Cactus
| Cacti Fossil range: 35-0 Ma Late Paleogene - Recent |
|
|---|---|
 |
|
| Ferocactus pilosus (Mexican Lime Cactus) growing south of Saltillo, Coahuila, northeast Mexico | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| (unranked): | Core eudicots |
| Order: | Caryophyllales |
| Family: | Cactaceae Juss. |
| Subfamilies | |
See also taxonomy of the Cactaceae |
|
A cactus (plural: cacti or cactuses) is a member of the plant family Cactaceae, native to the Americas (with one exception, Rhipsalis baccifera, which is native to parts of the Old World). They are often used as ornamental plants, and some are also crop plants for fodder, forage, fruits, cochineal, and other uses. Numerous species have been used since ancient times by indigenous peoples for their psychedelic effects. Cacti are part of the plant order Caryophyllales, which also includes members like beets, gypsophila, spinach, amaranth, tumbleweeds, carnations, rhubarb, buckwheat, plumbago, bougainvillea, chickweed and knotgrass.
Cacti are unusual and distinctive plants, which are adapted to extremely arid and/or semi-arid hot environments, as well as tropical environments as epiphytes or hemi-epiphytes.[1][2][3] They show a wide range of anatomical and physiological features which conserve water. Their stems have adapted to become photosynthetic and succulent, while the leaves have become the spines for which cacti are well known.
Cacti come in a wide range of shapes and sizes. The tallest is Pachycereus pringlei, with a maximum recorded height of 19.2 m,[4] and the smallest is Blossfeldia liliputiana, only about 1 cm diameter at maturity.[5] Cactus flowers are large, and like the spines and branches arise from areoles.
Cacti are perennial and grow as trees, shrubs, or vines. Most species are terrestrial, but there are also many epiphytic species, especially in the tribes Rhipsalideae and Hylocereeae. In most species, except for the sub-family Pereskioideae (see image), the leaves are greatly or entirely reduced. The leaves may also be tiny and deciduous as can be seen on new shoots of Opuntia. Spines grown by the cacti are actually modified leaves; the stems (the green "pads" of many cacti) have also evolved to photosynthesize. The study of when spines grow and how they can be used to tell the cactus' age is called acanthochronology. The flowers, mostly radially symmetrical and bisexual, bloom either by day or by night, depending on the species. Their shape varies from tube-like through bell-like to wheel-shaped, and their size from 0.2 to 15–30 centimetres. Most of them have numerous sepals (from 5 to 50 or more), and change form from outside to inside, from bracts to petals. They have stamens in great numbers (from 50 to 1,500, rarely fewer). Nearly all species of cacti have a bitter mucilaginous sap contained within them. The berry-like fruits may contain few to many (3,000), seeds, which can be between 0.4 and 12 mm long.[6]
The life of a cactus is seldom longer than 300 years, but may be as short as 25 years, (although these flower as early as their second year). The Saguaro cactus (Carnegiea gigantea) grows to a height of up to 15 metres (the record is 17 metres 67 cm), but in its first ten years, it grows only 10 centimeters. The "mother-in-law's cushion" (Echinocactus grusonii) reaches a height of 2.5 meters and a diameter of 1 metre and – at least on the Canaries – is already capable of flowering after 6 years. The diameter of cactus flowers ranges from 5 to 30 cm; the colors are often conspicuous and spectacular.
The cactus family is endemic to the Americas with one exception, Rhipsalis baccifera; this species has a pantropical distribution, occurring in the Old World tropical Africa, Madagascar and Sri Lanka as well as in tropical America. This plant is thought to be a relatively recent colonist in the Old World (within the last few thousand years), probably carried as seeds in the digestive tracts of migratory birds. Many other cacti have become naturalized to similar environments in other parts of the world after being introduced by people, e.g. Australia, Hawaii, and the Mediterranean region. The Tehuacán Valley of Mexico has one of the richest occurrences of cacti in the world.[7] Species diversity decreases as one travels north; hundreds of species can be found in areas of Mexico, dozens of species are found in the Sonora and Mojave deserts of the southwestern U.S., and only several species are found in the eastern plains and dry valleys of Montana and Alberta.
Cacti are believed to have evolved in the last 30 to 40 million years. Long ago, the Americas were joined to the other continents, but separated due to continental drift. Unique species in the New World must have developed after the continents had moved apart or began developing just prior to the separation. Significant distance between the continents was only achieved in around the last 50 million years. This may explain why cacti are so rare in Africa as the continents had already separated when cacti evolved. Many succulent plants in both the Old and New World bear a striking resemblance to cacti, and are often called "cactus" in common usage.
Contents |
Adaptations to dry environment
Some environments, such as deserts, semi-deserts, and dry steppes, receive little water in the form of precipitation. Plants that inhabit these dry areas are known as xerophytes, and many of them are succulents, with thick or reduced, "succulent", leaves. Apart from a few exceptions (for example, the genus Pereskia) all cacti are succulent plants. Like other succulents, cacti have a range of specific adaptations that enable them to survive in these environments.
Cacti have never lost their leaves completely; they have only reduced the size so that they reduce the surface area through which water can be lost by transpiration. In some species the leaves are still remarkably large and ordinary while in other species they have become microscopic but they still contain the stomata, xylem and phloem. Certain cactus species have also developed ephemeral or deciduous leaves, which are leaves that last for a short period of time when the stem is still in its early stages of development. A good example is Opuntia ficus-indica, better known as the prickly pear. Cacti have also developed spines which allow less water to evaporate through transpiration by shading the plant, and defend the cactus against water-seeking animals. The spines grow from specialized structures called areoles, homologous to the nodes on other plants. Very few members of the family have leaves, and when present these are usually rudimentary and soon fall off; they are typically awl-shaped and only 1–3 mm. long. Two genera, Pereskia and Pereskiopsis, do however retain large, non-succulent leaves 5–25 cm. long, and non-succulent stems. Pereskia has now been determined to be the ancestral genus from which all other cacti evolved.[8] Enlarged stems carry out photosynthesis and store water. Unlike many other succulents, the stem is the only part of a true cactus where this takes place. Much like many other plants that have waxy coatings on their leaves, cacti often have a waxy coating on their stems to prevent water loss and potentially repel water from their stems.
The bodies of many cacti have become thickened during the course of evolution, and form water-retentive tissue and in many cases assume the optimal shape of a sphere or cylinder (combining highest possible volume with lowest possible surface area). By reducing its surface area, the body of the plant is also protected against excessive sunlight.
Most cacti have a short growing season and long dormancy. For example, a fully-grown Saguaro cactus (Carnegiea gigantea) can absorb up to 3,000 litres of water in ten days. This is helped by the ability to form new roots quickly. Two hours after rain following a relatively long drought, root formation begins in response to the moisture. Apart from a few exceptions, an extensively ramified root system is formed, which spreads out immediately beneath the surface. The salt concentration in the root cells is relatively high, so that when moisture is encountered, water can immediately be absorbed in the greatest possible quantity.
But the plant body itself is also capable of absorbing moisture (through the epidermis and the thorns), which for plants that are exposed to moisture almost entirely or indeed in some cases solely, in the form of fog, is of the greatest importance for sustaining life.
Most cacti have very shallow roots that can spread out widely close to the surface of the ground to collect water, an adaptation to infrequent rains; in one examination, a young Saguaro only 12 cm. tall had a root system covering an area 2 meters in diameter, but with no roots more than 10 cm. deep.[9] The larger columnar cacti also develop a taproot, primarily for anchoring but also to reach deeper water supplies and mineral nutrients.[9]
One feature distinguishes the cacti from all other plants: cacti possess areoles, as they are known. The areole appears like a cushion with a diameter of up to 15 mm. and is formed by two opposing buds in the angles of a leaf. From the upper bud develops either a blossom or a side shoot, from the lower bud develop thorns. The two buds of the areoles can lie very close together, but they can also sometimes be separated by several centimeters.
Like other succulents in the families of the Crassulaceae, Agavaceae (agaves), Euphorbiaceae (euphorbias), Liliaceae (lilies), Orchidaceae (orchids) and Vitaceae (vines), cacti reduce water loss through transpiration by Crassulacean acid metabolism.[9] Here, transpiration does not take place during the day at the same time as photosynthesis, but at night. The plant stores the carbon dioxide chemically linked to malic acid until the daytime. During the day the stomata are closed and the plant releases the stored CO2 and uses it for photosynthesis. Because transpiration takes place during the cool humid night hours, water loss through transpiration is significantly reduced.
Reproductive ecology
Some cactus flowers form long tubes (up to 30 centimetres) so that only moths can reach the nectar and therefore pollinate the blossoms. There are also specializations for bats, hummingbirds and particular species of bees. The duration of flowering is very variable. Many flowers, for example those of Selenicereus grandiflorus (Queen of the Night) are only fully open for two hours at night. Other cacti 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 become a completely protected area, protected by thorns, hairs and scales. Seed formation is very prolific, and the fruits are mostly fleshy, pleasant tasting and conspicuously coloured. Goats, birds, ants, mice and bats contribute significantly to the spreading of the seeds.
Because of the plants' high water-retention ability, detached parts of the plant can survive for long periods and are able to grow new roots anywhere on the plant body.
History
Among the remains of the Aztec civilization, cactus-like plants can be found in pictorial representations, sculpture and drawings, with many depictions resembling Echinocactus grusonii. Tenochtitlan (the earlier name of Mexico City) means "place of the sacred cactus." The coat of arms of Mexico to this day shows an eagle perched on a cactus while holding a snake, an image which is at the center of the Aztec origin myth.[10]
Economic exploitation of the cactus can also be traced back to the Aztecs. The North American Indians utilize the alkaloid content of several cacti species for religious ceremonies. Today, besides their use as foodstuffs (jam, fruit, vegetables), their principal use is as a host for the cochineal insect, from which a red dye (carmine) is obtained which is used in Campari or high-quality lipsticks. Particularly in South America dead pillar cacti yield valuable wood for construction. Some cacti are also of pharmaceutical significance.
From the moment the early European explorers sighted them, cacti have aroused much interest: Christopher Columbus brought the first melocactus to Europe. Scientific interest in them began in the 17th century. By 1737, twenty-four species were known, which Linnaeus grouped together as the genus "Cactaceae".
From the beginning of the 20th century, interest in cacti has increased steadily. This was accompanied by a rising commercial interest, the negative consequences of which culminated in raids on their native habitats. Through the great number of cactus admirers, whether their interest is scientific or hobby-oriented, new species and varieties are even today discovered every year.
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.
Some countries have a rather contradictory attitude to species protection. In Mexico for example to be caught in the act of digging up cacti carries a prison sentence, but cactus habitats are destroyed for the construction of new roads and electricity lines.
The Moche people of ancient Peru worshipped agriculture and often depicted the cactus in their art.[11]
Uses
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. Some, such as the Golden Barrel dekha Cactus, Echinocactus grusonii, are prominent in garden design. Cacti are commonly 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 Masai 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. The aesthetic characteristics of some species, in conjunction with their home security qualities, makes them a considerable alternative to artificial fences and walls.
As well as garden plants, many cactus species have important commercial uses, some cacti bear edible fruit, such as the prickly pear and Hylocereus, which produces Dragon fruit or Pitaya. According to Reuters, the edible cactus, or nopal, industry in Mexico is worth $150 million each year and approximately 10,000 farmers cultivate the plant.[12] Opuntia are also used as host plants for cochineal bugs in the cochineal dye industry in Central America.
The Peyote, Lophophora williamsii, is a well-known psychoactive agent used by Native Americans in the Southwest of the United States of America. Some species of Echinopsis (previously Trichocereus) also have psychoactive properties. For example, the San Pedro cactus, a common specimen found in many garden centers, is known to contain mescaline.
Etymology
The word cactus is derived through Latin from the Greek κάκτος kaktos, which referred to the cardoon (Cynara cardunculus). Linnaeus in 1753 applied this name generically to a genus he called Cactus, which was later reassigned as a family, Cactaceae, and subdivided into multiple genera.[13] Cactuses, the Latin plural cacti, and the uninflected plural cactus are all used in English.[14]
See also
- Ariocarpus scaphirostris
- List of foliage plant diseases (Cactaceae)
References
- ↑ P.S. Nobel. 1988. Environmental Biology of Agaves and Cacti. Cambridge University Press, New York.
- ↑ P.S. Nobel. 1994. Remarkable Agaves and Cacti. Oxford University Press, New York. 166 pp. Spanish Translation by E. García Moya. 1998. Los Incomparables Agaves y Cactos. Editorial Trillas, Mexico City.
- ↑ P.S. Nobel. 2010. DESERT WISDOM/AGAVES and CACTI: CO2, Water, Climate Change. iUniverse, Bloomington, IN.
- ↑ Salak, M. (2000). In search of the tallest cactus. Cactus and Succulent Journal 72 (3).
- ↑ Mauseth Cactus research: Blossfeldia liliputiana
- ↑ Description of the Family in the Flora of North America.
- ↑ "Mexico: Tehuacán-Cuicatlán Biosphere Reserve." (PDF). http://www.parkswatch.org/parkprofiles/pdf/tcbr_eng.pdf.
- ↑ How did cacti evolve?
- ↑ 9.0 9.1 9.2 Dalhousie University: Biology of Cacti
- ↑ "The Awesome Aztecs for Kids - Place of the Prickly Pear Cactus". Aztecs.mrdonn.org. http://aztecs.mrdonn.org/place.html. Retrieved 2010-05-22.
- ↑ Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.
- ↑ Frank Jack Daniel (2007-02-19). "Cactus-eating moth threatens favorite Mexican food". Reuters. http://www.reuters.com/article/oddlyEnoughNews/idUSN2G28324120070219. Retrieved 2010-05-22.
- ↑ Oxford English Dictionary, s.v. "cactus", http://dictionary.oed.com/cgi/entry/50030958.
- ↑ Merriam-Webster's Online Dictionary, s.v. "cactus", http://www.merriam-webster.com/dictionary/cactus.
External links
Cactus at the Open Directory Project