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| Caesalpinioideae | ||||||||||
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| GRIN-CA 2002-09-01 |
Fabaceae is the botanical name of a large and economically important family of flowering plants, which includes the plants commonly known as legumes. In its broadest circumscription, used here, the Fabaceae sensu lato, also known by the acceptable alternative name Leguminosae, is the third largest family of flowering plants (after Asteraceae and Orchidaceae) 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.
Some plants of this family are important pests. For example, Pueraria lobata (kudzu), an east Asian species originally planted in the U.S. southeast for soil improvement and as a cattle feed, has there become extremely invasive.
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.
According to the classification system being consulted, the name "Fabaceae" can have one of two different meanings:
- As used here, it can refer to a large family, Fabaceae sensu lato, which consists of three subfamilies, Mimosoideae, Caesalpinioideae, and Faboideae (often called Papilionoideae). The International Code of Botanical Nomenclature allows the use of Leguminosae as an equivalent botanical name to this larger family. This meaning is used by the APG system and many floras.
- Alternatively, it can refer to the subfamily Faboideae treated at the family level. In this circumscription, the other two subfamilies become the families Mimosaceae and Caesalpiniaceae. This circumscription is used in the Cronquist system and elsewhere. The smaller Fabaceae in this system can be referred to as "Papilionaceae", a name also approved by International Code of Botanical Nomenclature.
In consulting any reference that uses the name Fabaceae, care should be taken to make sure what group it applies to.
Subfamilies
The Fabaceae are traditionally classified into three subfamilies (raised in the alternate classification 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 a
re the most showy part of the flower.
- Faboideae or Papilionoideae (Fabaceae sensu strictu or Papilionaceae): One petal (the banner) is large and has a crease in it, the two adjacent petals (wings) are on the sides, and the two bottom petals are joined together at the bottom, forming a boat-like structure (keel).
Nitrogen fixation
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.