The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, are a large and economically important family of flowering plants. It includes trees, shrubs, and perennial or annual herbaceous plants, which are easily recognized by their fruit (legume) and their compound, stipulate leaves. Many legumes have characteristic flowers and fruits. The family is widely distributed, and is the third-largest land plant family in number of species, behind only the Orchidaceae and Asteraceae, with about 751 genera and about 19,000 known species. The five largest of the genera are Astragalus (over 3,000 species), Acacia (over 1000 species), Indigofera (around 700 species), Crotalaria (around 700 species), and Mimosa (around 400 species), which constitute about a quarter of all legume species. The ca. 19,000 known legume species amount to about 7% of flowering plant species. Fabaceae is the most common family found in tropical rainforests and in dry forests in the Americas and Africa.
The family Fabaceae includes a number of important agricultural and food plants, including Glycine max (soybean), Phaseolus (beans), Pisum sativum (pea), Cicer arietinum (chickpeas), Medicago sativa (alfalfa), Arachis hypogaea (peanut), Ceratonia siliqua (carob), and Glycyrrhiza glabra (liquorice). A number of species are also weedy pests in different parts of the world, including: Cytisus scoparius (broom), Robinia pseudoacacia (black locust), Ulex europaeus (gorse), Pueraria montana (kudzu), and a number of Lupinus species.
Many Fabaceae host bacteria in their roots within structures called root nodules. These bacteria, known as rhizobia, have the ability to take nitrogen gas (N2) out of the air and convert it to a form of nitrogen that is usable to the host plant ( NO3− or NH3 ). This process is called nitrogen fixation. The legume, acting as a host, and rhizobia, acting as a provider of usable nitrate, form a symbiotic relationship.