FABALES

order of dicotyledonous flowering plants, a division of the subclass Rosidae. The characteristic fruit of many members is a pod (legume) consisting, in essence, of an ovary that is a tightly folded leaf, as in a pea pod. The pod normally splits into two halves when mature. Fabales are especially important as human and animal food. The Fabales (legumes) may be considered a single family (Leguminosae or Fabaceae) as defined in 1981 by the British botanist R.M. Polhill and the American P.H. Raven and in this article, or they may be divided into three families as in the taxonomic system proposed by Arthur Cronquist. Including about 18,000 species in approximately 650 genera, the Leguminosae is the third largest family of angiosperms, exceeded only by the Asteraceae (aster or sunflower family) and Orchidaceae (orchid family). Although approximately coeval (contemporary) with the grass family (Poaceae), the legume family is the most important of any in the production of food for humans and livestock, as well as in the production of industrial products. Because they develop bacteria-harbouring root nodules that maintain the nitrogen balance in the soil which is necessary for plant growth, the legumes are also an essential element in nature and in agriculture. Legumes are perhaps best known by their more common cultivated names, such as peas, beans, soybeans, peanuts (groundnuts), alfalfa (lucerne), and clover. In any large group of organisms, evolutionary byways give exceptions to any useful generalities. This will be true of the following discussion. The common name legume used here much of the time is synonymous with the order Fabales as a whole. order of flowering plants, belonging to the class Magnoliopsida (dicotyledons; characterized by two seed leaves). It consists of approximately 650 genera and about 18,000 species in a single family, Leguminosae, or Fabaceae, divided into three subfamilies: Caesalpinioideae, Mimosoideae, and Papilionoideae, or Faboideae. (The widely accepted taxonomic system proposed by Arthur Cronquist, however, divides the order into three families.) The characteristic fruit of many members is a pod or legume consisting, in essence, of an ovary that is a tightly folded leaf, as in a pea pod. It normally splits into two halves when mature. In economic importance, this order is surpassed only by the grass and sedge order (Cyperales). Roots of most members of the Fabales harbour nitrogen-fixing bacteria in a cooperative arrangement whereby otherwise unusable nitrogen gas in the atmosphere is converted into the soluble nitrates that are required for the synthesis of proteins. All leguminous plants leave residues of nitrates in the soil, thereby benefiting other crops grown in rotation. Thus they are an essential element in nature and in agriculture. The legume family is the most important of any in the production of food for humans and livestock. The cultivation of crops such as peanuts and soybeans directly increases the world's protein supply; animals grazing on fodder plants such as alfalfa (lucerne) and clovers provide protein less directly. Fabales are widely distributed on all habitable continents. Many genera are predominantly tropical or subtropical, with significant extensions into the temperate zones. Members such as the broad, or fava, bean (Vicia faba) produce better crops in Canada than in warmer temperate regions. Representatives that are adapted to dry regions include the mesquite (Prosopis) of the southwestern United States and the wattle (Acacia) of Australia. The herb Neptunia oleracea of India is grown underwater. Leaves of many members appear feathery; leaflets arise on both sides of an axis (central stalk), producing pinnate, or in some cases bipinnate, leaves. Leaf blades are connected to the stem by a petiole (leaf stalk) with two stipules (leaflike structures) placed at the juncture of petiole and stem. Stipules may be larger than leaflets (as in peas) or in the form of spines (as in many genera of desert plants). Flowers in this order are almost universally showyif not individually, then by clustering. In the Mimosoideae petals are alike and radiately displayed, but in the Caesalpinioideae and Papilionoideae flowers have a papilionaceous (butterfly) shape unique to legumes. The production of a pod (legume), even by members of the Fabales that do not possess a butterfly-shaped flower, emphasizes the unity of the order. The subfamily Mimosoideae contains about 3,000 species in some 60 genera and consists chiefly of trees and shrubs. Most of them possess bipinnate leaves, small flowers (showy, when clustered), and petals alike in shape and size (radially symmetrical). The fruits of some members remain closed (indehiscent) at maturity. Acacia, with about 1,200 species, and Mimosa, about 400 species, generally inhabit deserts and savannas of subtropical regions. Economically important species of Acacia include the silver wattle (A. dealbata), cultivated for its fragrant oils; A. senegal, which yields gum arabic; and A. catechu, which yields black cutch, a khaki dye. Leaves and leaflets of the sensitive plant (Mimosa pudica), a common greenhouse plant, fold rapidly when touched. A swollen region at the base of each unit has greater internal space on one side than on the other. A touch stimulates the draining of water from loosely packed cells, causing the swollen region to lose pressure. This results in the folding of leaflets at these hinge joints. Other legumes respond similarly, and leaves of many legumes fold at night. Leaves of the Caesalpinioideae are once- or twice-pinnate; in the bud, the largest petal is innermost, and the keel petals are fully separate. Fruits may ripen dry as pods or as samaras (winged fruits); a few are drupes (having a stony layer covering the seed). The approximately 150 genera and 2,000 species constituting this family are nearly all trees or shrubs that are distributed worldwide, primarily in the tropics. Genera of the Caesalpinioideae valued for timber include the honey locust (Gleditsia triacanthos) and the Kentucky coffee tree (Gymnocladus dioica), both native to warmer areas of eastern North America, where they attain heights of more than 30 m (100 feet). Seeds of the latter have been used as a substitute for coffee beans. Both are valuable shade trees. Valued plants of the Caesalpinioideae include many of the more than 500 species of the genus Cassia, native to eastern North America, northeast Africa, and India. Leaves of several species are laxative, and C. acutifolia is cultivated for medicinal use in India. Other species are grown as ornamentals or for timber or firewood. Cassias are among the showiest trees and shrubs in the tropics. The subfamily Papilionoideae constitutes one of the greatest assemblages of agriculturally important plants. It consists of some 12,000 species in about 420 genera with worldwide distribution. Many genera, such as the cultivars (horticultural varieties) of the garden pea (Pisum sativum) and alfalfa (Medicago sativa), are adapted to cool climates. Members of the Papilionoideae are distinguished from the Mimosoideae by their papilionaceous flowers and from the Caesalpinioideae by the basal joining of keel petals. Most bear leaves that are pinnate or are reduced to three leaflets. Pods (legumes) of many genera are constricted, allowing the dispersal of segments containing one seed each. Other species produce drupes, samaras, or burrs. Peanuts (groundnuts; Arachis hypogaea) and soybeans (Glycine max) support the economies of many nations in temperate and tropical regions, largely because of proteins and edible oils contained in their seeds. Green and dry fodder is provided by alfalfa and by several clovers (red, Trifolium; white, Melilotus), vetches (Vicia species), and cowpeas (Vigna species). The broad bean, or fava bean (Vicia faba), native in northern Africa and southwest Asia, is the bean of antiquity. This cool-weather plant is widely used as a cover crop, green manure, and forage. Central America was the centre of origin for Phaseolus beans; P. vulgaris, the kidney, snap, green, and pinto beans; P. lunatus, the sieva, or lima, bean; P. limensis, the lima bean, with its many bush forms; and P. coccineus, the perennial scarlet runner bean, with its bright scarlet flowers. Seeds of Phaseolus and other legumes produce phytohemagglutinin, a large-molecule protein that clots certain human blood types and has other medical applications. Useful ornamental shrubs of the Papilionoideae include several cultivars of Amorpha fruticosa, native from Saskatchewan to Florida. Other shrubby plants with showy flowers are the brooms, Cytisus and Genista. Herbaceous (nonwoody) ornamental members include species of Canavalia (jack bean), Lupinus (lupine), Dolichos (hyacinth bean), and Vigna (snail flower). A few of the roughly 2,000 species of Astragalus are poisonous to livestock on the prairies of the central United States. Some species of Astragalus are commonly referred to as locoweed in North America, because after excessive consumption cattle seem to become unmanageable. Astragalus is poisonous in any of three ways: by promoting selenium absorption, through the toxin locoine, or through several nitrogen-containing toxins. In all three subfamilies flowers occur in inflorescences (clusters) rather than singly. They may be elongate, branched, compacted into heads, or grouped in loose clusters. Flower parts are generally in fives. Sepals form a lobed, bell-shaped calyx, and petals are either separate and uniform or papilionaceous. Flower colours in the order Fabales range from white to purple. The 10 stamens, useful in classification, are separate or joined in various combinations in consistent patterns according to the genus. The pistil (female) consists of a single carpel with a flattened ovary and, often, a style that ends in a pollen-receptive stigmatic surface. Insects, especially bees, are the agents of pollination, but flowers of the tropical genus Parkia are pollinated by bats. Following pollination, the ovule becomes the seed and the ovary the fruit. In the peanut, the ovary is pushed into the ground following pollination, when its base elongates. The indehiscent pod matures below ground. Additional reading O.N. Allen and Ethel K. Allen, The Leguminosae: A Source Book of Characteristics, Uses, and Nodulation (1981), a descriptive summary, with an alphabetical listing of legume genera; J.B. Harborne, D. Boulter, and B.L. Turner (eds.), Chemotaxonomy of the Leguminosae (1971); R.M. Polhill and P.H. Raven (eds.), Advances in Legume Systematics (1981 ), an updated evolutionary classification of the Fabales; R.J. Summerfield and A.L. Bunting (eds.), Advances in Legume Science (1980); C.H. Stirton and J.L. Zarucchi (eds.), Advances in Legume Biology (1989); James A. Duke, Handbook of Legumes of World Economic Importance (1981), also with an alphabetical listing of genera; D. Isely, Leguminosae and Homo sapiens, Economic Botany, 36(1):4670 (1982), a succinct summary of the contributions of legumes to human welfare; National Research Council (U.S.), Advisory Committee on Technology Innovation, Tropical Legumes: Resources for the Future (1979), a descriptive listing of utilized but undeveloped tropical legumes; J. Smartt, Tropical Pulses (1976); Harold J. Evans (ed.), Enhancing Biological Nitrogen Fixation (1975); J.R. Postgate, The Fundamentals of Nitrogen Fixation (1982); R.O.D. Dixon and C.T. Wheeler, Nitrogen Fixation in Plants (1986); J.M. Vincent (ed.), Nitrogen Fixation in Legumes (1982); W.J. Broughton (ed.), Nitrogen Fixation, vol. 3, Legumes (1982); J. Smartt, Grain Legumes: Evolution and Genetic Resources (1990); A. Geoffrey Norman (ed.), Soybean Physiology, Agronomy, and Utilization (1978); C. Webb and G. Hawtin (eds.), Lentils (1981).