Legume

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Legume, common name for a plant family, the only member of the third largest order of flowering plants in terms of species, with some 18,000; and the second most important economically, after the grasses.

The family is worldwide in distribution, but its greatest concentration is in tropical and subtropical regions. The plants show great diversity in both vegetative and floral form; woody, perennial species predominate, but numerous herbaceous forms and even a few aquatics also occur. The fruit is the feature by which the family is best characterized. Technically known as a legume, it is a single-chambered, flattened seedpod with two sutures. It usually splits open along the two sutures, as in the common pea. The seeds are attached along one of the sutures. The legume may be indehiscent (not splitting), as in the peanut, which matures underground; or explosively dehiscent, as in broom or lupine. It also may range from only a few millimeters long to more than 30 cm (more than 12 in) and may be single or many seeded and brightly or dully colored.

Legume flowers are quite variable, but in all of them the bases of the five sepals (outer floral whorls) and five petals (inner floral whorls) and the stamens (male floral parts) are fused to form a cup (hypanthium) about the base of the ovary (female floral structure). Usually ten stamens are found; they either are fused into a single structure or occur as two groups, one containing nine stamens and one containing a single stamen. The ovary, which matures into the fruit, consists of a single carpel (egg-bearing structure) and is superior—that is, borne above the other floral parts.

A common feature in the family is the presence of root nodules containing bacteria of the genus Rhizobium. These bacteria are capable of converting atmospheric nitrogen, which cannot be used by the plants, into nitrate (NO3-), a form that can be used. Legumes are often planted specifically to renew nitrogen supplies in soils. See Nitrogen Fixation.

The family is divided into three closely related subfamilies, which are often treated as three separate families. One subfamily is mostly herbaceous and is characterized by simple leaves and highly irregular flowers with ten stamens in two clusters. About 12,000 species exist, including such plants as peas, beans, peanuts, and soybeans; clover and alfalfa; and sweet pea, broom, and lupine. The second subfamily contains mostly trees and shrubs and is characterized by bipinnately compound (doubly branching) leaves and regular (radially symmetrical) flowers with ten or more stamens extending beyond the petals. This subfamily contains about 3,000 species and includes acacias and mimosas. The third subfamily is also mostly woody, but with leaves pinnately compound, and slightly to highly irregular flowers with ten stamens in one cluster. This subfamily contains about 3,000 species and includes such plants as brazilwood, carob, honey locust, Judas tree, logwood, and tamarind.

Scientific classification: Legumes make up the family Fabaceae (formerly Leguminosae). The three subfamilies are Papilionoideae, which includes peas and beans; Mimosoideae, which includes acacias and mimosas; and Caesalpinioideae, which includes brazilwood and carob.
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