cacti

A cactus (, or less commonly, 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 word (''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. Although some species live in quite humid environments, 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. 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. 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.

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, which 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 , and the smallest is '' Blossfeldia liliputiana'', only about in diameter at maturity. A fully grown saguaro ('' Carnegiea gigantea'') is said to be able to absorb as much as of water during a rainstorm. 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 referred to as "cactus".

Morphology

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 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. 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.

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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''.

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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.

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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.

; Other forms
Other cacti have a quite different appearance. In tropical regions, some grow as forest climbers and epiphytes. Their stems are typically flattened and almost leaf-like in appearance, with few or even no spines. Climbing cacti can be very large; a specimen of '' Hylocereus'' was reported as long from root to the most distant stem. Epiphytic cacti, such as species of '' Rhipsalis'' or '' Schlumbergera'', often hang downwards, forming dense clumps where they grow in trees high above the ground.

Pereskia aculeata5.jpg, Treelike habit ('' Pereskia aculeata'')
Cardon Pachycereus pringlei.jpg, Tall treelike habit ('' Pachycereus pringlei'')
Cephalocereus columna-trajani rev.jpg, Tall unbranched columnar habit ('' Cephalocereus'')
Ferocactus1.jpg, Shorter clustered columnar habit (''Ferocactus pilosus'')
Ferocactus echidne var victoriensis 1.jpg, Solitary globular habit (''Ferocactus echidne'')
Rebutia flavistylus 2 rev.jpg, Clustered globular habit ('' Rebutia'' species)
Rhipsalis paradoxa.jpg, Epiphytic cactus ('' Rhipsalis paradoxa'')

Stems

The leafless, spiny stem is the characteristic feature of the majority of cacti (and all of those belonging to the largest subfamily, the Cactoideae). The stem is typically succulent, meaning it is adapted to store water. The surface of the stem may be smooth (as in some species of ''Opuntia'') or covered with protuberances of various kinds, which are usually called tubercles. These vary from small "bumps" to prominent, nipple-like shapes in the genus '' Mammillaria'' and outgrowths almost like leaves in '' Ariocarpus'' species. The stem may also be ribbed or fluted in shape. The prominence of these ribs depends on how much water the stem is storing: when full (up to 90% of the mass of a cactus may be water), the ribs may be almost invisible on the swollen stem, whereas when the cactus is short of water and the stems shrink, the ribs may be very visible.

The stems of most cacti are some shade of green, often bluish or brownish green. Such stems contain chlorophyll and are able to carry out photosynthesis; they also have stomata (small structures that can open and close to allow passage of gases). Cactus stems are often visibly waxy.

Areoles

Areoles are structures unique to cacti. Although variable, they typically appear as woolly or hairy areas on the stems from which spines emerge. Flowers are also produced from areoles. In the genus '' Leuenbergeria'', believed similar to the ancestor of all cacti, the areoles occur in the axils of leaves (i.e. in the angle between the leaf stalk and the stem). In leafless cacti, areoles are often borne on raised areas on the stem where leaf bases would have been.

Areoles are highly specialized and very condensed shoots or branches. In a normal shoot, nodes bearing leaves or flowers would be separated by lengths of stem (internodes). In an areole, the nodes are so close together, they form a single structure. The areole may be circular, elongated into an oval shape, or even separated into two parts; the two parts may be visibly connected in some way (e.g. by a groove in the stem) or appear entirely separate (a dimorphic areole). The part nearer the top of the stem then produces flowers, the other part spines. Areoles often have multicellular hairs ( trichomes) that give the areole a hairy or woolly appearance, sometimes of a distinct color such as yellow or brown.

In most cacti, the areoles produce new spines or flowers only for a few years and then become inactive. This results in a relatively fixed number of spines, with flowers being produced only from the ends of stems, which are still growing and forming new areoles. In ''Pereskia'', a genus close to the ancestor of cacti, areoles remain active for much longer; this is also the case in ''Opuntia'' and '' Neoraimondia''.

Leaves

The great majority of cacti have no visible leaves; photosynthesis takes place in the stems (which may be flattened and leaflike in some species). Exceptions occur in three (taxonomically, four) groups of cacti. All the species of '' Leuenbergeria'', '' Pereskia'' and '' Rhodocactus'' are superficially like normal trees or shrubs and have numerous leaves with a midrib and a flattened blade (lamina) on either side. This group is paraphyletic, forming two taxonomic clades. Many cacti in the opuntia group (subfamily Opuntioideae) also have visible leaves, which may be long-lasting (as in '' Pereskiopsis'' species) or produced only during the growing season and then lost (as in many species of ''Opuntia''). The small genus '' Maihuenia'' also relies on leaves for photosynthesis. The structure of the leaves varies somewhat between these groups. Opuntioids and ''Maihuenia'' have leaves that appear to consist only of a midrib.

Even those cacti without visible photosynthetic leaves do usually have very small leaves, less than long in about half of the species studied and almost always less than long. The function of such leaves cannot be photosynthesis; a role in the production of plant hormones, such as auxin, and in defining axillary buds has been suggested.

Spines

Botanically, " spines" are distinguished from "thorns": spines are modified leaves, and thorns are modified branches. Cacti produce spines, always from areoles as noted above. Spines are present even in those cacti with leaves, such as '' Pereskia'', '' Pereskiopsis'' and '' Maihuenia'', so they clearly evolved before complete leaflessness. Some cacti only have spines when young, possibly only when seedlings. This is particularly true of tree-living cacti, such as '' Rhipsalis'' and '' Schlumbergera'', but also of some ground-living cacti, such as '' Ariocarpus''.

The spines of cacti are often useful in identification, since they vary greatly between species in number, color, size, shape and hardness, as well as in whether all the spines produced by an areole are similar or whether they are of distinct kinds. Most spines are straight or at most slightly curved, and are described as hair-like, bristle-like, needle-like or awl-like, depending on their length and thickness. Some cacti have flattened spines (e.g. '' Sclerocactus papyracanthus''). Other cacti have hooked spines. Sometimes, one or more central spines are hooked, while outer spines are straight (e.g., '' Mammillaria rekoi'').

In addition to normal-length spines, members of the subfamily Opuntioideae have relatively short spines, called glochids, that are barbed along their length and easily shed. These enter the skin and are difficult to remove due to being very fine and easily broken, causing long-lasting irritation.

Ferocactus1001.jpg, Varied spines of a '' Ferocactus''
Hakendornen IMGP7026 WP.jpg, Hooked central spine (cf. '' Mammillaria rekoi'')
Toumeya papyracantha fh 087 3 AZ BB.jpg, Unusual flattened spines of '' Sclerocactus papyracanthus''
Opuntia microdasys 01 ies cropped.jpg, Glochids of '' Opuntia microdasys''

Roots

Most ground-living cacti have only fine roots, which spread out around the base of the plant for varying distances, close to the surface. Some cacti have taproots; in genera such as '' Ariocarpus'', these are considerably larger and of a greater volume than the body. Taproots may aid in stabilizing the larger columnar cacti. Climbing, creeping and epiphytic cacti may have only adventitious roots, produced along the stems where these come into contact with a rooting medium.

Flowers

Like their spines, cactus flowers are variable. Typically, the ovary is surrounded by material derived from stem or receptacle tissue, forming a structure called a pericarpel. Tissue derived from the petals and sepals continues the pericarpel, forming a composite tube—the whole may be called a floral tube, although strictly speaking only the part furthest from the base is floral in origin. The outside of the tubular structure often has areoles that produce wool and spines. Typically, the tube also has small scale-like bracts, which gradually change into sepal-like and then petal-like structures, so the sepals and petals cannot be clearly differentiated (and hence are often called " tepals"). Some cacti produce floral tubes without wool or spines (e.g. '' Gymnocalycium'') or completely devoid of any external structures (e.g. '' Mammillaria''). Unlike the flowers of most other cacti, '' Pereskia'' flowers may be borne in clusters.

Cactus flowers usually have many stamens, but only a single style, which may branch at the end into more than one stigma. The stamens usually arise from all over the inner surface of the upper part of the floral tube, although in some cacti, the stamens are produced in one or more distinct "series" in more specific areas of the inside of the floral tube.

The flower as a whole is usually radially symmetrical ( actinomorphic), but may be bilaterally symmetrical ( zygomorphic) in some species. Flower colors range from white through yellow and red to magenta.

Adaptations for water conservation

All cacti have some adaptations to promote efficient water use. Most cacti— opuntias and cactoids—specialize in surviving in hot and dry environments (i.e. are xerophytes), but the first ancestors of modern cacti were already adapted to periods of intermittent drought. A small number of cactus species in the tribes Hylocereeae and Rhipsalideae have become adapted to life as climbers or epiphytes, often in tropical forests, where water conservation is less important.

Leaves and spines

The absence of visible leaves is one of the most striking features of most cacti. '' Pereskia'' (which is close to the ancestral species from which all cacti evolved) does have long-lasting leaves, which are, however, thickened and succulent in many species. Other species of cactus with long-lasting leaves, such as the opuntioid '' Pereskiopsis'', also have succulent leaves. A key issue in retaining water is the ratio of surface area to volume. Water loss is proportional to surface area, whereas the amount of water present is proportional to volume. Structures with a high surface area-to-volume ratio, such as thin leaves, necessarily lose water at a higher rate than structures with a low area-to-volume ratio, such as thickened stems.

Spines, which are modified leaves, are present on even those cacti with true leaves, showing the evolution of spines preceded the loss of leaves. Although spines have a high surface area-to-volume ratio, at maturity they contain little or no water, being composed of fibers made up of dead cells. Spines provide protection from herbivores and camouflage in some species, and assist in water conservation in several ways. They trap air near the surface of the cactus, creating a moister layer that reduces evaporation and transpiration. They can provide some shade, which lowers the temperature of the surface of the cactus, also reducing water loss. When sufficiently moist air is present, such as during fog or early morning mist, spines can condense moisture, which then drips onto the ground and is absorbed by the roots.

Stems

The majority of cacti are stem succulents, i.e., plants in which the stem is the main organ used to store water. Water may form up to 90% of the total mass of a cactus. Stem shapes vary considerably among cacti. The cylindrical shape of columnar cacti and the spherical shape of globular cacti produce a low surface area-to-volume ratio, thus reducing water loss, as well as minimizing the heating effects of sunlight. The ribbed or fluted stems of many cacti allow the stem to shrink during periods of drought and then swell as it fills with water during periods of availability. A mature saguaro ('' Carnegiea gigantea'') is said to be able to absorb as much as of water during a rainstorm. The outer layer of the stem usually has a tough cuticle, reinforced with waxy layers, which reduce water loss. These layers are responsible for the grayish or bluish tinge to the stem color of many cacti.

The stems of most cacti have adaptations to allow them to conduct photosynthesis in the absence of leaves. This is discussed further below under Metabolism.

Roots

Many cacti have roots that spread out widely, but only penetrate a short distance into the soil. In one case, a young saguaro only tall had a root system with a diameter of , but no more than deep. Cacti can also form new roots quickly when rain falls after a drought. The concentration of salts in the root cells of cacti is relatively high. All these adaptations enable cacti to absorb water rapidly during periods of brief or light rainfall. Thus, '' Ferocactus cylindraceus'' reportedly can take up a significant amount of water within 12 hours from as little as of rainfall, becoming fully hydrated in a few days.

Although in most cacti, the stem acts as the main organ for storing water, some cacti have in addition large taproots. These may be several times the length of the above-ground body in the case of species such as '' Copiapoa atacamensis'', which grows in one of the driest places in the world, the Atacama Desert in northern Chile.

Metabolism

Photosynthesis requires plants to take in carbon dioxide gas (). As they do so, they lose water through transpiration. Like other types of succulents, cacti reduce this water loss by the way in which they carry out photosynthesis. "Normal" leafy plants use the C₃ mechanism: during daylight hours, is continually drawn out of the air present in spaces inside leaves and converted first into a compound containing three carbon atoms ( 3-phosphoglycerate) and then into products such as carbohydrates. The access of air to internal spaces within a plant is controlled by stomata, which are able to open and close. The need for a continuous supply of during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C₃ mechanism lose as much as 97% of the water taken up through their roots in this way. A further problem is that as temperatures rise, the enzyme that captures starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%.

Crassulacean acid metabolism (CAM) is a mechanism adopted by cacti and other succulents to avoid the problems of the C₃ mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. enters the plant and is captured in the form of organic acids stored inside cells (in vacuoles). The stomata remain closed throughout the day, and photosynthesis uses only this stored . CAM uses water much more efficiently at the price of limiting the amount of carbon fixed from the atmosphere and thus available for growth. CAM-cycling is a less water-efficient system whereby stomata open in the day, just as in plants using the C₃ mechanism. At night, or when the plant is short of water, the stomata close and the CAM mechanism is used to store produced by respiration for use later in photosynthesis. CAM-cycling is present in ''Pereskia'' species.

By studying the ratio of ¹⁴C to ¹³C incorporated into a plant—its isotopic signature—it is possible to deduce how much is taken up at night and how much in the daytime. Using this approach, most of the ''Pereskia'' species investigated exhibit some degree of CAM-cycling, suggesting this ability was present in the ancestor of all cacti. ''Pereskia'' leaves are claimed to only have the C₃ mechanism with CAM restricted to stems. More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; ''Pereskia'' species are described as having "C₃ with inducible CAM." Leafless cacti carry out all their photosynthesis in the stem, using full CAM. , it is not clear whether stem-based CAM evolved once only in the core cacti, or separately in the opuntias and cactoids; CAM is known to have evolved convergently many times.

To carry out photosynthesis, cactus stems have undergone many adaptations. Early in their evolutionary history, the ancestors of modern cacti (other than ''Leuenbergeria'' species) developed stomata on their stems and began to delay developing bark. However, this alone was not sufficient; cacti with only these adaptations appear to do very little photosynthesis in their stems. Stems needed to develop structures similar to those normally found only in leaves. Immediately below the outer epidermis, a hypodermal layer developed made up of cells with thickened walls, offering mechanical support. Air spaces were needed between the cells to allow carbon dioxide to diffuse inwards. The center of the stem, the cortex, developed " chlorenchyma" – a plant tissue made up of relatively unspecialized cells containing chloroplasts, arranged into a "spongy layer" and a " palisade layer" where most of the photosynthesis occurs.

Taxonomy and classification

Naming and classifying cacti has been both difficult and controversial since the first cacti were discovered for science. The difficulties began with Carl Linnaeus. In 1737, he placed the cacti he knew into two genera, ''Cactus'' and ''Pereskia''. However, when he published ''Species Plantarum'' in 1753—the starting point for modern botanical nomenclature—he relegated them all to one genus, ''Cactus''. The word "cactus" is derived through Latin from the Ancient Greek (''kaktos''), a name used by Theophrastus for a spiny plant, which may have been the cardoon (''Cynara cardunculus'').

Later botanists, such as Philip Miller in 1754, divided cacti into several genera, which, in 1789, Antoine Laurent de Jussieu placed in his newly created family Cactaceae. By the early 20th century, botanists came to feel Linnaeus's name ''Cactus'' had become so confused as to its meaning (was it the genus or the family?) 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.

The difficulties continued, partly because giving plants scientific names relies on "type specimens". Ultimately, if botanists want to know whether a particular plant is an example of, say, ''Mammillaria mammillaris'', they should be able to compare it with the type specimen to which this name is permanently attached. Type specimens are normally prepared by compression and drying, after which they are stored in herbaria to act as definitive references. However, cacti are very difficult to preserve in this way; they have evolved to resist drying and their bodies do not easily compress. A further difficulty is that many cacti were given names by growers and horticulturalists rather than botanists; as a result, the provisions of the ''International Code of Nomenclature for algae, fungi, and plants'' (which governs the names of cacti, as well as other plants) were often ignored. Curt Backeberg, in particular, is said to have named or renamed 1,200 species without one of his names ever being attached to a specimen, which, according to David Hunt, ensured he "left a trail of nomenclatural chaos that will probably vex cactus taxonomists for centuries."

Classification

In 1984, it was decided that the Cactaceae Section of the International Organization for Succulent Plant Study should set up a working party, now called the International Cactaceae Systematics Group (ICSG), to produce consensus classifications down to the level of genera. Their system has been used as the basis of subsequent classifications. Detailed treatments published in the 21st century have divided the family into around 125–130 genera and 1,400–1,500 species, which are then arranged into a number of tribes and subfamilies. The ICSG classification of the cactus family recognized four subfamilies, the largest of which was divided into nine tribes. The subfamilies were:

* Subfamily Pereskioideae K. Schumann
:The only genus in the ICSG classification was ''Pereskia''. It has features considered closest to the ancestors of the Cactaceae. 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 (CAM)—an "advanced" feature of cacti and other succulents that conserves water.
:Molecular phylogenetic studies showed that when broadly circumscribed, ''Pereskia'' was not monophyletic, and it has been split into three genera, '' Leuenbergeria'', '' Rhodocactus'' and a narrowly circumscribed '' Pereskia''. ''Leuenbergeria'' is then placed on its own in a separate monogeneric subfamily, Leuenbergerioideae.
* Subfamily Opuntioideae K. Schumann
:Some 15 genera are included in this subfamily. They may have leaves when they are young, but these are lost later. Their stems are usually divided into distinct "joints" or "pads" ( cladodes). Plants vary in size from the small cushions of '' Maihueniopsis'' to treelike species of ''Opuntia'', rising to or more.
* Subfamily Maihuenioideae P. Fearn
:The only genus is ''Maihuenia'', with two species, both of which form low-growing mats. It has some features that are primitive within the cacti. Plants have leaves, and crassulean acid metabolism is wholly absent.
* Subfamily Cactoideae
:Divided into nine tribes, this is the largest subfamily, including all the "typical" cacti. Members are highly variable in habit, varying from tree-like to epiphytic. Leaves are normally absent, although sometimes very reduced leaves are produced by young plants. Stems are usually not divided into segments, and are ribbed or tuberculate. Two of the tribes, Hylocereeae and Rhipsalideae, contain climbing or epiphytic forms with a rather different appearance; their stems are flattened and may be divided into segments.

Molecular phylogenetic studies have supported the monophyly of three of these subfamilies (not Pereskioideae), but have not supported all of the tribes or even genera below this level; indeed, a 2011 study found only 39% of the genera in the subfamily Cactoideae sampled in the research were monophyletic. Classification of the cacti currently remains uncertain and is likely to change.

Phylogeny and evolution

Phylogeny

A 2005 study suggested the genus '' Pereskia'' as then circumscribed (''Pereskia'' sensu lato) was basal within the Cactaceae, but confirmed earlier suggestions it was not monophyletic, i.e., did not include all the descendants of a common ancestor. The Bayesian consensus cladogram from this study is shown below with subsequent generic changes added.

A 2011 study using fewer genes but more species also found that ''Pereskia'' s.l. was divided into the same clades, but was unable to resolve the members of the "core cacti" clade. It was accepted that the relationships shown above are "the most robust to date."

'' Leuenbergeria'' species (''Pereskia'' s.l. Clade A) always lack two key features of the stem present in most of the remaining "caulocacti": like most non-cacti, their stems begin to form bark early in the plants' life and also lack stomata—structures that control admission of air into a plant and hence control photosynthesis. By contrast, caulocacti, including species of '' Rhodocactus'' and the remaining species of ''Pereskia'' s.s., typically delay forming bark and have stomata on their stems, thus giving the stem the potential to become a major organ for photosynthesis. (The two highly specialized species of '' Maihuenia'' are something of an exception.)

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, although they would have appeared superficially similar to other trees growing nearby, they had already evolved strategies to conserve water (some of which are present in members of other families in the order Caryophyllales). 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, where carbon dioxide is used continuously in photosynthesis, to CAM cycling, in which when the stomata are closed, carbon dioxide produced by respiration is stored for later use in photosynthesis.

The clade containing ''Rhodocactus'' and ''Pereskia'' s.s. marks the beginnings of an evolutionary switch to using stems as photosynthetic organs. Stems have stomata and the formation of bark takes place later than in normal trees. The "core cacti" show a steady increase in both stem succulence and photosynthesis accompanied by multiple losses of leaves, more-or-less complete in the Cactoideae. One evolutionary question at present unanswered is whether the switch to full CAM photosynthesis in stems occurred only once in the core cacti, in which case it has been lost in ''Maihuenia'', or separately in Opuntioideae and Cactoideae, in which case it never evolved in ''Maihuenia''.

Understanding evolution within the core cacti clade is difficult , since phylogenetic relationships are still uncertain and not well related to current classifications. Thus, a 2011 study found "an extraordinarily high proportion of genera" were not monophyletic, so were not all descendants of a single common ancestor. For example, of the 36 genera in the subfamily Cactoideae sampled in the research, 22 (61%) were found not monophyletic. Nine tribes are recognized within Cactoideae in the International Cactaceae Systematics Group (ICSG) classification; one, Calymmantheae, comprises a single genus, ''Calymmanthium''. Only two of the remaining eight – Cacteae and Rhipsalideae – were shown to be monophyletic in a 2011 study by Hernández-Hernández et al. For a more detailed discussion of the phylogeny of the cacti, see Classification of the Cactaceae.

Evolutionary history

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, which occurred during the Early Cretaceous, around . Precisely when after this split cacti evolved is less clear. Older sources suggest an early origin around 90 – 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 35–30 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, whereas 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 25–20 million years ago, which was associated with increasing and varying aridity. However, the current species diversity of cacti is thought to have arisen only in the last 10–5 million years (from the late Miocene into the Pliocene

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