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77-456: See text . The Echinocereeae are a tribe of cacti in the subfamily Cactoideae . Since 2006, the tribe has included the former tribe Pachycereeae in many treatments of cactus classification. The exact circumscription of the tribe has been subject to considerable change, particularly since molecular phylogenetic approaches have been used in determining classifications, and remains uncertain. The tribe includes large treelike species, such as

154-453: 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

231-464: 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 )

308-547: A cluster may share a common root. 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 100 meters (330 ft) 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

385-452: A common ancestor), so that Pereskioideae is not monophyletic although the three other subfamilies are. The Bayesian consensus cladogram from a 2005 study is shown below: Pereskia Clade A Pereskia Clade B Opuntioideae Maihuenioideae Cactoideae Five tribes have been recognized within the subfamily Opuntioideae : Tephrocacteae , Pterocacteae , Austrocylindropuntieae , Cylindropuntieae and Opuntieae . All but

462-464: A continuous supply of CO 2 during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C 3 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 CO 2 starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%. Crassulacean acid metabolism (CAM)

539-429: A division into tribes of what is now the subfamily Cactoideae, then Cereoideae. Two of these tribes were Pachycereeae (which he spelt "Pachycereae") and Echinocereeae (which he spelt "Echinocereae"). Pachycereeae was a newly published taxon; Echinocereeae was an elevation of the subtribe Echinocereinae ("Echinocereanae") established by Britton and Rose in 1922. Studies based on chloroplast DNA showed that Echinocereus ,

616-411: 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 7 mm (0.3 in) of rainfall, becoming fully hydrated in a few days. Although in most cacti,

693-415: 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. Like their spines, cactus flowers are variable. Typically, the ovary is surrounded by material derived from stem or receptacle tissue, forming a structure called

770-415: 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

847-418: 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

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924-407: A number of tribes and subfamilies. The ICSG classification of the family recognizes four subfamilies: Pereskioideae (consisting only of the genus Pereskia ), Opuntioideae , Maihuenioideae (consisting only of the genus Maihuenia ) and Cactoideae . Molecular phylogenetic studies suggest that Pereskia is not monophyletic (i.e. its species are not the complete set of descendants of

1001-466: 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

1078-657: A single genus, Calymmanthium . Of the remaining eight, only two ( Cacteae and Rhipsalideae ) have been shown to be monophyletic. A summary of the cladograms for the Cactoideae presented in a 2011 paper is shown below (ICSG tribes in bold). Blossfeldia Cacteae Calymmantheae (i.e. Calymmanthium ), species of Copiapoa and Frailea including members of Pachycereeae , Hylocereeae and Browningieae Rhipsalideae other core Cactoideae II, including members of Notocacteae , Trichocereeae , Cereeae and Browningieae The classification of

1155-620: A specimen, which, according to David Hunt , ensured he "left a trail of nomenclatural chaos that will probably vex cactus taxonomists for centuries." 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

1232-554: 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

1309-433: Is a mechanism adopted by cacti and other succulents to avoid the problems of the C 3 mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. CO 2 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 CO 2 . CAM uses water much more efficiently at

1386-401: 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

1463-546: 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

1540-537: Is derived through Latin from the Ancient Greek κάκτος ( 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

1617-835: 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

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1694-474: Is present in Pereskia species. By studying the ratio of C to C incorporated into a plant—its isotopic signature —it is possible to deduce how much CO 2 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

1771-557: Is said to be able to absorb as much as 200 U.S. gallons (760 L; 170 imp gal) 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

1848-416: Is said to be able to absorb as much as 200 U.S. gallons (760 L; 170 imp gal) 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

1925-461: 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

2002-721: 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". 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

2079-402: The C 3 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 "C 3 with inducible CAM." Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012 , it is not clear whether stem-based CAM evolved once only in

2156-443: 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. A 2005 study suggested the genus Pereskia as then circumscribed ( Pereskia sensu lato)

2233-633: The saguaro ( Carnegiea gigantea ), as well as shorter shrubby species. Most members of the tribe are found in desert regions, particularly in Mexico and the southwestern United States. The tribe includes large treelike species, as well as shorter shrubby species. Some species can grow to be over 15 m (50 ft) tall, like the saguaro ( Carnegiea gigantea ) and Cephalocereus macrocephalus (syn. Neobuxbaumia macrocephala ). Their stems are ribbed and columnar, not divided into segments. Most have flowers that open at night. In 1958, Franz Buxbaum published

2310-441: 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

2387-484: 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. 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

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2464-535: The Cactaceae The classification of the family Cactaceae remains uncertain as of August 2023 . Since the mid-1990s, the system produced by the International Cactaceae Systematics Group (ICSG) of the International Organization for Succulent Plant Study has been used as the basis of many published classifications. Detailed treatments produced in the 21st century have divided the family into around 125–130 genera and 1,400–1,500 species, which are then arranged into

2541-480: The Cactaceae A cactus ( pl. : cacti , cactuses , or less commonly, cactus ) is a member of the plant family Cactaceae ( / k æ k ˈ t eɪ s i . iː , - ˌ aɪ / ), 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 κάκτος ( káktos ), a name originally used by Theophrastus for

2618-542: The Cactaceae thus remains subject to change, from the genus level upwards. In 2016, David Hunt wrote that "the advent of molecular systematic studies in the past two decades will hopefully have a stabilizing effect on classification in due course, but has so far had the opposite effect". In 2010, Nyffler and Eggli produced a classification based on an explicit phylogeny. The broad outlines of their cladogram agree with that of Hern et al. (2011), shown above, although some details are different. Their classification corresponds to

2695-523: The Echinocereeae are native from the southwestern United States, through Mexico , Central America and the Caribbean to northern South America. Many are classic columnar desert cacti of Mexico and the southwestern US, like the saguaro ( Carnegiea gigantea ). A few have a wider distribution: Stenocereus griseus is found from northeastern Mexico to Colombia . Cactus See also Classification of

2772-633: The Hylocereeae in 2017 and 2020. Austrocactus , Corryocactus , Eulychnia Armatocereus , Leptocereus , Neoraimondia , Strophocactus (including Pseudoacanthocereus ) Core Echinocereeae Hylocereeae "Core Echinocereeae" recovered in molecular phylogenetic analyses in 2005 and 2011 contains the following genera, often only in part compared to earlier generic circumscriptions. Some genera were further placed in two subtribes, Pachycereinae and Echinocereinae. The position of some genera remained unclear, as of October 2023. Brachycereus

2849-448: The ICSG classification of the subfamily Cactoideae (as presented by Anderson in 2001 ) with a 2002 classification of the subfamily Opuntioideae. There are differences in the genera recognized in the systems; where the older system does not recognize the genus, "–" is shown. Unless otherwise indicated, the subfamily placement of the genera listed here is based on Anderson's 2001 presentation of

2926-403: The absence of leaves. This is discussed further below under Metabolism . Many cacti have roots that spread out widely, but only penetrate a short distance into the soil. In one case, a young saguaro only 12 cm (4.7 in) tall had a root system with a diameter of 2 m (7 ft), but no more than 10 cm (4 in) deep. Cacti can also form new roots quickly when rain falls after

3003-413: 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

3080-573: The cactus and providing some shade. In the absence of true leaves, cacti's enlarged stems carry out photosynthesis . 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

3157-478: The cladogram shown below. Pereskia A Pereskia B Maihuenia incertae sedis Cylindropuntieae Opuntieae Blossfeldia ( Blossfeldieae ) Cacteae incertae sedis Phyllocacteae Rhipsalideae Notocacteae Cereeae The table below shows how the genera recognized by Nyffler and Eggli are placed into their subfamilies and tribes (they also use some subtribes, not shown here). The column headed "Older tribe (if different)" shows

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3234-600: 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

3311-521: 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

3388-669: 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. 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

3465-490: 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 . 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

3542-418: The family Cactaceae. However, some authors continue to use the name Pachycereeae for the tribe. Genera placed at one time or another within the Echinocereeae (or Pachycereeae) fell within the "Core Cactoideae I" clade in the 2011 molecular phylogenetic study by Barcenas et al. Possible phylogenetic relationships within this clade are shown in the cladogram below, based on Guerrero et al. (2019) and revisions to

3619-404: 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. The absence of visible leaves is one of the most striking features of most cacti. Pereskia (which

3696-972: The first, Tephrocacteae , were shown to be "essentially monophyletic" in molecular phylogenetic study in 2009. A summary of the consensus Bayesian tree is shown below (tribes are bold; the number of species in the study is shown in parentheses). Puna (1), Maihueniopsis (3), Cumulopuntia (1) =  Tephrocacteae (part) Pterocactus (6) =  Pterocacteae Maihueniopsis (1), Puna (1), Tephrocactus (7) =  Tephrocacteae (part) Maihueniopsis (1), Cumulopuntia (5), Puna (1), Austrocylindropuntia (4) ≈  Austrocylindropuntieae Quiabentia (1), Pereskiopsis (2), Micropuntia (2), Cylindropuntia (12), Grusonia s.s. (1), Corynopuntia (6) ≈  Cylindropuntieae Miqueliopuntia (1), Tunilla (2), Maihueniopsis (1) Opuntia (2) Brasiliopuntia (1), Opuntia (1), Tacinga (2) Opuntia (28), Consolea (6), Nopalea (7) Many of

3773-570: 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

3850-573: The genera within the Opuntioideae are not monophyletic. Maihueniopsis is highly polyphyletic , appearing in four separate lineages. The two largest genera within the subfamily, Opuntia and Cylindropuntia are also not monophyletic. The classification of the Opuntioideae is thus uncertain as of March 2012 ; Griffith and Porter say that changes in classification will require "broad information (of multiple data types) regarding all species of opuntioid cacti". The ICSG classification divides

3927-874: 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 are structures unique to cacti. Although variable, they typically appear as woolly or hairy areas on

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4004-529: 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

4081-558: The ground or from branches very low down, such as in Stenocereus thurberi . 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,

4158-552: The ground surface. Cactus stems are often ribbed or fluted with a number of ribs which corresponds to a number in the Fibonacci numbers (2, 3, 5, 8, 13, 21, 34 etc.). This 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

4235-501: The ground. The leafless, spiny stem is the characteristic feature of the majority of cacti (all 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

4312-441: 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 0.5 mm (0.02 in) long in about half of

4389-401: 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 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana , only about 1 cm (0.4 in) in diameter at maturity. A fully grown saguaro ( Carnegiea gigantea )

4466-483: 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: Molecular phylogenetic studies have supported

4543-455: 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

4620-421: 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

4697-449: The photosynthesis occurs. 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"

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4774-442: 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

4851-453: 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. Cacti show a wide variety of growth habits , which are difficult to divide into clear, simple categories. Cacti can be tree-like (arborescent), meaning they typically have a single more-or-less woody trunk topped by several to many branches . In

4928-453: 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 C 3 mechanism. At night, or when the plant is short of water, the stomata close and the CAM mechanism is used to store CO 2 produced by respiration for use later in photosynthesis. CAM-cycling

5005-431: 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

5082-481: 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

5159-666: The species studied and almost always less than 1.5 mm (0.06 in) 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. 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

5236-513: 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. Photosynthesis requires plants to take in carbon dioxide gas (CO 2 ). As they do so, they lose water through transpiration . Like other types of succulents , cacti reduce this water loss by

5313-433: 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

5390-428: The subfamily Cactoideae into nine tribes. However, phylogenetic research has not supported most of these tribes, nor even the genera of which they are composed. A 2011 study found that "an extraordinarily high proportion of genera" were not monophyletic, including 22 (61%) of the 36 genera in the subfamily Cactoideae sampled in the research. Of the nine tribes recognized within Cactoideae, one, Calymmantheae , comprises

5467-493: 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. 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

5544-410: 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. 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

5621-422: 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

5698-541: The type genus of the Echinocereeae, was nested within the Pachycereeae, as then circumscribed , and Pachycereeae was expanded to include Echinocereeae. In 2006, David Hunt and the International Cactaceae Systematics Group used the name Echinocereeae rather than Pachycereeae for the tribe. This has been followed in much subsequent work, including a major 2011 molecular phylogenetic analysis of

5775-460: The way in which they carry out photosynthesis. "Normal" leafy plants use the C 3 mechanism : during daylight hours, CO 2 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

5852-432: 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. Pereskia s.l. Clade A → Leuenbergeria Pereskia s.l. Clade B → Rhodocactus + Pereskia s.s. Opuntioideae Maihuenia Classification of

5929-470: Was not included in many studies, but has been placed with Armatocereus and Jasminocereus , both included in Echinocereeae. The 2011 study found the tribe Hylocereeae to be embedded within a wider Echinocereeae, but studies in 2017 and 2020 asserted the independence of the Hylocereeae, including within it two genera placed within Pachycereeae in earlier classifications: Acanthocereus and Pseudoacanthocereus (sunk into Strophocactus ). Members of

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