Cactus

This article is about the plant family. For the former genus Cactus, see Mammillaria, Melocactus, and Opuntia. For other meanings, see Cactus (disambiguation)

"Cacti" redirects here. For the software, see Cacti (software).

Cactus Temporal range: 35-0 Ma PreЄ Є O S D C P T J K Pg N ↓ Late Paleogene - Recent
Echinopsis mamillosa
Scientific classification
Kingdom:	Plantae
clade:	Angiosperms
clade:	Eudicots
Order:	Caryophyllales
Family:	Cactaceae Juss.
Subfamilies
Cactoideae Maihuenioideae Opuntioideae Pereskioideae See also Classification of the Cactaceae

A cactus is a member of the plant family Cactaceae, within the order Caryophyllales. The plural of cactus varies; the Latin cacti, the English cactuses and the uninflected plural cactus are all used. Cacti have a variety of uses: some species are used as ornamental plants, others are grown for fodder or forage, others for food (particularly their fruit). Cochineal is the product of an insect that lives on some cacti.

Most cacti live in habitats which are subject to at least some degree of drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines reduce air flow close to the cactus and provide some shade, both of which help to prevent water loss. 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 multi-petaled.

In the absence of leaves, enlarged stems carry out photosynthesis. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Cactus stems also store water, often being ribbed or fluted which allows them to expand and contract easily. Cacti come in a wide range of shapes and sizes. 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. The smaller cacti usually have globe-shaped stems, combining the highest possible volume with the lowest possible surface area. Many cacti have a short growing season and a long dormancy and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb up to 3,000 liters (790 US gal; 660 imp gal) of water in 10 days.^{[citation needed]}

Like other succulent plants, most cacti employ a special mechanism called Crassulacean acid metabolism (CAM) as part of photosynthesis. Transpiration, during which carbon dioxide enters the plant, does not take place during the day at the same time as photosynthesis, but instead 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.

A few members of the family are significantly different in appearance from most cacti. At least superficially, plants of the genus Pereskia resemble the other trees and shrubs which grow 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 have many adaptations for water conservation. Pereskia is considered to be close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers and epiphytes (plants which grow on trees). Their stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines, like the well known Christmas cactus or Thanksgiving cactus (in the genus Schlumbergera).

Many species of cactus have long, sharp spines, like this Opuntia.

Morphology

Ferocactus species, a "cactoid", in its native Arizona habitat

There are some 1,500–1,800 species of cacti, most of which 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 which are major organs of photosynthesis; absent, small or transient leaves; and flowers with ovaries which lie below the sepals and petals, often deeply sunken into a fleshy receptacle (the part of the stem from which the flower parts grow). All cacti have areoles – highly specialized short shoots with extremely short internodes, from which spines, normal shoots and flowers are produced.

The remaining cacti fall into only two genera, Pereskia and Maihuenia, and are rather different, which means that any description of cacti as a whole must frequently make exceptions for them. Pereskia species superficially resemble other tropical forest trees. When mature they have woody stems which may be covered with bark; leaves which are long-lasting and are the main means of photosynthesis; flowers which may have superior ovaries (i.e. which are above the points of attachment of the sepals and petals); and areoles which produce further leaves. The two species of Maihuenia have small globe-shaped bodies with prominent leaves at the top.

Growth habit

Cacti show a wide variety of growth habits, which are difficult to divide into clear, simple categories. Cacti can be treelike (arborescent), meaning that they typically have a single more-or-less woody trunk topped by several to many branches. In the genus Pereskia the branches are covered with leaves, so that species of this genus may not be recognized as cacti. In most other cacti the branches are more typically cactus-like, bare of leaves and bark and covered with spines, as in Pachycereus pringlei or the larger opuntias. Some cacti may become tree-sized but without branches, such as larger specimens of Echinocactus platyacanthus. Cacti may also be described as shrubby, with several stems coming from the ground or from branches very low down, such as in Stenocereus thurberi.

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 treelike 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 treelike. In some cases the "columns" may be horizontal rather than vertical. Thus Stenocereus eruca has stems that grow along the ground, rooting at intervals.

Cacti whose stems are smaller still may be described as globular (or globose); they consist of stems which are shorter and more "ball-shaped" than those of columnar cacti. Globular cacti may be solitary, such as Ferocactus latispinus, or their stems may form clusters, possibly creating quite large mounds. All or some of the stems in a cluster may have a common root.

Other cacti have a quite different appearance. In tropical regions, some cacti grow as forest climbers and epiphytes. Their stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines. Climbing cacti can be very large; a specimen of Hylocereus was said to be 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 the ground.

Growth habits of cacti
Pachycereus pringlei, showing a tall treelike habit Cephalocereus, showing a tall unbranched columnar habit Ferocactus pilosus, showing a shorter clustered columnar habit Ferocactus echidne, showing a solitary globular habit

Rebutia species, showing a clustered globular habit Rhipsalis paradoxa, an epiphytic cactus Pereskia aculeata, showing its treelike habit

Stems

Stem of Mammillaria longimamma, showing tubercles

The leafless spiny stem is the characteristic feature of the majority of cacti (and all of those belonging to the largest subfamily, the Cactoideae). The stem is typically succulent, meaning that 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" through prominent nipple-like shapes in the genus Mammillaria to structures which are 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 which can open and close to allow the passage of gases). Cactus stems are often visibly waxy.

Areoles

Cactus areoles
Areole of Pereskia grandifolia showing its position relative to leaves Cross-section of Cereus showing areoles with spines and wool

Areoles of an Echinopsis species Close-up of an areole of Astrophytum capricorne showing fine wool Flowers appear from the upper part of an areole, spines from the lower

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

Areoles are highly specialized and very condensed shoots or branches. In a normal shoot, nodes bearing leaves or flowers would be separated by lengths of stem (internodes). In an areole, the nodes are so close together that 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) which 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, and flowers being produced only from the ends of stems, which are still growing and forming new areoles. In Pereskia, a genus close to the ancestor of cacti, areoles remain active for much longer; this is also the case in Opuntia and Neoraimondia.

Leaves

The great majority of cacti are leafless; photosynthesis takes place in the stems (which may be flattened and leaflike in some species). Exceptions occur in three groups of cacti. All the species of Pereskia are superficially like normal trees or shrubs and have numerous leaves. Many cacti in the opuntia group (subfamily Opuntioideae) also have leaves, which may be long lasting (as in Pereskiopsis species) or be produced only during the growing season and then be lost (as in many species of Opuntia). The small genus Maihuenia also relies on leaves for photosynthesis.

Spines

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

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. Schlerocactus papyracanthus). Other cacti have spines which are hooked; sometimes one or more central spines will be hooked while outer spines are straight (e.g. Mammillaria rekoi).

As well as normal length spines, members of the subfamily Opuntioideae have relatively short spines, called "glochids", which are barbed along their length and easily shed. These enter the skin and are then difficult to remove, causing long-lasting irritation.

Cactus spines
Varied spines of a Ferocactus Hooked central spine (cf. Mammillaria rekoi) Unusual flattened spines of Schlerocactus papyracanthus Glochids of Opuntia microdasys

Roots

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

Flowers

Large flowers of an Echinopsis

Like their spines, cactus flowers are variable. Typically the ovary is surrounded by tissue derived from stem or receptacle tissue, forming a structure called a pericarpel. Tissue derived from the petals and sepals continues the pericarpel, forming a composite tube – the whole may be called a "floral tube", although strictly speaking only the part furthest from the base is floral in origin. The outside of the tubular structure often has areoles which produce wool and spines. There are typically also small scale-like bracts on the tube, which gradually change into sepal-like and then petal-like structures, so that 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 which are completely devoid of any external structures (e.g. Mammillaria). Unlike the flowers of other cacti, Pereskia flowers may be borne in clusters.

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

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

Adaptations for water conservation

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

Leaves and spines

The absence of leaves is one of the most striking features of most cacti. Pereskia, which is close to the ancestral species from which all cacti evolved, does have long-lasting leaves, which are, however, thickened and succulent in many species. Other species of cactus with long-lasting leaves, such as the opuntioid Pereskiopsis, also have leaves which are succulent. 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 which do have true leaves, showing that the evolution of spines preceded that of leaves. Although spines have a high surface area-to-volume ratio, they contain little or no water.^{[citation needed]} Apart from providing protection from herbivores and camouflage in some species, spines assist in water conservation in several ways. They trap air near to the surface of the cactus, creating a moister layer which reduces evaporation and transpiration. They can provide some shade which lowers the temperature of the surface of the cactus, also reducing water loss. When sufficiently moist air is present, such as during fog or early morning mist, spines can condense moisture which then drips onto the ground to be absorbed by the roots.

Stems

Stem of Cereus hildmannianus subsp. uruguayanus, showing ribbing and waxy coating

The majority of cacti are "stem succulents", i.e. plants in which the stem is the main organ used to store water. Water may form up to 90% of the total mass of a cactus. Stem shapes vary considerably among cacti. The cylindrical shape of columnar cacti and the spherical shape of globular cacti produce a low surface area-to-volume ratio, thus reducing water loss as well as minimizing the heating effects of sunlight. The ribbed or fluted stems of many cacti allow the stem to shrink during periods of drought and then swell as it fills with water during periods of availability. A mature saguaro (Carnegiea gigantea) is said to be able to absorb as much as 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 the absence of leaves. This is discussed further below under Metabolism.

Roots

Many cacti have roots which 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 (3.9 in) deep. Cacti can also form new roots quickly when rain falls after a drought. The concentration of salts in the root cells of cacti is relatively high. All these adaptations enable cacti to absorb water rapidly during periods of brief or light rainfall. Thus it has been reported that Ferocactus cylindraceus can take up a significant amount of water within 12 hours of rainfall amounting to as little as 7 mm (0.3 in), becoming fully hydrated in a few days.

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

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.

Metabolism

Like other types of succulent, cacti reduce water loss through transpiration by the way in which they perform photosynthesis. "Normal" leafy plants use the C₃ mechanism: during daylight hours, carbon dioxide gas (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 as carbohydrates. The access of air to internal spaces within a plant is controlled by stomata, which are able to open and close. The need for a continual supply of CO₂ during photosynthesis means that the stomata must be open, so that water vapor is continuously lost. Plants using the C₃ mechanism lose as much as 97% of the water taken up through their roots in this way. A further problem is that as temperatures rise, the enzyme which captures CO₂ starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%.

Crassulacean acid metabolism (CAM) is a mechanism adopted by cacti and other succulents to avoid the problems of the C₃ mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. CO₂ 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 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. CAM cycling is a less 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 by respiration for use later in photosynthesis. CAM cycling is present in Pereskia species.

By studying the ratio of ¹⁴C to ¹³C incorporated into a plant – its isotopic signature – it is possible to deduce how much CO₂ is taken up at night and how much in the daytime. Using this approach, it has been shown that most of the Pereskia species that have been investigated exhibit some degree of CAM cycling, suggesting that this ability was present in the ancestor of all cacti. It has been claimed that Pereskia leaves only have the C₃ mechanism with CAM restricted to stems. More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; Pereskia species are described as having "C₃ with inducible CAM". Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012, it is not clear whether stem-based CAM evolved once only in the "core cacti", or separately in the opuntias and cactoids; CAM itself is known to have evolved convergently many times.

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

Taxonomy and classification

(Above) the spiny heads of the wild cardoon, once known as cactus; (below) a Melocactus, likely to be the first genus seen by Europeans

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

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 that Linnaeus' name Cactus had become so confused as to its meaning (was it the genus or the family?) that it should not be used as a genus name. The 1905 Vienna botanical congress rejected the name Cactus and instead declared that Mammillaria was the type genus of the family Cactaceae. It did, however, conserve the name "Cactaceae", leading to the unusual situation in which the family Cactaceae no longer contains the genus after which it was named.

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

Classification

Main article: Classification of the Cactaceae

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, to produce consensus classifications down to the level of genera. Their classification of the cactus family recognizes four subfamilies, the largest of which is divided into nine tribes. The subfamilies are:

The four cactus subfamilies
Pereskioideae: Pereskia aculeata Opuntioideae: Opuntia chlorotica

Maihuenioideae: Maihuenia poeppigii Cactoideae: Mammillaria elongata

• Subfamily Pereskioideae K. Schumann

The only genus is Pereskia. It has features considered to be closest to the ancestors of the Cactaceae. Plants are trees or shrubs with leaves; their stems are smoothly round in cross section, rather than being ribbed or having tubercles. Two systems may be used in photosynthesis, both the "normal" C₃ mechanism and crassulean acid metabolism (CAM) – an "advanced" feature of cacti and other succulents which conserves water.

• Subfamily Opuntioideae K. Schumann

Some 15 genera are included in this subfamily. They may have leaves when they are young, but these are lost later. Their stems are usually divided into distinct "joints" or "pads" (cladodes). Plants vary in size from the small cushions of Maihueniopsis to treelike species of Opuntia, rising to 10 m (33 ft) or more.

• Subfamily Maihuenioideae P. Fearn

The only genus is Maihuenia, with two species, both of which form low-growing mats. It has some features which are primitive within the cacti. Plants have leaves, and crassulean acid metabolism is wholly absent.

• Subfamily Cactoideae

Divided into nine tribes, this is the largest subfamily, including all the "typical" cacti. Members are highly variable in habit, varying from treelike to epiphytic. Leaves are normally absent, although sometimes very reduced leaves are produced by young plants. Stems are usually not divided into segments, and are ribbed or tuberculate. Two of the tribes, Hylocereeae and Rhipsalideae, contain climbing or epiphytic forms which have a rather different appearance; their stems are flattened and may be divided into segments.

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

Phylogeny and evolution

Phylogeny

Pereskia grandifolia is weakly succulent, possesses leaves, and is believed to be similar to the ancestor of all cacti.

A 2005 study suggested that the genus Pereskia was basal within the Cactaceae, but confirmed earlier suggestions that 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.

Cactaceae	Pereskia Clade A caulocacti Pereskia Clade B core cacti Opuntioideae Maihuenia Cactoideae

A more recent 2011 study using fewer genes but more species also found that Pereskia was divided into these two 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".

The two clades of Pereskia differ in their geographical distribution: with one exception, Clade A is found around the Gulf of Mexico and the Caribbean Sea whereas Clade B occurs south of the Amazon Basin. Species of Pereskia within Clade A always lack two key features of the stem present in most of the remaining "caulocacti": like most non-cacti, their stems begin to form bark early in the plant's life and also lack stomata – structures which control the admission of air into a plant and hence control photosynthesis. By contrast, caulocacti, including species of Pereskia Clade B, typically delay forming bark and have stomata on their stems, thus giving the stem the potential to become a major organ for photosynthesis. (The two highly specialized species of Maihuenia are something of an exception.)

The first cacti seem to have been only slightly succulent shrubs or small trees whose leaves were the organs which carried out photosynthesis. They lived in tropical areas which experienced periodic drought. If Pereskia Clade A 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 the related Portulacaceae). These strategies included being able to respond rapidly to periods of rain, and keeping transpiration low by using water very efficiently during photosynthesis. This latter was achieved by tightly controlling the opening of stomata. Like Pereskia species today, early ancestors may have been able to switch from the normal C₃ mechanism, where carbon dioxide is used continuously in photosynthesis, to "CAM cycling", where the stomata are closed at night (unlike full CAM in which they are open) with carbon dioxide produced by respiration being stored for later use in photosynthesis.

Pereskia Clade B 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 which is at present unanswered is whether the switch to full CAM photosynthesis in stems occurred only once in the core cacti, in which case it has been lost in Maihuenia, or separately in Opuntioideae and Cactoideae, in which case it never evolved in Maihuenia.

Understanding evolution within the core cacti clade is difficult as of February 2012, since phylogenetic relationships are still uncertain and not well related to current classifications. Thus a 2011 study found that "an extraordinarily high proportion of genera" were not monophyletic, and 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 to be monophyletic. Nine tribes are recognized within Cactoideae in the International Cactaceae Systematics Group (ICSG) classification; one, Calymmantheae, comprises a single genus, Calymmanthium. Of the remaining eight, only two (Cacteae and Rhipsalideae) were shown to be monophyletic in a 2011 study by Hernández-Hernández et al.; a summary of the cladograms for the Cactoideae presented in their paper (using the ICSG tribes) is shown below.

Cactoideae	Blossfeldia Tribe Cacteae core Cactoideae species of Calymmanthium, Copiapoa, Frailea core Cactoideae I including members of Tribes Pachycereeae, Hylocereeae and Browningieae core Cactoideae II Tribe Rhipsalideae other core Cactoideae II, including members of Tribes Notocacteae, Trichocereeae, Cereeae and Browningieae

Evolutionary history

There are no known fossils of cacti which throw light on their evolutionary history. However, the geographical distribution of cacti offers some evidence. Except for a relatively recent spread of Rhipsalis baccifera to parts of the Old World, cacti are plants of South America and mainly southern regions of North America. This suggests that the family must have evolved after the ancient continent of Gondwana split into South America and Africa, which occurred during the Early Cretaceous, around 146 to 100 million years ago. Precisely when after this split cacti evolved is less clear. Older sources suggest an early origin around 90–65 million years ago, during the Late Cretaceous. More recent molecular studies suggest a much younger origin, perhaps in the mid-Tertiary, around 35–30 million years ago. Based on the phylogeny of the cacti, it may be that the earliest diverging group (Pereskia Clade A) originated in Central America and northern South America, whereas the caulocacti – those with more or less succulent stems – evolved later in the southern part of South America, and then moved northwards. Core cacti – those with strongly succulent stems – are estimated to have evolved around 25 million years ago. A possible stimulus to their evolution may have been uplifting in the central Andes, some 25–20 million years ago, which was associated with increasing and varying aridity. However, the current species diversity of cacti is thought to have arisen only in the last 10–5 million years (from the late Miocene into the Pliocene). Other succulent plants, such as the Aizoaceae in South Africa, the Didiereaceae in Madagascar and the genus Agave in the Americas, appear to have diversified at the same time, which coincided with a global expansion of arid environments.

Distribution

Distribution of cacti; blue=Rhipsalis baccifera, green=all other cacti

Cacti inhabit diverse regions, from coastal plains to high mountain areas. With one exception, they are native to the Americas, where their range extends from Patagonia to British Columbia and Alberta in western Canada. There are a number of centers of diversity. For cacti adapted to drought, the three main centers are Mexico and the southwestern United States; the southwestern Andes, where they are found in Peru, Bolivia, Chile and Argentina; and eastern Brazil, away from the Amazon Basin. Tree-living epiphytic and climbing cacti necessarily have different centers of diversity as they require moister environments. They are mainly found in the coastal mountains and Atlantic forests of southeastern Brazil; in Bolivia, which is the center of diversity for the subfamily Rhipsalideae; and in forested regions of Central America, where the climbing Hylocereeae are most diverse.

Rhipsalis baccifera is the exception; it is native to both the Americas and the Old World, where it is found in tropical Africa, Madagascar, and Sri Lanka. It was probably spread by being carried as seeds in the digestive tracts of migratory birds; the seeds of Rhipsalis are adapted for bird distribution. Old World populations are polyploid, and regarded as distinct subspecies, suggesting that the spread was not recent.

Many other species have become naturalized outside the Americas after having been introduced by people, especially in Australia, Hawaii, and the Mediterranean region. In Australia, species of Opuntia, particularly Opuntia stricta, were introduced in the 19th century for use as natural agricultural fences and in an attempt to establish a cochineal industry. They rapidly became a major weed problem, but are now controlled by biological agents, particularly the moth Cactoblastis cactorum.

Reproductive ecology

Closeup image of a cactus flower (Echinopsis spachiana) showing large number of stamens.

Some cactus flowers form long tubes (up to 30 cm) so only certain species of moths can reach the nectar, and therefore pollinate the blossoms. There are also specializations for species of bats, hummingbirds and bees. The duration of flowering is highly variable. Some flowers, such as those of Selenicereus grandiflorus (Queen of the Night), are only fully open for two hours at night, while other species may flower for a whole week. Most cacti are self-incompatible, and thus require a pollinator. A few are autogamous and are able to pollinate themselves. Fraileas only open their flowers completely in exceptional circumstances; they mostly pollinate themselves or others with their flowers closed ("cleistogamy"). The flower itself has also undergone a further development: the ovary tends to be highly protected by thorns, hairs and scales. Seed formation is prolific, and the fruits are mostly fleshy, pleasant tasting and conspicuously colored. Goats, birds, ants, mice and bats may contribute to seed dispersal.

Uses

Early history

As of March 2012, 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 in Chile has been dated to around 15,000 years ago, suggesting that cacti would have been encountered before then. Early evidence of the use of cacti includes cave paintings in the Serra da Capivara in Brazil, and seeds found in ancient middens (waste dumps) in Mexico and Peru, with dates estimated at 12,000–9,000 years ago. It is likely that hunter-gatherers collected cactus fruits in the wild and brought them back to their camps.

The central image in this extract from the Codex Osuna of 1565 shows an opuntia used as a symbol for Mexico, i.e. Tenochtitlan.

It is not known when cacti were first cultivated. Opuntias (prickly pears) were used for a variety of purposes by the Aztecs whose empire, lasting from the 14th to the 16th century, had a complex system of horticulture. Their capital from the 15th century was Tenochtitlan (now Mexico City); one explanation for the origin of the name is that it includes the Nahuatl word nōchtli, referring to the fruit of an opuntia. The coat of arms of Mexico shows an eagle perched on a cactus while holding a snake, an image which is at the center of the myth of the founding of Tenochtitlan. The Aztecs symbolically linked the ripe red fruits of an opuntia to human hearts; just as the fruit quenches thirst, so offering human hearts to the sun god ensured the sun would keep moving.

Europeans first encountered cacti when they arrived in the New World late in the fifteenth century. Their first landfalls were in the West Indies where relatively few cactus genera are found; one of the commonest is the genus Melocactus. It is thus possible that melocacti were among the first cacti seen by Europeans. Melocactus species were present in English collections of cacti before the end of the sixteenth century (by 1570 according to one source), where they were called "Echinomelocactus", a name later shortened to Melocactus by Joseph Pitton de Tourneville in the early eighteenth century. Cacti, both purely ornamental species and those with edible fruit, continued to arrive in Europe so that by 1753, Linnaeus was able to name 22 species. One of these, his Cactus opuntia (now part of Opuntia 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.

Food

Peeled fruits of the Indian fig cactus at various stages of ripeness on sale in Mexico

The plant now known as Opuntia 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 name tuna, the latter under the name nopal. Cultivated forms are often significantly less spiny or even spineless. The nopal industry in Mexico was said to be worth US$150 million in 2007. 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 that early botanists assumed that it was native to the area. Outside the Americas, the Indian fig cactus is an important commercial crop in Sicily, Algeria and other north African countries.

Psychoactive agents

The peyote, Lophophora williamsii, is a well-known psychoactive agent used by Native Americans in the southwestern United States. Some species of Echinopsis also have psychoactive properties. For example, the San Pedro cactus, a common specimen found in many garden centers, is known to contain mescaline.

Ornamental plants

Cacti, cultivated by people worldwide, are a familiar sight as potted plants, houseplants or in ornamental gardens in warmer climates. They often form part of xeriphytic (dry) gardens in arid regions, or raised rockeries. Some countries, such as Australia, have water restrictions in many cities, so drought-resistant plants are increasing in popularity. Numerous species have entered widespread cultivation, including members of Echinopsis, Mammillaria and Cereus among others. Less drought resistant epiphytes such as Schlumbergera (the Thanksgiving or Christmas cactus) and Hatiora (the Easter cactus) are also widely cultivated.

Other uses

Cacti can be used for fencing material where there is a lack of either natural resources or financial means to construct a permanent fence. This is often seen in arid and warm climates, such as the Maasai Mara in Kenya. This is known as a cactus fence. Cactus fences are often used by homeowners and landscape architects for home security purposes. The sharp thorns of the cactus deter unauthorized persons from entering private properties, and may prevent break-ins if planted under windows and near drainpipes.

Opuntia are also used as host plants for cochineal bugs in the cochineal dye industry in Central America. Particularly in South America dead pillar cacti can yield valuable wood for construction. Some cacti are also of pharmaceutical significance.

Conservation

Some species have become endangered in the wild because of overharvesting for sale as an ornamental plant. All cacti are covered by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, and many species, by virtue of their inclusion in Appendix 1, are fully protected.

Cultivation

Cultivated Notocactus warasii on display at the San Diego County Fair, California, USA

With few exceptions, the vast majority of cacti in habitat almost always are found growing in mineral based soils. Epiphytic cacti are the exception and prefer soils rich in organic materials, however, cacti in this group which prefer these types of soils produce healthier plants when allowed to dry completely between waterings.^{[citation needed]} Some species of cacti such as Toumaya papyracantha (formerly Pediocactus papyracantha – found in the high mesas of New Mexico) are mycorrhizal symbionts with the roots of various species of grasses and grow underground on the roots of these wild grasses, only emerging above ground and initiating photosynthesis for sexual reproduction and flowering, forming deciduous cactus bodies that die back in winter or during droughts.

The vast majority of commercial "cactus soil mixes" which contain organic materials are unsuitable for growing most terrestrial cacti^{[citation needed]}, and many contain pumice, which exudes traces of heavy metals which will rot the roots of cacti if grown in these medium over a period of years^{[citation needed]}. Surprisingly, most cacti prefer a mineral based soil cut with about half sand and allowed to dry out completely between waterings.

Although cacti are adapted to hot deserts and other xerophytic environments, most cacti evolved in mountainous areas^{[citation needed]} and require moderate cold and/or cool night temperatures for some period of the year to initiate regular flowering. Overwatering of cacti is the single biggest cause of plant loss.^{[citation needed]} Cacti are subject to Fusarium infections in their vascular cambium (the bundle of fibers and the "ring" inside the center of a cactus visible when cut in cross section) when overwatered or maintained in soils with high organic content^{[citation needed]}. Fusarium mycelia typically grow into the cambium channels and plug up the plant's transport system, causing tissue death and the classic rotting and collapsing observed when a cactus plant is overwatered. Cacti are easy to grow if allowed to dry completely between waterings.^{[citation needed]} Many species have specific periods of dormancy and should not be watered during these periods.

See also

• List of foliage plant diseases (Cactaceae)

Bibliography

• Anderson, Edward F. (2001), The Cactus Family, Pentland, Oregon: Timber Press, ISBN 978-0-88192-498-5 

External links

Cactus at the Open Directory Project