Fabaceae

The Fabaceae or Leguminosae are a large and economically important family of flowering plants, which is commonly known as the legume family, pea family, bean family or pulse family. The name 'Fabaceae' comes from the defunct genus Faba, now included into Vicia. 'Leguminosae' is an older name still considered valid according to ICBN Art. 18.5 (Vienna Code) and it refers to the typical fruit of these plants: the legume.

It is the third largest family of flowering plants (after Orchidaceae and Asteraceae) with 730 genera and over 19,400 species, according to the Royal Botanical Gardens. The largest genera are Astragalus (more than 2,000 species), Acacia (more than 900 species), Indigofera (700 species), Crotalaria (600 species), Mimosa (500 species).

Glycine max (soya bean), Phaseolus (bean), Pisum sativum (pea), Medicago sativa (alfalfa) are amongst the most well-known Fabaceae.

The family is cosmopolitan.

Taxonomy
The Fabaceae are placed into the order Fabales according to most taxonomical systems, including the APG system.

The Fabaceae comprise three subfamilies (with distribution and some representative species):


 * Mimosoideae: 80 genera and 3,200 species. Mostly tropical and warm temperate Asia and America. Mimosa, Acacia.
 * Caesalpinioideae: 170 genera and 2,000 species, cosmopolitan. Senna, Cassia.
 * Faboideae: 470 genera and 14,000 species, cosmopolitan. Astragalus, Lupinus.

These three subfamilies have been alternatively treated at family level, as in the Cronquist and Dahlgren systems. In this circumscription, the Mimosoideae become Mimosaceae, the Caesalpinioideae become Caesalpiniaceae and the Faboideae become Fabaceae (sensu stricto, s.s.), which can be referred to as Papilionaceae, a name also approved by International Code of Botanical Nomenclature as is 'Leguminosae' for the Fabaceae s.l..

While the Mimosoideae and the Faboideae are largely monophyletic, the Caesalpinioideae appear to be paraphyletic and the tribe Cercideae is probably sister to the rest of the family. Moreover, there are a number of genera whose placement into the Caesalpinioideae is not always agreed on (e.g. Dimorphandra).

Description
Fabaceae can be trees to annual herbs. Amongst the most evident apomorphies there are the pinnately compound leaves with stipules, the flowers with one carpel and five free petals (two of which may be connate), the fruit that is typically a legume.

Leaves
The leaves are usually alternate and compound. Most often they are even- or odd-pinnately compound (e.g. Caragana and Robinia respectively), often trifoliate (e.g. Trifolium, Medicago) and rarely palmately compound (e.g. Lupinus), in the Mimosoideae and the Caesalpinioideae commonly bipinnate (e.g. Acacia, Mimosa). They always have stipules, which can be leaf-like (e.g. Pisum), thorn-like (e.g. Robinia) or be rather inconspicuous. The leaflets have sometimes evolved into tendrils (e.g. Vicia). Their margin is entire or, occasionally, serrate. Both the leaves and the leaflets often have wrinkled pulvini to permit nastic movements.

In many species the leaves have structures evolved to attract ants, that, being predatory, protect the plant from herbivore insects (a foprm of coevolution). Extrafloral nectaries are common among the Mimosoideae an the Caesalpinionideae and are also found in some Faboideae (e.g. Vicia sativa). In some Acacia the modified hollow stipules are inhabited by ants.

Flowers
The flowers always have five generally fused sepals and five free petals. The are generally hermaphrodite and have a short hypanthium, usually cup shaped. There are normally ten stamens and one elongated superior ovary, with a curved style. They are usually arranged in indeterminate inflorescences. Fabaceae are typically entomophilous plants (i.e. they are pollinated by insects) and the flower are usually showy to attract the pollinators.

In the Mimosoideae the flowers are actinomorphic and arranged in globose inflorescences. The petals are small and the stamens, which can be more than just ten, have long coloured filaments which are the most showy part of the flower.

In the Caesalpinioideae the flowers are most often zygomorphic, but very variable, like closely resembling Faboideae flowers in Cercis or symmetrical with five equal petals in Bauhinia. The petals are normally the showy part of the flower.

In the Faboideae the flowers are always zygomorphic and have a specialized structure. The upper petal, called the banner, is large and envelops the rest of the petals in bud, often reflexing when the flower blooms. The two adjacent petals, the wings, surround the two bottom petals. The two bottom petals are fused together at the apex (remaining free at the base), forming a boat-like structure called the keel. The stamens are always ten in number and their filaments can be fused (monadelphous flower) or in a group of nine stamens plus one separate stamen (diadelphous flower).

Fruit


The ovary most typically develops in a legume. A legume is a simple dry fruit that usually dehisces (opens along a seam) on two sides. A common name for this type of fruit is a "pod", although also applied to a few other fruit types. Much rarer are samarae, loments, follicles, indehiscent legumes, achenes, drupes, berries. 

Roots
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.

Uses
The history of legumes is tied in closely with that of human civilization, appearing early in Asia, the Americas (the common bean, several varieties) and Europe (broad beans) by 6,000 BC, where they became a staple, essential for supplementing protein where there was not enough meat.

Their ability to fix atmospheric nitrogen reduces fertilizer costs for farmers and gardeners who grow legumes, and means that legumes can be used in a crop rotation to replenish soil that has been depleted of nitrogen. Legume seed and foliage has a comparatively higher protein content than non-legume material, due to the additional nitrogen that legumes receive through the process.

Farmed legumes can belong to numerous classes including forage, grain, blooms, pharmaceutical/industrial, fallow/green manure and timber species, with most commercially farmed species filling two or more roles simultaneously.

There are of two broad types of forage legumes. Some, like alfalfa, clover, vetch, Arachis, are sown in pasture and grazed by livestock. Other forage legumes such as Leucaena or Albizia are woody shrub or tree species that are either broken down by livestock or regularly cut by humans to provide stock feed. Grain legumes are cultivated for their seeds, and are also called pulses. The seeds are used for human and animal consumption or for the production of oils for industrial uses. Grain legumes include both herbaceous plants like beans, lentils, lupins, peas and peanuts. and trees such as carob and tamarind.

Bloom legume species include species such as lupin, which are farmed commercially for their blooms as well as being popular in gardens worldwide. Laburnum, Robinia, Gleditsia, Acacia, Mimosa, and Delonix are ornamental trees and shrubs.

Industrial farmed legumes include Indigofera, cultivated for the production of indigo, Acacia, for gum arabic and Derris for the insecticide action of rotenonte, a compound it produces.

Fallow or green manure legume species are cultivated to be tilled back into the soil in order exploit the high nitrogen levels found in most legumes. Numerous legumes are farmed for this purpose including Leucaena, Cyamopsis and Sesbania.

Various legume species are farmed for timber production worldwide including numerous Acacia species, Erythroxylum and Castanospermum australe.

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.

References and sources

 * 1) Chapphill 1994; Tucker & Douglas 1004; Doyle 1983; Doyle & al. 1997.
 * 2) J. J. Doyle & al. 2000 and references; Bruneau & al. 2001