cacti have been shown to contain psychoactive agents. Two species have a long history of use by the indigenous peoples of the Americas: peyote Lophophora williamsii in North America and the San Pedro cactus Echinopsis pachanoi in South America. Both contain mescaline. Evidence indicates peyote was in use more than 5,500 years ago. Dried peyote buttons presumed to be from a site on the Rio Grande Texas were radiocarbon dated to around 3780 to 3660 BC. Archaeological evidence of the use of this cactus appears to date back to 2,000 to 2,300 years ago with carvings and ceramic objects showing columnar cacti.
Cacti were cultivated as ornamental plants from the time they were first brought from the New World. By the early 1800s enthusiasts in Europe had large collections often including other succulents alongside cacti. Rare plants were sold for very high prices. Suppliers of cacti and other succulents employed collectors to obtain plants from the wild in addition to growing their own. In the late 1800s collectors turned to orchids and cacti became less popular although never disappearing from cultivation.
Commercially huge numbers of cacti are produced annually. For example in 2002 in Korea alone 49 million plants were propagated with a value of almost US$9 million. Most of them 31 million plants were propagated by grafting. The nopal industry in Mexico was said to be worth US$150 million in 2007. Fruits of other opuntias are also eaten generally under the same name tuna. Flower buds particularly of Cylindropuntia species are also consumed. The plant now known as Opuntia ficus-indica has long been an important source of food. It spread rapidly in the Mediterranean area both naturally and by being introduced so much so that early botanists assumed it was native to the area.
Cacti are used for human food and as fodder for animals usually after burning off their spines. Cochineal is a red dye produced by a scale insect that lives on species of Opuntia. Long used by the peoples of Central and North America demand fell rapidly when European manufacturers began to produce synthetic dyes in the middle of the 19th century. Commercial production has now increased following a rise in demand for natural dyes. Cacti are used as construction materials. Living cactus fences are employed as barricades around buildings to prevent people breaking in.
All cacti are included in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora CITES which lists species that are not necessarily now threatened with extinction but that may become so unless trade is closely controlled. Control is exercised by making international trade in most specimens of cacti illegal unless permits have been issued at least for exports. Some exceptions are allowed such as for naturalized or artificially propagated plants. Some cacti such as all Ariocarpus and Discocactus species are included in the more restrictive Appendix I used for the most endangered species. These may only be moved between countries for non-commercial purposes and only then when accompanied by both export and import permits.
The three main threats to cacti in the wild are development grazing and over-collection. Development takes many forms. The construction of a dam near Zimapan Mexico caused the destruction of a large part of the natural habitat of Echinocactus grusonii. Urban development and highways have destroyed cactus habitats in parts of Mexico New Mexico and Arizona including the Sonoran Desert. The conversion of land to agriculture has affected populations of Ariocarpus kotschoubeyanus in Mexico where dry plains were plowed for maize cultivation. Grazing in many areas by introduced animals such as goats has caused serious damage to populations of cacti. Over-collection of cacti for sale has greatly affected some species. For example the type locality of Pelecyphora strobiliformis near Miquihuana Mexico was virtually denuded of plants which were dug up for sale in Europe.
Continue Browsing
Common questions
When did the first cacti evolve according to recent molecular studies?
Recent molecular studies suggest that cacti evolved around 35 to 30 million years ago during the very Late Eocene to early Oligocene periods. Older sources previously suggested an origin between 90 and 66 million years ago during the Late Cretaceous.
Where are the three main centers of diversity for drought-adapted cacti located?
The three main centers of diversity for drought-adapted cacti are the deserts of North America, the southwestern Andes found in Peru Bolivia Chile and Argentina, and the Caatinga of eastern Brazil. These regions support the highest concentration of species adapted to arid conditions.
Which two cactus species contain mescaline and have a long history of use by indigenous peoples?
Peyote Lophophora williamsii in North America and the San Pedro cactus Echinopsis pachanoi in South America both contain mescaline. Evidence indicates peyote was in use more than 5,500 years ago with radiocarbon dating showing dried buttons from 3780 to 3660 BC.
How many genera and species of cacti exist according to detailed treatments published in the 21st century?
Detailed treatments published in the 21st century divide the family into around 125 to 130 genera and 1,400 to 1,500 species. The International Cactaceae Systematics Group recognizes four subfamilies within this total count.
Why is Rhipsalis baccifera considered an exception among cacti regarding geographical distribution?
Rhipsalis baccifera is the only cactus native to both the Americas and the Old World including tropical Africa Madagascar and Sri Lanka. One theory suggests it spread by being carried as seeds in the digestive tracts of migratory birds.