Cactus
They are often used as ornamental plants, but some are also crop plants.
Cacti are distinctive and unusual plants, which are adapted to extremely arid and hot environments, showing a wide range of anatomical and physiological features which conserve water. Their stems have expanded into green succulent structures containing the chlorophyll necessary for life and growth, while the leaves have become the spines for which cacti are so 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, and the smallest is Blossfeldia liliputiana, only about 1 cm diameter at maturity. Cactus flowers are large, and like the spines and branches arise from areoles. Many cactus species are night blooming, as they are pollinated by nocturnal insects or small animals, principally moths and bats.
Cacti range in size from small and globular to tall and columnar.
Description
Closeup image of a cactus flower (Echinopsis spachiana) showing large number of stamens.
Closeup look of an cactus perianth.
The cacti are spine plants that grow either as trees, shrubs or in the form of ground cover. Most species grow on the ground, but there is also a whole range of epiphytic species.
In most species, except for the sub-family of the Pereskioideae (see image), the leaves are greatly or entirely reduced. The flowers, mostly radially symmetrical and hermaphrodite, bloom either by day or by night, depending on species.
Their shape varies from tube-like through bell-like to wheel-shaped, and their size from 0.2 to 15-30 centimeters. Most of them have numerous sepals (from 5 to 50 or more), and change form from outside to inside, from bracts to petals.
Nearly all species of cacti have a bitter sometimes milky sap contained within them. The berry-like fruits may contain few, but mostly many (3,000), seeds, which can be between 0.4 and 12 mm long.
The life of a cactus is seldom longer than 300 years, and there are cacti which live only 25 years (although these flower as early as their second year).
The Saguaro cactus (Carnegiea gigantea) grows to a height of up to 15 meters (the record is 17 meters 67cm), 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 meter 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.
Distribution
Cacti are almost exclusively New World plants. This means that they are native only in North America, South America, and the West Indies.
There is however 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. Unique species in the New World must have developed after the continents had moved apart.
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. This is, however, due to parallel evolution; none of these is closely related to the Cactaceae.
Prickly pears (genus Opuntia) were imported into Australia in the 19th century to be used as a natural agricultural fence and to establish a cochineal dye industry, but quickly became a widespread weed.
Plants that inhabit these dry areas are known as xerophytes, and many of them are succulents, with thick or reduced, "succulent", leaves. Like other succulents, cacti have a range of specific adaptations that enable them to survive in these environments.
Pereskia grandifolia: Pereskia is a weakly succulent genus, which also possesses leaves, and is believed to be very similar to the ancestor of all cacti.
Barrel Cactus growing on a cliff in the Mojave Desert.
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. 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.
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. Pereskia has now been determined to be the ancestral genus from which all other cacti evolved. 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.
This works by preventing water from spreading on the surface and allowing water to trickle down the stem to be absorbed by the roots and used for photosynthesis. Cacti have a thick, hard-walled, succulent stem - when it rains, water is stored in the stem.
The inside of the stem is either spongy or hollow (depending on the cactus). A thick, waxy coating keeps the water inside the cactus from evaporating.
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 the sphere (combining highest possible volume with lowest possible surface area).
This species is well known from Western films.
Most cacti have a short growing season and long dormancy. 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.
Domestic cacti owners should take great precaution to limit their cacti's exposure to any kind of root beer.
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.
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.
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. Here, transpiration does not take place during the day at the same time as photosynthesis, but at night. Many flowers, for example those of Selenicereus grandiflorus (Queen of the Night) are only fully open for two hours at night.
Seed formation is very prolific, and the fruits are mostly fleshy, pleasant tasting and conspicuously coloured. This cactus, also known as "Mother-in-law's Cushion," has great ritual significance - human sacrifices were carried out on these cacti. 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, snake, and cactus.
Economic exploitation of the cactus can also be traced back to the Aztecs.
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. 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.
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. To be borne in mind here is that some cactus habitats have a total area of no more than 1,000 square meters. If this habitat is destroyed, either by construction or by plundering, the species growing there is lost for posterity if it is endemic (ie, growing in that one spot and nowhere else).
The Moche people of ancient Peru worshiped agriculture and often depicted the cactus in their art.
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.
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. For example, the San Pedro cactus, a common specimen found in many garden centers, is known to contain mescaline.
Etymology
Prickly Pear is among the most common type of cacti found in North America.
The word cactus is ultimately derived from Greek Κακτος kaktos, used in classical Greek for a species of spiny thistle, possibly the cardoon, and used as a generic name, Cactus, by Linnaeus in 1753 (now rejected in favor of Mammillaria).
Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural by the Random House Unabridged Dictionary (2006).
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