Fabaceae is the botanical name of a plant family. As circumscribed by the APG system, it is a large family: Fabaceae sensu lato. The International Code of Botanical Nomenclature allows the use of Fabaceae s.l. and Leguminosae as equivalent botanical names at the rank of family. The APG-system uses the name Fabaceae.

However, the family Fabaceae can be circumscribed differently as Fabaceae sensu stricto, for example in the Cronquist system. In such classifications the subfamilies Mimosoideae and Caesalpinioideae are raised to the rank of families Mimosaceae and Caesalpiniaceae. The remaining group has the equivalent botanical names of Fabaceae and Papilionaceae (but not Leguminosae). The APG treats this group at the rank of subfamily, with the name Faboideae (its equivalent in the Leguminosae is Papilionoideae).

In consulting any book that uses the name Fabaceae, care should be taken to make sure what group it applies to. The names Leguminosae or Papilionaceae are unambiguous, and taxonomists dealing with this group mostly use Leguminosae.

The Leguminosae (or Fabaceae sensu lato) is the second largest families of flowering plants with 650 genera and over 18,000 species. These are commonly called legumes or pulses 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 Sub-Families

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The legumous plants are traditionally classified into three subfamilies, sometimes raised to the rank of family in the order Fabales, on the basis of flower morphology (specifically, petal shape):

• 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.
• Faboideae or Papilionoideae (Fabaceae sensu strictu or Papilionaceae): 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.

Nitrogen fixation

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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.

External links

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• International Legume Database & Information Service
• Beanref - Links to research on Beans

Fabaceae | Legumes | Plant families | Nitrogen metabolism

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