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Cactus

CACTUS

CACTUS. Cacti are succulent perennials that are native to arid and semi-arid regions and are cultivated extensively, except where freezes regularly occur. The land area devoted to cactus cultivation in 2001 was about 1.8 million hectares (4.4 million acres), mostly for fodder, and over half of which was in northern Africa and northeastern Brazil. Cacti are also cultivated in over twenty countries for their fruits, which commercially fall into three categories: cactus pears, which are the fruits of the prickly pear Opuntia ficus-indica and certain other cacti with flat stems (cladodes), and represent over 90% of the cactus fruits sold; pitahayas, which are the fruits of vine cacti in the genera Hylocereus and Selenicereus ; and pitayas, which are the fruits of columnar cacti. Young cladodes are consumed as a vegetable (nopalitos), particularly in Mexico. Nearly all cacti employ a photosynthetic pathway known as Crassulacean acid metabolism (CAM), in which the stomates (shoot pores that allow CO2 entry) open primarily at night, when temperatures are lower and water loss is lower than for the overwhelming majority of plants, whose stomates open during the daytime. The best known edible CAM plant is pineapple, which is cultivated on about half as much area as cacti. Because of their lower water loss, cacti and other CAM plants thrive in dry regions (and also require little or no irrigation when cultivated in other regions.

History

Although evidence for cacti in human diets goes back more than 8,000 years in present-day Mexico, worldwide consumption has developed only in the last few hundred years. Cacti were introduced into Europe in 1495 from the second trip of Christopher Columbus to the New World. Opuntia ficus-indica spread across the Mediterranean region in the sixteenth century, where it readily grew under the local semi-arid conditions. Also in the sixteenth century, Spaniards introduced Hylocereus undatus into the Philippines, whence it spread throughout southeast Asia. In the nineteenth century, it became established in Viet Nam and is now extensively cultivated in the Mekong Delta, where its tasty fruit with red peel and white pulp is called "dragon fruit." Also in the nineteenth century, the columnar Stenocereus queretaroensis was domesticated in Jalisco, Mexico. None of these species received much agronomic attention until the end of the twentieth century, and even then the money for research and development was meager. Both fruit crops and young cladodes used as vegetables require much hand labor. Although machines have been developed to remove the irritating small spines (termed "glochids") from cactus pears, many improvements in their cultivation await future research.

Fruits

Fruits of many cacti are edible. Indeed, the Seri Indians of the southwestern United States and northwestern Mexico consumed fruits from over twenty species, including those of the saguaro cactus (Carnegiea gigantea ), used by various Native Americans for fruits and wine. Fruits collected from the wild influenced the species selected for domestication. Such selections involved various species of Opuntia in Mexico, eventually leading to the presently planted cultivars.

Cactus pear. The fruits of Opuntia ficus-indica and a few other prickly pears are harvested in the summer from plants that are one to three meters tall. Harvest can be delayed by removing the early flowers, as is commonly done in Sicily, leading to a second harvest in the autumn that is more valuable per fruit due to lessened competition from other species. One-year-old cladodes can bear five to fifteen fruits each; terminal cladodes with fewer fruits tend to bear larger ones (over 150 g each), which command higher prices. After harvesting, the fruits must have the glochids removed mechanically, after which they are often packaged by color and weight. Fruits with red pulp are prized in the United States and certain European countries, whereas greenish pulp for mature fruits is generally preferred in Mexico. Although sold in supermarkets worldwide, fruits are also sold by street vendors, who slice the peel and provide the exposed pulp directly to the consumer. The relatively large seeds are a detriment to fruit consumption by many, but the seeds are harmless and readily swallowed by aficionados.

The country with the greatest land area devoted to cactus pear cultivation is Mexico (Table 1). Annual production can be over fifteen tons fresh weight per hectare under intensive management. In Mexico, Sicily, Israel, and the United States, most production is from commercial plantations, whereas in other Latin American countries and in northern Africa, a large amount of the fruit is collected from hedges and other informal plantings.

Pitahayas and pitayas. The most widely cultivated pitahaya is Hylocereus undatus, which in 2001 was cultivated on about 12,000 hectares in many countries, including Viet Nam, Taiwan, the Philippines, Mexico, Guatemala,

Land areas and harvests for fruit production by Opuntia ficus-indica and closely related species in 2001
Country Area (hectares) Annual harvest (tons fresh weight)
Argentina 900 8,000
Bolivia 1,300 3,500
Chile 1,200 9,000
Israel 400 7,000
Italy 7,500 80,000
Mexico 70,000 400,000
Northern Africa (Algeria, Morocco, Tunisia) ~20,000
South Africa 200 1,500
United States 200 3,600

Peru, Colombia, and Israel. It is a vine that is trained to grow on posts, trellises, or arbors. Its relatively large fruits (generally 250 to 500 g) are harvested after the peel, which has no spines or glochids, turns red. The pulp is

whitish with small black seeds. Other species of Hylocereus and Selenicereus megalanthus have peels and pulps of various colors, leading to a wide choice of tasty and visually appealing fruits.

Although their cultivation is expanding rapidly, in 2001 pitayas were harvested on only about 3,000 hectares worldwide, mostly in Mexico, from species like Cereus peruvianus, and especially Stenocereus queretaroensis and other Stenocereus species. Fruits grow along the main stem and branches about two to six meters above the ground, requiring a pole with a basket-like attachment for harvest of individual fruits. Fruits of Stenocereus queretaroensis have an attractive and tasty dark red or purple pulp with small seeds (like those in kiwis) that are easily swallowed. However, the fruits tend to split within two or three days after harvest, requiring rapid local consumption.

Vegetables

Tender young cladodes about 10 to 15 cm long of Opuntia ficus-indica, Opuntia robusta, and a few related species are used in Mexico as nopalitos. About 6,000 hectares were cultivated for this purpose in 2001, and nopalitos are also prepared from plants in the wild or growing around houses, or as hedges. The raised portions of the stem containing spines and glochids are readily removed with a knife or by machine. The cladodes are then generally sliced or diced and blanched in a weak saline solution for a few minutes to remove excess mucilage. After draining, the material can be cooked, yielding a vegetable with a taste not unlike string beans or okra. Because of their high fructose and mucilage content, nopalitos are highly recommended for people with type II diabetes. Often the blanched material is pickled and used as a relish or in salads. More than thirty companies sold pickled nopalitos in Mexico in 2001, and this product is in supermarkets worldwide.

Other Uses

Other uses of cacti range from candy made from the stems of barrel cacti that have been infused with a sugar solution to peyote from dried stems of Lophophora williamsii, used by Native Americans for ceremonial purposes. Flowers have been used for medicinal purposes and to make perfume. The seeds of cacti such as Opuntia ficus-indica have been dried, ground, and then used as a flavoring paste for cooking. Carminic acid, an important red dye for food coloring, can be extracted from dried cochineal insects that feed on Opuntia ficus-indica. Although most cactus pears are consumed fresh, sorbets and marmalades are also prepared from the fruits. The strained pulp of fresh fruits is used as a fruit drink or fermented to make wine. Fruits of cactus pears are also partially dried and sold in brick-sized blocks in Mexico. More than thirty brands of dried and powdered cladodes are sold in Mexico as a dietary supplement. The range of edible products from cacti is indeed great and their use is steadily increasing, as more people become willing to try new and natural foods, and growers search for crops that do not need irrigation.

BIBLIOGRAPHY

Mizrahi, Yosef, Avinoam Nerd, and Park S. Nobel. "Cacti as Crops." In Horticultural Reviews 18 (1997): 291319.

Nobel, Park S. Los Incomparables Agaves y Cactos. Translated by Edmundo Garcia Moya. Mexico City: Editorial Trillas, 1998.

Nobel, Park S. Remarkable Agaves and Cacti. New York: Oxford University Press, 1994.

Nobel, Park S., editor. Cacti : Biology and Uses. Berkeley, California: University of California Press, 2002.

Valles Septién, Carmen, editor. Succulentas Mexicanas/Cactáceas. Mexico City: CVS Publicaciones, 1997.

Park S. Nobel

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cactus

cactus, any plant of the family Cactaceae, a large group of succulents found almost entirely in the New World. A cactus plant is conspicuous for its fleshy green stem, which performs the functions of leaves (commonly insignificant or absent), and for the spines (not always present) of various colors, shapes, and arrangements. Cactus flowers are notably delicate in appearance although usually large and showy; they are commonly yellow, white, or shades of red and purple. Many species are pollinated by bats. Cactus fruits are berries and are usually edible. A cactus plant appears on the coat of arms of Mexico, and the blossom of the giant cactus, or saguaro (Cereus giganteus), is the state flower of Arizona.

The plants vary from small, round globes to epiphytes, vines, and large treelike forms. The reduced leaf surface, the enlarged fleshy stem, which is well fitted to store water and to retain it, and the ramified and extensive root system (much reduced in cultivated cacti) make the plant particularly adapted to regions of high temperature and long dry periods. Cacti are not restricted to desert regions, however, for in America they range from the tropics into Canada.

Most cacti bloom in the spring for a very short period, sometimes for only a few hours. The blossoms are noticeably sensitive to light, and often different species blossom only at specific times of the day. One of the most famous of the cacti is the night-blooming cereus usually classified as Selenicereus or C. grandiflora (several other night-blooming cactus species bear the same common name). Its fragrant blossoms unfold at a visible rate after sunset and last only a single night. In many of its native habitats the flowering of this cactus is celebrated with festivals.

Economic Importance

The largest cactus genus is Opuntia, jointed-stemmed species recognizable by the fleshy stems made up of either cylindrical (in the cane cacti and the chollas) or flattened (in the prickly pears) joints called pads. The large pear-shaped berries of several of these species are edible, e.g., the cultivated varieties of the Indian fig and the tuna. This fruit is common in Mexican markets; the plants have been widely naturalized in the Mediterranean countries, Australia, and elsewhere as a source of food. Most opuntias grow so rapidly to a large and ungainly size that they are unsuitable for cultivation as ornamentals, and in the wild often become weeds.

However, the major economic importance of the cactus family is in the florists' trade. Among those cultivated for their showy blossoms are the Christmas cactus (Zygocactus) and species of Echinocereus and of Epiphyllum, the orchid cactus. The pincushion cacti (Mammillaria), the golden ball cactus (Echinocactus), and the hedgehog cactus (Echinopsis) are among the many grown as oddities for their curious appearance.

The nopal (Nopalea coccinellifera) is the cactus traditionally cultivated as a host for the cochineal insect, and the hallucinatory drug mescaline occurs in the genera Lophophora (peyote) and Trichocereus. Other cacti are used as a substitute for wood, as stock feed, and for hedges.

Classification

Cactus is classified in the division Magnoliophyta, class Magnoliopsida, order Caryophyllales, family Cactaceae.

Bibliography

See L. Benson, The Cacti of the United States and Canada (1982) and A. C. Gibson and P. S. Nobel, The Cactus Primer (1986).

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cactus

cactus Any of 2000 or so species of succulents, found particularly in hot desert regions of the Western Hemisphere. A cactus' long roots enable it to absorb moisture from desert terrains and the fleshy green stem is adapted for water storage with a waxy coating to restrict evaporation. Stems are usually spiny, cylindrical and branched. Cactus flowers are usually borne singly in a wide range of colours. Height: from less than 2.5cm (1in) to more than 15m (50ft). See also xerophyte

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cactus

cac·tus / ˈkaktəs/ • n. (pl. cacti / ˈkakˌtī; -ˌtē/ or cactuses ) a succulent New World plant (family Cactaceae), chiefly of arid regions, with a thick, fleshy stem that typically bears spines and has brilliantly colored flowers.

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cactus

cactus †cardoon XVII; prickly plant with thick fleshy stems XVIII. — L. — Gr. káktos cardoon or Spanish artichoke (of Sicily); the name was adopted by Linnaeus for a genus of entirely different prickly plants.

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cactus

cactus •cactus • saltus • Diophantus • Sanctus •Rastus, Theophrastusaltostratus, cirrostratus, nimbostratus, stratus •conspectus, prospectus •momentous, portentous •asbestos, Festus •apparatus, Donatus, hiatus, status •acetous, boletus, Cetus, Epictetus, fetus, Miletus, quietus •Hephaestus •Benedictus, ictus, rictus •Quintus • linctus • eucalyptus • cistus •coitus •circuitous, fortuitous, gratuitous •Hippolytus • calamitous • tinnitus •Iapetus • crepitus •precipitous, serendipitous •impetus • emeritus • spiritous •Democritus, Theocritus •Tacitus • necessitous •duplicitous, felicitous, solicitous •covetous •iniquitous, ubiquitous •detritus, Heraclitus, Polyclitus, Titus, Vitus •Pocahontas, PontusPlautus, tortoise •cobaltous •Duns Scotus, lotus •hostess •arbutus, Brutus •Eustace • conductus • cultus •coitus interruptus • Augustus •riotous • Herodotus • Oireachtas

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Cactus

Cactus ★★½ 1986

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Cactus

Cactus

Biology of cacti

Species of cacti in North America

Economic importance of cacti

Resources

The cactus family or Cactaceae is made up of about 2,000 species of perennial plants with succulent stems, most of which are well-armed with sharp spines. The natural distribution of most cacti is American, ranging from southern British Columbia and southern Ontario in Canada, through much of the United States, to the tip of southern South America. One genus, Rhipsalis, occurs in Africa, Madagascar, and India, and is probably native there. Cacti usually inhabit deserts and other dry, open places. The major use of cacti by humans is as attractive, ornamental plants in gardens, or as indoor house plants. A few species produce edible fruits, and one yields peyote, a hallucinogenic drug.

Biology of cacti

Cacti are perennial plants. Their stems are fleshy or succulent, and are cylindrical or flattened in shape. The stems are green-colored, and are photosynthetic, usually performing this function instead of leaves, which are greatly reduced in abundance or even absent in most mature cacti. Most species of cactus are well-protected by sharp bristles and spines, which serve to deter most herbivores.

The stems of cactus plants have numerous cushion- or pit-like structures known as areoles on their surface, from which usually emerge clusters of spines. In terms of developmental biology, areoles are usually interpreted as being incompletely developed, axillary stem branches. The spines are actually modified leaves. The areoles may also be protected by hook-like barbs known as glochidia. The roots of cacti are shallow and may be widely spread in the soil.

The flowers of cacti are usually perfect (bisexual), containing both male reproductive organs (stamens) and female parts (a pistil). The flowers occur singly, rather than in groups, although many discrete flowers may be present on a cactus at the same time. The flowers of most species of cacti are large and showy, and they can be colored white, red, pink, orange, or yellow, but not blue. The sepals of the calyx are petal-like in shape and color, and they combine with the numerous petals to form an attractive, often richly scented, nectar-producing flower, designed to lure such pollinators as hawk-moths, bees, bats, and birds, especially hummingbirds and small doves. The fruit is a many-seeded berry.

Cacti are xerophytic plants, meaning they are physiologically and morphologically adapted to coping with the extreme water deficiencies of dry habitats, such as deserts. The xerophytic adaptations of cacti include: (1) their succulent, water-retaining stems, (2) a thick, waxy cuticle and few or no leaves to greatly reduce the losses of water through transpiration, (3) stems that are photosynthetic, so leaves are not required to execute this function, (4) stems that are cylindrical or spherical in shape, which reduces the surface to volume ratio, and helps to preserve moisture, (5) tolerance of high tissue temperatures, (6) protection of the biomass and moisture reserves from herbivores by an armament of stout spines, (7) a physiological tolerance of long periods of drought, and (8) a periodic pattern of growth, productivity, and flowering, which takes advantage of the availability of moisture during the brief, rainy season, while the plant remains dormant at drier times of the year.

Cacti have a so-called crassulacean-acid metabolism, in which atmospheric carbon dioxide is only taken up during the night, when the stomates are open. The carbon dioxide is fixed into four-carbon, organic acids,

and can later be released within the plant, to be fixed into sugars by photosynthesis when the sun is shining during the daylight hours. Because this system allows stomates to be kept tightly closed during the day, crassulacean-acid metabolism is an efficient way of conserving water in dry environments.

Some plant species of dry habitats that are not related to cacti are nevertheless remarkably similar in appearance (at least, apart from their flowers and fruits, which are always distinctive among plant families). This is the result of convergent evolution, the similar evolutionary development of unrelated species or families that are subjected to comparable types of environmental selective pressures. Some species of spurges (family Euphorbiaceae) that grow in dry habitats are commonly thought by non-botanists to be cacti, even though they are quite unrelated.

Species of cacti in North America

Species of cacti are prominent in many arid and semi-arid habitats in the Americas. Cacti provide important elements of the habitat for many species of animals, especially larger species such as saguaro and candelabra cacti.

One of the most familiar groups of cacti are the prickly-pears, beaver-tails, or chollas (Opuntia spp.), of which there are about 300 species. These species have flattened, succulent, segmented stems (sometimes known as stem-joints), and are usually well-armed with spines of various sizes. Opuntia lindheimeri is a red- or yellow-flowered species that grows in Louisiana, Texas, and northeastern Mexico. This plant can reach a height of almost 13 ft (4 m) and can sometimes form dense thickets. Opuntia macrorhiza is a yellow-flowered species that grows in dry prairies from Kansas and Missouri to Texas. Opuntia imbricata has cylindrical instead of flattened stems, grows as tall as 6.6 ft (2 m), has red- or purple-colored flowers, and is commonly known as the tree or candelabra cactus. Opuntia compressa or the beaver-tail is a low-growing, yellow-flowered, eastern species that ranges from Massachusetts to Georgia. Opuntia fulgida or cholla occurs in the Sonora and other deserts of the southwestern United States and Mexico.

The pin-cushion cacti (Mammillaria spp.) are about 300 species of relatively small cacti that have spherical stems, with numerous, small, spiny, nipple-like protuberances on their surface. Mammillaria microcarpa and M. thornberi are species native to the southwestern states and Mexico.

The hedge or candelabra cacti (Cereus spp.) are made up of about 40 species. The barbed-wire cactus (Cereus pentagonus ) is an arching, sometimes climbing species that grows in southern Florida, while the organ-pipe cactus (C. thurberi ) is an erect, multi-stemmed species of deserts of Arizona and Mexico, which can achieve a height greater than 39 ft (12 m). The desert night-blooming cereus (C. greggii ) occurs in deserts of the southwestern United States and Mexico. The odorous, nectar-rich, white flowers of this species open synchronously on only a few nights each year, and are pollinated by bats and hawk moths.

The saguaro, giant, or tall cactus (Carnegiea giganteus, sometimes known as Cereus giganteus ) is a spectacular, multi-columnar species that dominates the landscape of deserts of Arizona and down into Mexico. This candelabra-like species can grow as tall as 49 ft (15 m) and has showy flowers that are pollinated by bats, birds, moths, and bees. The saguaro is an important component of the habitat of many species of animals. The gila woodpecker (Centurus uropygalis ) and gilded flicker (Colaptes chrysoides ) excavate nesting cavities in the saguaro cactus, and when these are abandoned they may be used secondarily by elf owls (Micrathene whitneyi ) and other species of birds. The cactus wren (Campylorhynchus brunneicapillus ) is another prominent species in saguaro-dominated deserts. In addition, many species of animals feed on the nectar of the saguaro, and on the bright-red, juicy pulp of its ripened fruits.

The barrel cacti (Echinocactus spp.) are seven species with stout, rotund, barrel-like stems. The barrel cactus (Echinocactus polycephalus ) is a relatively large species of the southwestern states and Mexico, while the horse crippler (E. texensis ) and star cactus (E. asterias ) are smaller species. The hedgehog cacti (Echinocereus spp.) are 70 species with relatively small, densely aggregated, spiny stems. The red-flowered hedgehog cactus (E. triglochidiatus ) occurs widely in arid habitats of the southwestern United States and Mexico. The organ-pipe cacti (Lemaireocereus spp.) are 25 species of tall, multi-stemmed, columnar cacti, including the candebobe (L. weberi ) of Mexico. The barrelcacti (Ferocactus spp.) are 35 species of stout, short-columnar species, including F. acanthodes, F. wislizenii, and F. covillei of the southwestern states and Mexico.

Economic importance of cacti

Many species of cacti are highly prized by horticulturalists as botanical oddities and ornamental plants. These may be cultivated for their beautiful flowers, the aesthetics of their stems and spines, or merely because the plants have a strange-looking appearance. In addition, many people like to grow cacti because they are relatively easy to maintainit does not matter much if you forget to water your cacti for a few days, or even a few weeks or more. In fact, over-watering is usually the greatest risk to most cacti that are kept as house plants, because too much moisture will pre-dispose these drought-adapted plants to developing fungal and bacterial diseases, such as soft-rot.

Virtually any of the native species of cacti of North America may be used in horticulture, as are many of the species of Central and South America. The genera Mammillaria and Opuntia are most commonly grown, but virtually any species may be found in cultivation around or in homes and greenhouses. One of the most common and familiar species is the Christmas cactus (Zygocactus elegans ), a flat-stemmed, red-, pink-, or white-flowered species that is grown as a garden and house plant. This species blooms during the winter, and florists often induce this plant to bloom around Christmas-time, when it is commonly sold as a living ornament to brighten homes during that festive season. The candelabra cactus (Cereus peruvianus ) is a tree-sized species native to South America that is commonly cultivated outdoors in hot climates, or in greenhouses in colder climates.

Many species of cacti can be rather easily transplanted from natural habitats into the vicinities of homes and businesses, where they may be used as central components of low-maintenance gardens in places where rainfall is sparse, and the development of grassy lawns would require an excessive use of scarce and expensive water. Wild cacti are also collected to grow in or around the home, and to develop private collections of these interesting plants.

Unfortunately, most species of cacti re-colonize disturbed sites very slowly and infrequently. Extensive losses of cactus habitat to industrial and residential developments, coupled with excessive collections of wild plants, have resulted in the populations of some species of cacti becoming endangered. In some areas, populations of wild cacti must be guarded against illegal, often nocturnal collecting of valuable plants for horticultural purposes. Unfortunately, it is difficult to protect many endangered cacti from poaching. This is because of the extensive areas that must be patrolled, in the face of multi-million-dollar profits that can potentially be made in the illicit cactus trade. Some species of cactus are now critically endangered in the wild because of excessive, illegal collecting, and this represents an important ecological problem in many areas.

The most commonly edible cactus fruit is that of Opuntia species, especially O. ficus-indica. The fruits of prickly-pears, sometimes known as apples or tunas, can be eaten directly or used to make a jelly. Prickly-pear fruits are considered to be a delicacy around Christmas time in some regions.

Peyote or mescal buttons (Lophophora williamsii ) is a cactus containing several alkaloids in its tissues that are used as a hallucinogen and folk medicine. Peyote is important in the culture of some tribes of native Amerindians in the southwestern United States and Mexico, especially in the vicinity of the Rio Grande River. These aboriginal peoples use peyote to induce religious experiences and revelations. Peyote is also commonly used as a recreational drug by many people, and by several religious cults.

Some species of spiny cacti, such as Opuntia, are used as living fences, for example, to keep livestock out

KEY TERMS

Berry A soft, multi-seeded fruit, developed from a single, compound ovary.

Cuticle A waxy, superficial layer that covers the foliage of vascular plants, and the stems of cacti.

Monoecious This is a plant breeding system in which male and female reproductive structures are present on the same plant, and in the case of cacti, in the same flowers.

Perfect In the botanical sense, this refers to flowers that are bisexual, containing both male and female reproductive parts.

Stomate These are microscopic pores in the leaf or stem cuticle, bordered by guard cells which control opening or closing of the pore.

Succulent Having thick, fleshy leaves or stems that conserve moisture.

Xerophyte A plant adapted to dry or drought prone habitats.

of gardens. The long, sharp spines of other cacti were used as needles in some of the earliest types of phonographs. The wood of the saguaro cactus has long been used by Amerindian peoples, and is still utilized to make crafts and novelty furniture.

A few species of cacti have become pests, or weeds, when they escaped from cultivation in places where they were not native, and were not controlled by diseases or herbivores. The best known example is that of a prickly pear cactus (Opuntia spp.) that was imported to Australia from North America for use as an ornamental plant and living fence, but became invasive and a serious weed of rangelands. This pest has now been almost completely controlled through the introduction of one of its natural herbivores, the moth Cactoblastis cactorum, whose larvae feed on the cactus.

See also Hallucinogens; Spurge family.

Resources

BOOKS

Benson, L. The Cacti of the United States and Canada. Stanford, CA: Stanford University Press, 1982.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Bill Freedman

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Cactus

Cactus

The cactus family or Cactaceae is made up of about 2,000 species of perennial plants with succulent stems, most of which are well-armed with sharp spines. The natural distribution of most cacti is American, ranging from southern British Columbia and southern Ontario in Canada, through much of the United States, to the tip of southern South America . One genus, Rhipsalis, occurs in Africa , Madagascar, and India, and is probably native there. Cacti usually inhabit deserts and other dry, open places. The major use of cacti by humans is as attractive, ornamental plants in gardens, or as indoor house plants. A few species produce edible fruits , and one yields peyote, a hallucinogenic drug.


Biology of cacti

Cacti are perennial plants. Their stems are fleshy or succulent, and are cylindrical or flattened in shape. The stems are green-colored, and are photosynthetic, usually performing this function instead of leaves, which are greatly reduced in abundance or even absent in most mature cacti. Most species of cactus are well-protected by sharp bristles and spines, which serve to deter most herbivores.

The stems of cactus plants have numerous cushion- or pit-like structures known as areoles on their surface, from which usually emerge clusters of spines. In terms of developmental biology , areoles are usually interpreted as being incompletely developed, axillary stem branches. The spines are actually modified leaves. The areoles may also be protected by hook-like barbs known as glochidia. The roots of cacti are shallow and may be widely spread in the soil .

The flowers of cacti are usually perfect (bisexual), containing both male reproductive organs (stamens) and female parts (a pistil). The flowers occur singly, rather than in groups, although many discrete flowers may be present on a cactus at the same time. The flowers of most species of cacti are large and showy, and they can be colored white, red, pink, orange, or yellow, but not blue. The sepals of the calyx are petal-like in shape and color , and they combine with the numerous petals to form an attractive, often richly scented, nectar-producing flower , designed to lure such pollinators as hawk-moths, bees , bats , and birds , especially hummingbirds and small doves. The fruit is a many-seeded berry.

Cacti are xerophytic plants, meaning they are physiologically and morphologically adapted to coping with the extreme water deficiencies of dry habitats, such as deserts. The xerophytic adaptations of cacti include: (1) their succulent, water-retaining stems, (2) a thick, waxy cuticle and few or no leaves to greatly reduce the losses of water through transpiration , (3) stems that are photosynthetic, so leaves are not required to execute this function, (4) stems that are cylindrical or spherical in shape, which reduces the surface to volume ratio, and helps to preserve moisture, (5) tolerance of high tissue temperatures, (6) protection of the biomass and moisture reserves from herbivores by an armament of stout spines, (7) a physiological tolerance of long periods of drought , and (8) a periodic pattern of growth, productivity, and flowering, which takes advantage of the availability of moisture during the brief, rainy season, while the plant remains dormant at drier times of the year.

Cacti have a so-called crassulacean-acid metabolism , in which atmospheric carbon dioxide is only taken up during the night, when the stomates are open. The carbon dioxide is fixed into four-carbon, organic acids, and can later be released within the plant, to be fixed into sugars by photosynthesis when the sun is shining during the daylight hours. Because this system allows stomates to be kept tightly closed during the day, crassulacean-acid metabolism is an efficient way of conserving water in dry environments.

Some plant species of dry habitats that are not related to cacti are nevertheless remarkably similar in appearance (at least, apart from their flowers and fruits, which are always distinctive among plant families). This is the result of convergent evolution, the similar evolutionary development of unrelated species or families that are subjected to comparable types of environmental selective pressures. Some species of spurges (family Euphorbiaceae) that grow in dry habitats are commonly thought by non-botanists to be cacti, even though they are quite unrelated.


Species of cacti in North America

Species of cacti are prominent in many arid and semiarid habitats in the Americas. Cacti provide important elements of the habitat for many species of animals, especially larger species such as saguaro and candelabra cacti.

One of the most familiar groups of cacti are the prickly-pears, beaver-tails, or chollas (Opuntia spp.), of which there are about 300 species. These species have flattened, succulent, segmented stems (sometimes known as stem-joints), and are usually well-armed with spines of various sizes. Opuntia lindheimeri is a red- or yellow-flowered species that grows in Louisiana, Texas, and northeastern Mexico. This plant can reach a height of almost 13 ft (4 m) and can sometimes form dense thickets. Opuntia macrorhiza is a yellow-flowered species that grows in dry prairies from Kansas and Missouri to Texas. Opuntia imbricata has cylindrical instead of flattened stems, grows as tall as 6.6 ft (2 m), has red- or purple-colored flowers, and is commonly known as the tree or candelabra cactus. Opuntia compressa or the beaver-tail is a low-growing, yellow-flowered, eastern species that ranges from Massachusetts to Georgia. Opuntia fulgida or cholla occurs in the Sonora and other deserts of the southwestern United States and Mexico.

The pin-cushion cacti (Mammillaria spp.) are about 300 species of relatively small cacti that have spherical stems, with numerous, small, spiny, nipple-like protuberances on their surface. Mammillaria microcarpa and M. thornberi are species native to the southwestern states and Mexico.

The hedge or candelabra cacti (Cereus spp.) are made up of about 40 species. The barbed-wire cactus (Cereus pentagonus) is an arching, sometimes climbing species that grows in southern Florida, while the organ-pipe cactus (C. thurberi) is an erect, multi-stemmed species of deserts of Arizona and Mexico, which can achieve a height greater than 39 ft (12 m). The desert night-blooming cereus (C. greggii) occurs in deserts of the southwestern United States and Mexico. The odorous, nectar-rich, white flowers of this species open synchronously on only a few nights each year, and are pollinated by bats and hawk moths .

The saguaro, giant, or tall cactus (Carnegiea giganteus, sometimes known as Cereus giganteus) is a spectacular, multi-columnar species that dominates the landscape of deserts of Arizona and down into Mexico. This candelabra-like species can grow as tall as 49 ft (15 m) and has showy flowers that are pollinated by bats, birds, moths, and bees. The saguaro is an important component of the habitat of many species of animals. The gila woodpecker (Centurus uropygalis) and gilded flicker (Colaptes chrysoides) excavate nesting cavities in the saguaro cactus, and when these are abandoned they may be used secondarily by elf owls (Micrathene whitneyi) and other species of birds. The cactus wren (Campylorhynchus brunneicapillus) is another prominent species in saguaro-dominated deserts. In addition, many species of animals feed on the nectar of the saguaro, and on the bright-red, juicy pulp of its ripened fruits.

The barrel cacti (Echinocactus spp.) are seven species with stout, rotund, barrel-like stems. The barrel cactus (Echinocactus polycephalus) is a relatively large species of the southwestern states and Mexico, while the horse crippler (E. texensis) and star cactus (E. asterias) are smaller species. The hedgehog cacti (Echinocereus spp.) are 70 species with relatively small, densely aggregated, spiny stems. The red-flowered hedgehog cactus (E. triglochidiatus) occurs widely in arid habitats of the southwestern United States and Mexico. The organ-pipe cacti (Lemaireocereus spp.) are 25 species of tall, multi-stemmed, columnar cacti, including the candebobe (L. weberi) of Mexico. The barrel cacti (Ferocactus spp.) are 35 species of stout, short-columnar species, including F. acanthodes, F. wislizenii, and F. covillei of the southwestern states and Mexico.


Economic importance of cacti

Many species of cacti are highly prized by horticulturalists as botanical oddities and ornamental plants. These may be cultivated for their beautiful flowers, the aesthetics of their stems and spines, or merely because the plants have a strange-looking appearance. In addition, many people like to grow cacti because they are relatively easy to maintain—it does not matter much if you forget to water your cacti for a few days, or even a few weeks or more. In fact, over-watering is usually the greatest risk to most cacti that are kept as house plants, because too much moisture will pre-dispose these drought-adapted plants to developing fungal and bacterial diseases, such as soft-rot.

Virtually any of the native species of cacti of North America may be used in horticulture , as are many of the species of Central and South America. The genera Mammillaria and Opuntia are most commonly grown, but virtually any species may be found in cultivation around or in homes and greenhouses. One of the most common and familiar species is the Christmas cactus (Zygocactus elegans), a flat-stemmed, red-, pink-, or white-flowered species that is grown as a garden and house plant. This species blooms during the winter, and florists often induce this plant to bloom around Christmas-time, when it is commonly sold as a living ornament to brighten homes during that festive season. The candelabra cactus (Cereus peruvianus) is a tree-sized species native to South America that is commonly cultivated outdoors in hot climates, or in greenhouses in colder climates.

Many species of cacti can be rather easily transplanted from natural habitats into the vicinities of homes and businesses, where they may be used as central components of low-maintenance gardens in places where rainfall is sparse, and the development of grassy lawns would require an excessive use of scarce and expensive water. Wild cacti are also collected to grow in or around the home, and to develop private collections of these interesting plants.

Unfortunately, most species of cacti re-colonize disturbed sites very slowly and infrequently. Extensive losses of cactus habitat to industrial and residential developments, coupled with excessive collections of wild plants, have resulted in the populations of some species of cacti becoming endangered. In some areas, populations of wild cacti must be guarded against illegal, often nocturnal collecting of valuable plants for horticultural purposes. Unfortunately, it is difficult to protect many endangered cacti from poaching. This is because of the extensive areas that must be patrolled, in the face of multi-million-dollar profits that can potentially be made in the illicit cactus trade. Some species of cactus are now critically endangered in the wild because of excessive, illegal collecting, and this represents an important ecological problem in many areas.

The most commonly edible cactus fruit is that of Opuntia species, especially O. ficus-indica. The fruits of prickly-pears, sometimes known as apples or tunas, can be eaten directly or used to make a jelly. Prickly-pear fruits are considered to be a delicacy around Christmas time in some regions.

Peyote or mescal buttons (Lophophora williamsii) is a cactus containing several alkaloids in its tissues that are used as a hallucinogen and folk medicine. Peyote is important in the culture of some tribes of native Amerindians in the southwestern United States and Mexico, especially in the vicinity of the Rio Grande River. These aboriginal peoples use peyote to induce religious experiences and revelations. Peyote is also commonly used as a recreational drug by many people, and by several religious cults.

Some species of spiny cacti, such as Opuntias, are used as living fences, for example, to keep livestock out of gardens. The long, sharp spines of other cacti were used as needles in some of the earliest types of phonographs. The "wood" of the saguaro cactus has long been used by Amerindian peoples, and is still utilized to make crafts and novelty furniture.

A few species of cacti have become pests , or weeds, when they escaped from cultivation in places where they were not native, and were not controlled by diseases or herbivores. The best known example is that of a prickly pear cactus (Opuntia spp.) that was imported to Australia from North America for use as an ornamental plant and living fence, but became invasive and a serious weed of rangelands. This pest has now been almost completely controlled through the introduction of one of its natural herbivores, the moth Cactoblastis cactorum, whose larvae feed on the cactus.

See also Hallucinogens; Spurge family.


Resources

books

Benson, L. The Cacti of the United States and Canada. Stanford, CA: Stanford University Press, 1982.

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Klein, R.M. The Green World. An Introduction to Plants andPeople. New York: Harper and Row, 1987.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Berry

—A soft, multi-seeded fruit, developed from a single, compound ovary.

Cuticle

—A waxy, superficial layer that covers the foliage of vascular plants, and the stems of cacti.

Monoecious

—This is a plant breeding system in which male and female reproductive structures are present on the same plant, and in the case of cacti, in the same flowers.

Perfect

—In the botanical sense, this refers to flowers that are bisexual, containing both male and female reproductive parts.

Stomate

—These are microscopic pores in the leaf or stem cuticle, bordered by guard cells which control opening or closing of the pore.

Succulent

—Having thick, fleshy leaves or stems that conserve moisture.

Xerophyte

—A plant adapted to dry or drought prone habitats.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Cactus." The Gale Encyclopedia of Science. . Encyclopedia.com. 10 Sep. 2018 <http://www.encyclopedia.com>.

"Cactus." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 10, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/cactus-0

"Cactus." The Gale Encyclopedia of Science. . Retrieved September 10, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/cactus-0

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

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Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.