CACTUS

plural Cacti, or Cactuses, dicotyledonous (characterized by two seed leaves) flowering plants belonging to the family Cactaceae, of the order Caryophyllales. (Some authorities place the cacti in a separate order, Cactales, with a single family.) Botanists estimate that there are about 1,650 species, grouped into 130 genera. The Cactaceae are native through most of the length of North and South America, from British Columbia and Alberta southward; the southernmost limit of their range extends far into Chile and Argentina. Mexico has the greatest number and variety of plants. The only cacti possibly native to the Old World are members of the tropical genus Rhipsalis, occurring in East Africa, Madagascar, and Sri Lanka. Many scholars, however, believe that the plants were introduced to these areas. Cacti are succulent perennial plants. Although a few species inhabit tropical or subtropical areas, most live in and are well adapted to dry regions. Cacti generally have thick herbaceous or woody chlorophyll-containing stems. In most species leaves are either absent or greatly reduced, minimizing the amount of surface area from which water can be lost, and the stem has taken over the photosynthetic functions of the plant. Only the tropical genera Pereskia and Pereskopsis have evident, functional leaves. The root systems are generally thin, fibrous, and shallow, ranging widely to absorb superficial moisture. Cacti vary greatly in size and general appearance, from buttonlike peyote (Lophophora) and low clumps of prickly pear (Opuntia) and hedgehog cactus (Echinocereus) to the upright columns of barrel cacti (Ferocactus and Echinocactus) and the imposing saguaro (Cereus giganteus, or Carnegia gigantea). Most cacti grow in the ground, but several tropical species, including Epiphyllum, Rhipsalis, and Schlumbergera, are epiphytes, growing on other plants; others live on hard substrates such as rocks. Epiphytic species tend to have thin, flattened stems. The appearance of the plant varies also according to whether the stem surface is smooth or ornamented with protruding tubercles, ridges, or grooves. Cacti can be distinguished from other succulent plants by the presence of areoles, small cushionlike structures. In almost all species, spines or barbed bristles (glochids) arise from the areoles, as do flowers, branches, and leaves (when present). The internal structure of cacti stems conforms to the pattern of dicot; a cambium layer of dividing cells, located between the woody inner tissues and those near the outside of the stem, is present. The bulk of the stem, however, consists of thin-walled storage cells that contain mucilaginous substances that prevent the loss of moisture. The stem of cacti is the main food-manufacturing and food-storage organ. A few cacti develop small plantlets at ground level that, as offsets, reproduce the species vegetatively. Tissues of cacti are broadly compatible so that terminal portions of one species may be grafted on top of another. The primary method of reproduction, however, is by seeds. Flowers, often large and colourful, are usually solitary. All genera have a floral tube, consisting of transitional and developed sepals and petals, that grows above a one-chambered ovary. A style topped by many pollen-receptive stigmas, also arises from the top of the ovary. Pollination and fertilization result in the development of the seed from the ovule (of which many are produced), and the development of the fruit, usually a berry, from the ovary. Soon after pollination, which may be effected by wind or by birds, the entire floral tube detaches from the top of the ovary to leave a prominent scar. Cacti are widely cultivated as ornamentals. In addition, various species, notably prickly pears and chollas (both Opuntia), are cultivated as food. In South America species of Opuntia, Cereus, and others are used as living fences, and wood from columnar cacti is used as fuel in some desert regions. Barrel cacti (Echinocactus and Ferocactus) are a source of water in emergencies. Peyote, from Lophophora williamsii, has been used ceremonially since pre-Columbian times for its hallucinogenic properties.