Many species of cactus have long, sharpspines, like thisOpuntia.
Acactus (pl.:cacti,cactuses, or less commonly,cactus)[3] is a member of theplant familyCactaceae (/kækˈteɪsi.iː,-ˌaɪ/),[a] a family of the orderCaryophyllales comprising about 127 genera with some 1,750 known species.[4] The wordcactus derives, through Latin, from theAncient Greek wordκάκτος (káktos), a name originally used byTheophrastus for a spiny plant whose identity is now not certain.[5] Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging fromPatagonia in the south to parts of westernCanada in the north, with the exception ofRhipsalis baccifera, which is also found inAfrica andSri Lanka. Cacti are adapted to live in very dry environments, including theAtacama Desert, one of the driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, almost all cacti aresucculents, 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 onlyspines, which are highly modified leaves. As well as defending againstherbivores, 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 enlargedstems carry outphotosynthesis.
Cactus spines are produced from specialized structures calledareoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise toflowers, which are usually tubular and multipetaled. Many cacti have shortgrowing 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 with a number of ribs which corresponds to a number in theFibonacci 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 longdrought periods. Like other succulent plants, most cacti employ a special mechanism called "crassulacean acid metabolism" (CAM) as part of photosynthesis.Transpiration, during whichcarbon 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 asmalic 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 fromtranspiration. The tallest[b] free-standing cactus isPachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft),[7] and the smallest isBlossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity.[8] A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 760 liters (200 U.S. gal) of water during a rainstorm.[9] A few species differ significantly in appearance from most of the family. At least superficially, plants of the generaLeuenbergeria,Rhodocactus andPereskia 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 forwater conservation.Leuenbergeria is considered close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers andepiphytes (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 genusSchlumbergera).
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.
The 1,500 to 1,800 species of cacti mostly fall into one of two groups of "core cacti": opuntias (subfamilyOpuntioideae) and "cactoids" (subfamilyCactoideae). Most members of these two groups are easily recognizable as cacti. They have fleshysucculentstems that are majororgans ofphotosynthesis. They have absent, small, or transientleaves. They haveflowers withovaries that lie below thesepals andpetals, often deeply sunken into a fleshyreceptacle (the part of the stem from which the flower parts grow). All cacti haveareoles—highly specialized shortshoots with extremely shortinternodes that producespines, normal shoots, and flowers.[10]
The remaining cacti fall into only two groups: three tree-like genera,Leuenbergeria,Pereskia andRhodocactus (all formerly placed inPereskia), and the much smallerMaihuenia. These two groups are rather different from other cacti,[10] 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, in that when mature, they have woody stems that may be covered withbark 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 ofMaihuenia have succulent but non-photosynthetic stems and prominent succulent leaves.[10]
Cacti show a wide variety of growthhabits, which are difficult to divide into clear, simple categories.
Arborescent cacti
Cacti can be tree-like (arborescent), meaning they typically have a single more-or-less woodytrunk topped by several to manybranches. In the generaLeuenbergeria,Pereskia andRhodocactus, 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 inPachycereus pringlei or the largeropuntias. Some cacti may become tree-sized but without branches, such as larger specimens ofEchinocactus platyacanthus. Cacti may also be described asshrubby, with several stems coming from the ground or from branches very low down, such as inStenocereus thurberi.[11]
Columnar cacti
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 ofCephalocereus 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.[11]
Globular cacti
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 asFerocactus latispinus, or their stems may form clusters that can create large mounds. All or some stems in a cluster may share a common root.[11]
Other forms
Other cacti have a quite different appearance. In tropical regions, some grow as forest climbers andepiphytes. 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 ofHylocereus was reported as 100 meters (330 ft) long from root to the most distant stem. Epiphytic cacti, such as species ofRhipsalis orSchlumbergera, often hang downwards, forming dense clumps where they grow in trees high above the ground.[11]
The leafless, spiny stem is the characteristic feature of the majority of cacti (all belonging to the largest subfamily, theCactoideae). The stem is typically succulent, meaning it is adapted to store water. The surface of the stem may be smooth (as in some species ofOpuntia) or covered with protuberances of various kinds, which are usually called tubercles. These vary from small "bumps" to prominent, nipple-like shapes in the genusMammillaria and outgrowths almost like leaves inAriocarpus 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.[11]
The stems of most cacti are some shade of green, often bluish or brownish green. Such stems containchlorophyll and are able to carry out photosynthesis; they also havestomata (small structures that can open and close to allow passage of gases). Cactus stems are often visibly waxy.[11]
Flowers appear from the upper part of an areole, spines from the lower (Cereus species).
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 genusLeuenbergeria, 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).[12] 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.[11]
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. InPereskia, a genus close to the ancestor of cacti, areoles remain active for much longer; this is also the case inOpuntia andNeoraimondia.[11]
The great majority of cacti have no visibleleaves; 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 ofLeuenbergeria,Pereskia andRhodocactus are superficially like normal trees or shrubs and have numerous leaves with a midrib and a flattened blade (lamina) on either side. This group isparaphyletic, forming two taxonomicclades. Many cacti in the opuntia group (subfamilyOpuntioideae) also have visible leaves, which may be long-lasting (as inPereskiopsis species) or produced only during the growing season and then lost (as in many species ofOpuntia).[11] The small genusMaihuenia also relies on leaves for photosynthesis.[13] The structure of the leaves varies somewhat between these groups. Opuntioids andMaihuenia have leaves that appear to consist only of a midrib.[14]
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 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 asauxin, and in definingaxillary buds has been suggested.[15]
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 asPereskia,Pereskiopsis andMaihuenia, 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 asRhipsalis andSchlumbergera, but also of some ground-living cacti, such asAriocarpus.[11]
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).[11]
In addition to normal-length spines, members of the subfamily Opuntioideae have relatively short spines, calledglochids, 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.[11]
Most ground-living cacti have onlyfine roots, which spread out around the base of the plant for varying distances, close to the surface. Some cacti havetaproots; in genera such asAriocarpus, these are considerably larger and of a greater volume than the body. Taproots may aid in stabilizing the larger columnar cacti.[16] Climbing, creeping and epiphytic cacti may have onlyadventitious roots, produced along the stems where these come into contact with a rooting medium.[11]
Like their spines, cactusflowers are variable. Typically, theovary is surrounded by material derived from stem orreceptacle tissue, forming a structure called apericarpel. Tissue derived from thepetals andsepals 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 hasareoles that produce wool and spines. Typically, the tube also has small scale-likebracts, 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").[11] Some cacti produce floral tubes without wool or spines (e.g.Gymnocalycium)[17] or completely devoid of any external structures (e.g.Mammillaria).[11] Unlike the flowers of most other cacti,Pereskia flowers may be borne in clusters.[12]
Cactus flowers usually have manystamens, but only a singlestyle, which may branch at the end into more than onestigma. 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.[11]
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.[11]
All cacti have some adaptations to promote efficient water use. Most cacti—opuntias andcactoids—specialize in surviving in hot and dry environments (i.e. arexerophytes), but the first ancestors of modern cacti were already adapted to periods of intermittent drought.[10] A small number of cactus species in the tribesHylocereeae andRhipsalideae have become adapted to life as climbers orepiphytes, 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 is close to the ancestral species from which all cacti evolved) does have long-lasting leaves, which are, however, thickened andsucculent in many species.[10] Other species of cactus with long-lasting leaves, such as the opuntioidPereskiopsis, also have succulent leaves.[18] 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.[14] Spines provide protection fromherbivores 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 andtranspiration. 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.[11]
Stem of youngCereus hildmannianus subsp.uruguayanus, showing ribbing and waxy coating
The majority of cacti arestem 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.[11] A mature saguaro (Carnegiea gigantea) is said to be able to absorb as much as 760 liters (200 U.S. gal) of water during a rainstorm.[9] The outer layer of the stem usually has a toughcuticle, 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.[11]
The stems of most cacti have adaptations to allow them to conduct photosynthesis in the absence of leaves[citation needed]. This is discussed further below underMetabolism.
Many cacti have roots that spread out widely, but only penetrate a short distance into the soil.[19] 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.[16] 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.[20] 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.[11]
Although in most cacti, the stem acts as the main organ for storing water, some cacti have in addition largetaproots.[11] These may be several times the length of the above-ground body in the case of species such asCopiapoa atacamensis,[11] which grows in one of the driest places in the world, theAtacama Desert in northern Chile.[21]
Photosynthesis requires plants to take incarbon dioxide gas (CO2). As they do so, they lose water throughtranspiration. Like other types ofsucculents, cacti reduce this water loss by the way in which they carry out photosynthesis. "Normal" leafy plants use theC3 mechanism: during daylight hours, CO2 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 ascarbohydrates. The access of air to internal spaces within a plant is controlled bystomata, which are able to open and close. The need for a continuous supply of CO2 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.[22] A further problem is that as temperatures rise, theenzyme that captures CO2 starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%.[23]
Schematic illustration of CAM
Night: stomata open; CO2 enters and is stored as malic acid; water vapor is able to escape.
Day: stomata close; malic acid is converted back to CO2 and used to make carbohydrate; water vapor is confined.
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. CO2 enters the plant and is captured in the form of organic acids stored inside cells (invacuoles). The stomata remain closed throughout the day, and photosynthesis uses only this stored CO2. CAM uses water much more efficiently at the price of limiting the amount of carbon fixed from the atmosphere and thus available for growth.[24] 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 CO2 produced byrespiration for use later in photosynthesis. CAM-cycling is present inPereskia species.[10]
By studying the ratio of14C to13C incorporated into a plant—itsisotopic signature—it is possible to deduce how much CO2 is taken up at night and how much in the daytime. Using this approach, most of thePereskia species investigated exhibit some degree of CAM-cycling, suggesting this ability was present in the ancestor of all cacti.[10]Pereskia leaves are claimed to only have the C3 mechanism with CAM restricted to stems.[25] 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."[10] Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012[update], it is not clear whether stem-based CAM evolved once only in the core cacti, or separately in the opuntias and cactoids;[10] CAM is known to haveevolved convergently many times.[24]
To carry out photosynthesis, cactus stems have undergone many adaptations. Early in their evolutionary history, the ancestors of modern cacti (other thanLeuenbergeria species) developedstomata 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 containingchloroplasts, arranged into a "spongy layer" and a "palisade layer" where most of the photosynthesis occurs.[26]
(Above) The spiny heads of thecardoon, possibly once known as "cactus"; (below) AMelocactus, likely the first genus seen by Europeans
Naming and classifying cacti has been both difficult and controversial since the first encounters with them by western observers. The difficulties began withCarl Linnaeus. In 1737, he placed the cacti he knew into two genera,Cactus andPereskia. However, when he publishedSpecies 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 theAncient Greekκάκτος (kaktos), a name used byTheophrastus for a spiny plant,[27] which may have been thecardoon (Cynara cardunculus).[28]
Later botanists, such asPhilip 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 nameCactus 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 nameCactus and instead declaredMammillaria 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.[29]
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 inherbaria 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.[30] A further difficulty is that many cacti were given names by growers and horticulturalists rather than botanists; as a result, the provisions of theInternational 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 toDavid Hunt, ensured he "left a trail of nomenclatural chaos that will probably vex cactus taxonomists for centuries."[31]
In 1984, it was decided that the Cactaceae Section of theInternational 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.[32][33][34] The ICSG classification of the cactus family recognized four subfamilies, the largest of which was divided into nine tribes. The subfamilies were:[32]
The only genus in the ICSG classification wasPereskia. 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.[32] Two systems may be used inphotosynthesis, both the "normal"C3 mechanism andcrassulean acid metabolism (CAM)—an "advanced" feature of cacti and othersucculents that conserves water.[10]
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).[32] Plants vary in size from the small cushions ofMaihueniopsis[39] to treelike species ofOpuntia, rising to 10 m (33 ft) or more.[40]
The only genus isMaihuenia, with two species, both of which form low-growing mats.[13] It has some features that are primitive within the cacti. Plants have leaves, and crassulean acid metabolism is wholly absent.[32]
Divided into nine tribes, this is the largest subfamily, including all the "typical" cacti. Members are highly variable in habit, varying from tree-like toepiphytic. 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 andRhipsalideae, contain climbing orepiphytic forms with a rather different appearance; their stems are flattened and may be divided into segments.[32]
Molecular phylogenetic studies have supported themonophyly of three of these subfamilies, but not Pereskioideae;[34][35] nor have they supported all of the tribes or even genera below this level. A 2011 study found only 39% of the genera in the subfamily Cactoideae sampled in the research were monophyletic.[34] Classification of the cacti currently remains uncertain and is likely to change.
A 2005 study suggested the genusPereskia as thencircumscribed (Pereskia sensu lato) wasbasal within the Cactaceae, but confirmed earlier suggestions it was notmonophyletic, i.e., did not include all the descendants of a common ancestor. TheBayesian consensuscladogram from this study is shown below with subsequent generic changes added.[35][36][37]
A 2011 study using fewer genes but more species also found thatPereskia 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."[34]
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 formbark early in the plants' life and also lackstomata—structures that control admission of air into a plant and hence controlphotosynthesis. By contrast, caulocacti, including species ofRhodocactus and the remaining species ofPereskia 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 ofMaihuenia are something of an exception.)[35][37]
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. IfLeuenbergeria 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 orderCaryophyllales). 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. LikePereskia 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.[10]
The clade containingRhodocactus andPereskia 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 inMaihuenia, or separately in Opuntioideae and Cactoideae, in which case it never evolved inMaihuenia.[10]
Understanding evolution within the core cacti clade is difficult as of February 2012[update], 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 notmonophyletic, 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.[34] Nine tribes are recognized within Cactoideae in the International Cactaceae Systematics Group (ICSG) classification; one, Calymmantheae, comprises a single genus,Calymmanthium.[32] 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, seeClassification of the Cactaceae.
No known fossils of cacti exist to throw light on their evolutionary history.[41] However, the geographical distribution of cacti offers some evidence. Except for a relatively recent spread ofRhipsalis baccifera to parts of theOld World, cacti are plants ofSouth America and mainly southern regions ofNorth America. This suggests the family must have evolved after the ancient continent ofGondwana split into South America andAfrica, which occurred during theEarly Cretaceous, around143 to 101 million years ago.[42] Precisely when after this split cacti evolved is less clear. Older sources suggest an early origin around 90 – 66 million years ago, during theLate Cretaceous. More recent molecular studies suggest a much younger origin, perhaps in very LateEocene to earlyOligocene periods, around 35–30 million years ago.[41][43] 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.[35] Core cacti, those with strongly succulent stems, are estimated to have evolved around 25 million years ago.[41] A possible stimulus to their evolution may have been uplifting in the centralAndes, some 25–20 million years ago, which was associated with increasing and varying aridity.[35] However, the current species diversity of cacti is thought to have arisen only in the last 10–5 million years (from the lateMiocene into thePliocene). Other succulent plants, such as theAizoaceae in South Africa, theDidiereaceae in Madagascar and the genusAgave in the Americas, appear to have diversified at the same time, which coincided with a global expansion of arid environments.[41]
Cacti inhabit diverse regions, fromcoastal plains to high mountain areas. With one exception, they are native to theAmericas, where their range extends fromPatagonia toBritish Columbia andAlberta in westernCanada. A number of centers of diversity exist. For cacti adapted to drought, the three main centers are thedeserts of North America; the southwesternAndes, where they are found inPeru,Bolivia,Chile andArgentina; and theCaatinga of easternBrazil. 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 subfamilyRhipsalideae; and in forested regions ofCentral America, where the climbingHylocereeae are most diverse.[44]
Rhipsalis baccifera is the exception; it is native to both the Americas and theOld World, where it is found in tropicalAfrica,Madagascar, andSri Lanka. One theory is it was spread by being carried asseeds in thedigestive tracts ofmigratorybirds; the seeds ofRhipsalis are adapted for bird distribution. Old World populations arepolyploid, and regarded as distinct subspecies, supporting the idea that the spread was not recent.[45] The alternative theory is the species initially crossed the Atlantic on European ships trading between South America and Africa, after which birds may have spread it more widely.[46]
Many other species have become naturalized outside the Americas after having been introduced by people, especially inAustralia,Hawaii, and theMediterranean region. In Australia, species ofOpuntia, particularlyOpuntia stricta, were introduced in the 19th century for use as natural agricultural fences and in an attempt to establish acochineal industry. They rapidly became a major weed problem, but are now controlled by biological agents, particularly the mothCactoblastis cactorum.[47] The weed potential ofOpuntia species in Australia continues however, leading to all opuntioid cacti exceptO. ficus-indica being declaredWeeds of National Significance by theAustralian Weeds Committee in April 2012.
TheArabian Peninsula has a wide variety of ever-increasing, introduced cactus populations. Some of these are cultivated,[48][49] some are escapes from cultivation, and some are invasives that are presumed to be ornamental escapes.[50][51][52]
Flower ofSchlumbergera cut in half, showing typical adaptations tobird pollinationFlowers ofsaguaro showing flattish white flowers adapted for bat pollination
Cactus flowers arepollinated by insects, birds and bats. None are known to be wind-pollinated and self-pollination occurs in only a very few species; for example the flowers of some species ofFrailea do not open (cleistogamy).[53] The need to attractpollinators has led to the evolution ofpollination syndromes, which are defined as groups of "floral traits, including rewards, associated with the attraction and utilization of a specific group of animals as pollinators."[54]
Bees are the most common pollinators of cacti; bee-pollination is considered to have been the first to evolve.[53] Day-flying butterflies and nocturnal moths are associated with different pollination syndromes. Butterfly-pollinated flowers are usually brightly colored, opening during the day, whereas moth-pollinated flowers are often white or pale in color, opening only in the evening and at night.[55] As an example,Lophocereus schottii is pollinated by a particular species of moth,Upiga virescens, which also lays its eggs among the developing seeds its caterpillars later consume.[55] The flowers of this cactus are funnel-shaped, white to deep pink, up to 4 cm (1.6 in) long, and open at night.[56]
Hummingbirds are significant pollinators of cacti. Species showing the typical hummingbird-pollination syndrome have flowers with colors towards the red end of the spectrum, anthers and stamens that protrude from the flower, and a shape that is notradially symmetrical, with a lower lip that bends downwards; they produce large amounts ofnectar with a relatively low sugar content.[57]Schlumbergera species, such asS. truncata, have flowers that correspond closely to this syndrome.[58] Other hummingbird-pollinated genera includeCleistocactus andDisocactus.[53]
Bat-pollination is relatively uncommon in flowering plants, but about a quarter of the genera of cacti are known to be pollinated bybats—an unusually high proportion, exceeded amongeudicots by only two other families, both with very few genera. Columnar cacti growing in semidesert areas are among those most likely to be bat-pollinated; this may be because bats are able to travel considerable distances, so are effective pollinators of plants growing widely separated from one another. The pollination syndrome associated with bats includes a tendency for flowers to open in the evening and at night, when bats are active. Other features include a relatively dull color, often white or green; a radially symmetrical shape, often tubular; a smell described as "musty"; and the production of a large amount of sugar-rich nectar.Carnegiea gigantea is an example of a bat-pollinated cactus, as are many species ofPachycereus andPilosocereus.[59]
Fruit ofSchlumbergera cut in half; the length is 1.0~1.5 cm.
Thefruits produced by cacti after the flowers have been fertilized vary considerably; many are fleshy, although some are dry. All contain a large number of seeds. Fleshy, colorful and sweet-tasting fruits are associated with seed dispersal by birds. The seeds pass through their digestive systems and are deposited in their droppings. Fruit that falls to the ground may be eaten by other animals;giant tortoises are reported to distributeOpuntia seeds in the Galápagos Islands.Ants appear to disperse the seeds of a few genera, such asBlossfeldia. Drier spiny fruits may cling to the fur of mammals or be moved around by the wind.[60]
As of March 2012[update], there is still controversy as to the precise dates when humans first entered those areas of the New World where cacti are commonly found, and hence when they might first have used them. An archaeological site inChile has been dated to around 15,000 years ago,[61] suggesting cacti would have been encountered before then. Early evidence of the use of cacti includes cave paintings in theSerra da Capivara inBrazil, and seeds found in ancientmiddens (waste dumps) inMexico andPeru, with dates estimated at 12,000–9,000 years ago. Hunter-gatherers likely collected cactus fruits in the wild and brought them back to their camps.[62]
The central image in this extract from theCodex Osuna of 1565 shows anopuntia used as a symbol forTenochtitlan (now in Mexico City)
It is not known when cacti were first cultivated. Opuntias (prickly pears) were used for a variety of purposes by theAztecs, whose empire, lasting from the 14th to the 16th century, had a complex system of horticulture. Their capital from the 15th century wasTenochtitlan (nowMexico City); one explanation for the origin of the name is that it includes theNahuatl wordnōchtli, referring to the fruit of an opuntia.[63] Thecoat of arms of Mexico shows an eagle perched on a cactus while holding a snake, an image at the center of the myth of the founding of Tenochtitlan.[64] 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.[65]
Europeans first encountered cacti when they arrived in the New World late in the 15th century. Their first landfalls were in theWest Indies, where relatively few cactus genera are found; one of the most common is the genusMelocactus.[66] Thus, melocacti were possibly among the first cacti seen by Europeans.Melocactus species were present in English collections of cacti before the end of the 16th century (by 1570 according to one source,[67]) where they were calledEchinomelocactus, later shortened toMelocactus by Joseph Pitton de Tourneville in the early 18th century.[68] Cacti, both purely ornamental species and those with edible fruit, continued to arrive in Europe, soCarl Linnaeus was able to name 22 species by 1753. One of these, hisCactus opuntia (now part ofOpuntia ficus-indica), was described as "fructu majore ... nunc in Hispania et Lusitania" (with larger fruit ... now in Spain and Portugal), indicative of its early use in Europe.[69][70]
Peeled fruits of theIndian fig cactus of different varieties on sale in Mexico
The plant now known asOpuntia ficus-indica, or the Indian fig cactus, has long been an important source of food. The original species is thought to have come from central Mexico, although this is now obscure because the indigenous people of southern North America developed and distributed a range of horticultural varieties (cultivars), including forms of the species and hybrids with other opuntias. Both the fruit and pads are eaten, the former often under the Spanish nametuna, the latter under the namenopal. Cultivated forms are often significantly less spiny or even spineless.[71] The nopal industry in Mexico was said to be worth US$150 million in 2007.[72] The Indian fig cactus was probably already present in the Caribbean when the Spanish arrived, and was soon after brought to Europe. It spread rapidly in the Mediterranean area, both naturally and by being introduced—so much so, early botanists assumed it was native to the area. Outside the Americas, the Indian fig cactus is an important commercial crop inSicily,Algeria and other North African countries.[70] Fruits of other opuntias are also eaten, generally under the same name,tuna. Flower buds, particularly ofCylindropuntia species, are also consumed.[73]
Almost any fleshy cactus fruit is edible. The wordpitaya orpitahaya (usually considered to have been taken into Spanish from Haitian creole[74]) can be applied to a range of "scaly fruit", particularly those of columnar cacti. The fruit of thesaguaro (Carnegiea gigantea) has long been important to the indigenous peoples of northwestern Mexico and the southwestern United States, including theSonoran Desert. It can be preserved by boiling to produce syrup and by drying. The syrup can also be fermented to produce an alcoholic drink. Fruits ofStenocereus species have also been important food sources in similar parts of North America;Stenocereus queretaroensis is cultivated for its fruit. In more tropical southern areas, the climberSelenicereus undatus providespitahaya orejona, now widely grown in Asia under the name dragon fruit. Other cacti providing edible fruit include species ofEchinocereus,Ferocactus,Mammillaria,Myrtillocactus,Pachycereus,Peniocereus andSelenicereus. The bodies of cacti other than opuntias are less often eaten, although Anderson reported thatNeowerdermannia vorwerkii is prepared and eaten like potatoes in upland Bolivia.[75]
A number of species of cacti have been shown to containpsychoactive agents, chemical compounds that can cause changes in mood, perception and cognition through their effects on the brain. 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 containmescaline.[76]
L. williamsii is native to northern Mexico and southern Texas. Individual stems are about 2–6 cm (0.8–2.4 in) high with a diameter of 4–11 cm (1.6–4.3 in), and may be found in clumps up to 1 m (3 ft) wide.[77] A large part of the stem is usually below ground. Mescaline is concentrated in the photosynthetic portion of the stem above ground. The center of the stem, which contains the growing point (the apicalmeristem), is sunken. Experienced collectors of peyote remove a thin slice from the top of the plant, leaving the growing point intact, thus allowing the plant to regenerate.[78] 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, wereradiocarbon dated to around 3780–3660 BC.[79] Peyote is perceived as a means of accessing the spirit world. Attempts by theRoman Catholic church to suppress its use after the Spanish conquest were largely unsuccessful, and by the middle of the 20th century, peyote was more widely used than ever by indigenous peoples as far north as Canada. It is now used formally by theNative American Church.[76]
Echinopsis pachanoi (syn.Trichocereus macrogonus var.pachanoi) is native to Ecuador and Peru. It is very different in appearance fromL. williamsii. It has tall stems, up to 6 m (20 ft) high, with a diameter of 6–15 cm (2.4–5.9 in), which branch from the base, giving the whole plant a shrubby or tree-like appearance.[80] Archaeological evidence of the use of this cactus appears to date back to 2,000–2,300 years ago, with carvings and ceramic objects showing columnar cacti.[81] Although church authorities under the Spanish attempted to suppress its use, this failed, as shown by the Christian element in the common name "San Pedro cactus"—Saint Peter cactus. Anderson attributes the name to the belief that just as St Peter holds the keys to heaven, the effects of the cactus allow users "to reach heaven while still on earth."[76] It continues to be used for its psychoactive effects, both for spiritual and for healing purposes, often combined with other psychoactive agents, such asDatura ferox and tobacco.[81] Several other species ofEchinopsis, includingE. peruviana andE. lageniformis, also contain mescaline.[76]
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.[82]
Cacti are often grown in greenhouses, particularly in regions unsuited to the cultivation of cacti outdoors, such the northern parts of Europe and North America. Here, they may be kept in pots or grown in the ground. Cacti are also grown ashouseplants, many being tolerant of the often dry atmosphere. Cacti in pots may be placed outside in the summer to ornament gardens or patios, and then kept under cover during the winter.[83] Less drought-resistantepiphytes, such asepiphyllum hybrids,Schlumbergera (the Thanksgiving or Christmas cactus) andHatiora (the Easter cactus), are widely cultivated as houseplants.
A planted cactus inGibraltar. The hot and relatively dry climate of the territory allows cacti to thrive.
Cacti may also be planted outdoors in regions with suitable climates. Concern for water conservation in arid regions has led to the promotion of gardens requiring less watering (xeriscaping). For example, inCalifornia, the East Bay Municipal Utility District sponsored the publication of a book on plants and landscapes for summer-dry climates.[84] Cacti are one group of drought-resistant plants recommended for dry landscape gardening.[85]
Cacti have many other uses. They are used for human food and as fodder for animals, usually after burning off their spines.[86] In addition to their use as psychoactive agents, some cacti are employed inherbal medicine. The practice of using various species ofOpuntia in this way has spread from the Americas, where they naturally occur, to other regions where they grow, such asIndia.[87]
Cochineal is a red dye produced by ascale insect that lives on species ofOpuntia. 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.[88]
Cacti are used as construction materials. Livingcactus fences are employed as barricades around buildings to prevent people breaking in.[89][90] They also used tocorral animals. The woody parts of cacti, such asCereus repandus andEchinopsis atacamensis, are used in buildings and in furniture. The frames ofwattle and daub houses built by theSeri people of Mexico may use parts of thesaguaro (Carnegiea gigantea). The very fine spines and hairs (trichomes) of some cacti were used as a source of fiber for filling pillows and in weaving.[91]
All cacti are included in Appendix II of theConvention 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.[92] Some exceptions are allowed, e.g., for "naturalized or artificially propagated plants".[93] Some cacti, such as allAriocarpus andDiscocactus species, are included in the more restrictive Appendix I,[93] 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.[92]
The three main threats to cacti in the wild are development, grazing and over-collection. Development takes many forms. The construction of a dam nearZimapan, Mexico, caused the destruction of a large part of the natural habitat ofEchinocactus grusonii. Urban development and highways have destroyed cactus habitats in parts of Mexico,New Mexico andArizona, including theSonoran Desert. The conversion of land to agriculture has affected populations ofAriocarpus kotschoubeyanus in Mexico, where dry plains were plowed for maize cultivation, and ofCopiapoa andEulychnia inChile, where valley slopes were planted with vines.[94] Grazing, in many areas by introduced animals, such as goats, has caused serious damage to populations of cacti (as well as other plants); two examples cited by Anderson are theGalápagos Islands generally and the effect onBrowningia candelaris inPeru. Over-collection of cacti for sale has greatly affected some species. For example, the type locality ofPelecyphora strobiliformis nearMiquihuana, Mexico, was virtually denuded of plants, which were dug up for sale in Europe. Illegal collecting of cacti from the wild continues to pose a threat.[95][96]
Conservation of cacti can bein situ orex situ.In situ conservation involves preserving habits through enforcement of legal protection and the creation of specially protected areas such as national parks and reserves. Examples of such protected areas in the United States includeBig Bend National Park, Texas;Joshua Tree National Park, California; andSaguaro National Park, Arizona. Latin American examples includeParque Nacional del Pinacate, Sonora, Mexico andPan de Azúcar National Park, Chile.Ex situ conservation aims to preserve plants and seeds outside their natural habitats, often with the intention of later reintroduction.Botanical gardens play an important role inex situ conservation; for example, seeds of cacti and other succulents are kept in long-term storage at theDesert Botanical Garden, Arizona.[97]
The popularity of cacti means many books are devoted to their cultivation. Cacti naturally occur in a wide range of habitats and are then grown in many countries with different climates, so precisely replicating the conditions in which a species normally grows is usually not practical.[82] A broad distinction can be made between semidesert cacti and epiphytic cacti, which need different conditions and are best grown separately.[98] This section is primarily concerned with the cultivation of semidesert cacti in containers and under protection, such as in a greenhouse or in the home, rather than cultivation outside in the ground in those climates that permit it. For the cultivation of epiphytic cacti, seeCultivation ofSchlumbergera (Christmas or Thanksgiving cacti), andCultivation of epiphyllum hybrids.
The purpose of the growing medium is to provide support and to store water, oxygen and dissolved minerals to feed the plant.[99] In the case of cacti, there is general agreement that an open medium with a high air content is important. When cacti are grown in containers, recommendations as to how this should be achieved vary greatly; Miles Anderson says that if asked to describe a perfect growing medium, "ten growers would give 20 different answers".[100] Roger Brown suggests a mixture of two parts commercial soillessgrowing medium, one parthydroponic clay and one part coarsepumice orperlite, with the addition of soil from earthworm castings.[99] The general recommendation of 25–75% organic-based material, the rest being inorganic such as pumice, perlite or grit, is supported by other sources.[100][101][102][103] However, the use of organic material is rejected altogether by others; Hecht says that cacti (other than epiphytes) "want soil that is low in or free ofhumus", and recommends coarse sand as the basis of a growing medium.[104]
Semi-desert cacti need careful watering. General advice is hard to give, since the frequency of watering required depends on where the cacti are being grown, the nature of the growing medium, and the original habitat of the cacti.[105] Brown says that more cacti are lost through the "untimely application of water than for any other reason" and that even during the dormant winter season, cacti need some water.[106] Other sources say that water can be withheld during winter (November to March in the Northern Hemisphere).[98] Another issue is the hardness of the water; where it is necessary to usehard water, regularre-potting is recommended to avoid the build up of salts.[106] The general advice given is that during the growing season, cacti should be allowed to dry out between thorough waterings.[106][107][98] A water meter can help in determining when the soil is dry.[107]
Although semi-desert cacti may be exposed to high light levels in the wild, they may still need some shading when subjected to the higher light levels and temperatures of a greenhouse in summer.[108][109] Allowing the temperature to rise above 32 °C (90 °F) is not recommended.[109] The minimum winter temperature required depends very much on the species of cactus involved. For a mixed collection, a minimum temperature of between 5 °C (41 °F) and 10 °C (50 °F) is often suggested, except for cold-sensitive genera such asMelocactus andDiscocactus.[110][98] Some cacti, particularly those from the highAndes, are fully frost-hardy when kept dry (e.g.Rebutia minuscula survives temperatures down to −9 °C (16 °F) in cultivation[111]) and may flower better when exposed to a period of cold.[112]
Cacti can be propagated by seed,cuttings orgrafting. Seed sown early in the year produces seedlings that benefit from a longer growing period.[113] Seed is sown in a moist growing medium and then kept in a covered environment, until 7–10 days after germination, to avoid drying out.[114] A very wet growing medium can cause both seeds and seedlings to rot.[115] A temperature range of 18–30 °C (64–86 °F) is suggested for germination; soil temperatures of around 22 °C (72 °F) promote the best root growth. Low light levels are sufficient during germination, but afterwards semi-desert cacti need higher light levels to produce strong growth, althoughacclimatization is needed to conditions in a greenhouse, such as higher temperatures and strong sunlight.[114]
Reproduction by cuttings makes use of parts of a plant that can grow roots. Some cacti produce "pads" or "joints" that can be detached or cleanly cut off. Other cacti produce offsets that can be removed.[113] Otherwise, stem cuttings can be made, ideally from relatively new growth. It is recommended that any cut surfaces be allowed to dry for a period of several days to several weeks until acallus forms over the cut surface. Rooting can then take place in an appropriate growing medium at a temperature of around 22 °C (72 °F).[113][114]
Trichocereus bridgesii monstrose
Grafting is used for species difficult to grow well in cultivation or that cannot grow independently, such as somechlorophyll-free forms with white, yellow or red bodies, or some forms that show abnormal growth (e.g.,cristate ormonstrose forms). For the host plant (thestock), growers choose one that grows strongly in cultivation and is compatible with the plant to be propagated: thescion. The grower makes cuts on both stock and scion and joins the two, binding them together while they unite. Various kinds of graft are used—flat grafts, where both scion and stock are of similar diameters, and cleft grafts, where a smaller scion is inserted into a cleft made in the stock.[116]
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.[117]
A range of pests attack cacti in cultivation. Those that feed on sap includemealybugs, living on both stems and roots;scale insects, generally only found on stems;whiteflies, which are said to be an "infrequent" pest of cacti;[118]red spider mites, which are very small but can occur in large numbers, constructing a fine web around themselves and badly marking the cactus via their sap sucking, even if they do not kill it; andthrips, which particularly attack flowers. Some of these pests areresistant to many insecticides, although there arebiological controls available. Roots of cacti can be eaten by the larvae ofsciarid flies andfungus gnats.Slugs andsnails also eat cacti.[119][120]
Fungi, bacteria and viruses attack cacti, the first two particularly when plants are over-watered.Fusarium rot can gain entry through a wound and cause rotting accompanied by red-violet mold. "Helminosporium rot" is caused byBipolaris cactivora (syn.Helminosporium cactivorum[121]);Phytophthora species also cause similar rotting in cacti.Fungicides may be of limited value in combating these diseases.[122] Several viruses have been found in cacti, includingcactus virus X. These appear to cause only limited visible symptoms, such aschlorotic (pale green) spots and mosaic effects (streaks and patches of paler color).[123] However, in anAgave species, cactus virus X has been shown to reduce growth, particularly when the roots are dry.[124] There are no treatments for viral diseases.[122]
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