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.

Organ Pipe Cactus National Monument: see National Parks and Monuments (table).

Any of the flowering plants that make up the family Cactaceae, containing about 1,650 species, native through most of North and South America, with the greatest number and variety in Mexico. Cacti are succulent perennials. Most live in and are well adapted to dry regions. Cacti generally have thick herbaceous or woody stems containing chlorophyll. Leaves usually are absent or greatly reduced, minimizing the surface area from which water can be lost; the stem is the site of photosynthesis. The generally thin, fibrous, shallow root systems range widely in area to absorb superficial moisture. Cacti vary greatly in size and appearance, from buttonlike peyote and low clumps of prickly pear and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro. Cacti can be distinguished from other succulent plants by the presence of small cushionlike structures (areoles) from which, in almost all species, spines arise, as do flowers, branches, and leaves (when present). Flowers, often large and colourful, are usually solitary. Cacti are widely cultivated as ornamentals. Various species, notably prickly pears and chollas, are cultivated as food. Barrel cacti are an emergency source of water for people.

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National monument, southwestern Arizona, U.S., at the Mexican border. It was established in 1937. With an area of 330,689 acres (133,929 hectares), it preserves segments of the mountainous Sonoran Desert and is named for the organ-pipe cactus. Wildlife includes Gila monsters, antelope, coyotes, and a variety of birds.

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