(Cactaceae), a family of dicotyledonous perennial plants usually having thickened, succulent, fleshy stems covered with spines, hairs, or bristles. Cacti may be treelike, shrubby, or vinelike; some, such as those of Pereskioideae are small trees. The stem may be spherical, ovoid, cylindrical, columnar, or occasionally flattened and segmented. As a rule, the stem does not have developed leaves (normally developed leaves are found only in the subfamily Pereskioideae), but it is adapted for photosynthesis and transpiration, as well as for the accumulation of moisture. The cactus stem has a well-developed water-bearing parenchyma (large cacti can hold up to 2, 000 liters of water), and the cells contain a mucous substance that impedes the loss of water. Many cacti have a waxy surface or a thick, sometimes woolly, covering. There are a few stomata below the surface, and almost all cacti have ribs or papillae (modified leaf bases). Cacti are distinguished by the presence on the stems of areoles, clearly defined areas with hair and a spine originating from the axillary buds and their scales.
The flowers are often large, sometimes with diameters of up to 25 cm, brightly colored, usually solitary (occasionally arranged in apical inflorescences), almost always bisexual, and usually regular. The perianth has many spirally arranged segments, the outermost of which are the sepals and the innermost, the staminal petals. In many genera the petals are fused at the base into a tube. There are many stamens, and the ovary is usually inferior. The fruit is usually polyspermous, fleshy, berrylike, and sometimes dry. Many cacti blossom only late in the evening or at night. They are pollinated by insects or birds (including hummingbirds), although sometimes by bats. The seedlings of many cacti have seed leaves.
There are about 85 genera (according to other sources 50-220), comprising more than 2, 000 species, found almost exclusively in North and South America. Half of all the species are found in Mexico. Only one genus, Rhipsalis, occurs in tropical Africa (probably imported), on Madagascar and the Mascarene Islands, and in Sri Lanka. Cacti usually grow in tropical and subtropical deserts, extending in some areas up to 4, 500 m into the mountains; they are rarely found in tropical rain forests, in savannas, or along seacoasts. Some cacti, such as various species of Opuntia, are naturalized in Australia, China, the Mediterranean region, and other countries; in the USSR they grow on the Southern Crimean Shore.
Cacti are used for food (the fruit and the flesh of the stems), fodder (particularly the spineless members of Opuntia), fuel, light building material (the woody stems of some species), and the making of hedgerows (especially prickly Opuntia and some members of the genus Cereus). Many cactus species are grown indoors and in greenhouses (Mammillaria, Opuntia, Cereus, Cephalocereus, and Epiphyllum). Cacti are propagated by seeds, cuttings, or graftings. Depending on the species, the seeds may retain their germinating potential for several years. The sprouts appear between three and 30 days after sowing. Grafting is done from May to August. For many cactus species, Trichocereus Spachianus, Trichocereus pachanoi, Eriocereus Jusberta, and Pe-reskia aculeata are used as stock.
Cacti are grown in a soil mixture composed of leaf mold, clay sod, large-grain sand (with an admixture of gravel), crushed charcoal, and lime, with the proportion depending on the species. Organic fertilizers are used only for epiphytes, such as Zygocactus and Epiphyllum; mineral fertilizers are used for others.
Cacti blossom from early spring to late autumn; species of Zygocactus and Rhipsalis flower in winter. When they are not flowering, most species are dormant. In winter most cacti grown indoors and in greenhouses are kept at a temperature of 8°-10°C, although some are maintained at 5°-6°C and epiphytes at 14°-16°C; they are watered infrequently. In summer cacti require a warm, sunny spot, fresh air, much watering, and regular spraying. During the budding period, some cacti should not be moved, as this may cause the buds to fall.
R. A. UDALOVA and S. G. SAAKOV