The Family Fabaceae (also as Family Leguminosae) is a grouping of plants in the Order Fabales, and one of the largest families of flowering plants with 650 genera and over 18,000 species. These plants are commonly called legumes and the family contains some of our most valuable food crops, such as beans, peas, peanuts, soybeans, and lentils. Other members of the family are important sources of animal feed or green manure, such as lupins, clover, alfalfa, cassia, and soybean. Some genera such as Laburnum, Robinia, Gleditsia, Acacia, Mimosa, and Delonix are ornamental trees and shrubs. Still other members of the family have medicinal or insecticidal properties (for instance Derris ) or yield important substances like gum arabic, tannin, dyes, or resins. Then there is kudzu, an east Asian species originally planted in the U.S. southeast for soil improvement and as a cattle feed, that has become a notorious invasive weed that tends to grow over everything.
All members of this family have five-petaled flowers in which the superior ovary ripens to form a "pod", technically called a legume, whose two sides split apart, releasing the seeds which are attached to one or both seams. The legumous plants are classified into three subfamilies, sometimes raised to the rank of family in the order Fabales, on the basis of flower morphology (specifically, petal shape):
Faboideae (Fabaceae), previously called Papilionoideae: One petal is large and has a crease in it, the two adjacent petals are on the sides, and the two bottom petals are joined together at the bottom, forming a boat-like structure.
Caesalpinioideae (Caesalpiniaceae): The flowers are zygomorphic , but are very variable, e.g. closely resembling Faboideae flowers in Cercis, while symmetrical with five equal petals in Bauhinia.
Mimosoideae (Mimosaceae): The petals are small, and are frequently globose or spicate and the stamens are the most showy part of the flower.
A significant characteristic of legumes is that they 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.