An order of flowering plants, division Magnoliophyta (Angiospermae), in the subclass Rosidae of the class Magnoliopsida (dicotyledons). The order consists of three closely related families (Mimosaceae, Caesalpiniaceae, Fabaceae) collectively called the legumes. Members of the order typically have stipulate, compound leaves, 10–many stamens which are often united by the filaments, and a single carpel which gives rise to a dry fruit (legume) that opens at maturity by splitting along two sutures, releasing the non-endospermous seeds. Many, or perhaps most, members of the order harbor symbiotic nitrogen-fixing bacteria in the roots.
Mimosaceae includes some 40 genera and 2000 species, most of which are woody and occur in the warmer regions of the world. The flowers are regular, and the stamens at least twice as many as the petals, and often very numerous. Common members of the family are Acacia, Albizia, Mimosa, and Prosopis (mesquite).
Caesalpiniaceae includes about 150 genera and 2200 species, most of which are woody and of tropical or subtropical regions. The flowers are somewhat irregular, typically with five (or fewer) unequal petals and 10 or fewer stamens. The family includes the redbud (Cercis), Kentucky coffee tree (Gymnocladus), honey locust (Gleditsia), and numerous tropical ornamentals.
Fabaceae (sometimes termed the Papilionoideae) has about 400 genera and at least 10,000 species of trees, shrubs, vines, and herbs, which are widespread in the tropical and temperate regions. The flowers are characteristically irregular, with a large upper petal (the banner or standard), two wing petals, and the lower two petals fused to form a boat-shaped keel. Usually there are 10 stamens, with 9 of them united by their filaments, and the tenth one more or less separate. Well-known members of the family include beans (Phaseolus), peas (Pisum), sweet peas (Lathyrus), clover (Trifolium), vetch (Vicia), alfalfa (Medicago), and lupine (Lupinus). See also: Alfalfa; Bean; Clover; Cowpea; Kentucky coffee tree; Kudzu; Lespedeza; Locust (forestry); Lupine; Pea; Peanut; Soybean; Vetch