Leguminosae

In botany, the pea family, which includes over 16,000 species of dicotyledonous trees, shrubs, climbers, and herbaceous plants. These plants are particularly valuable as crops because they do not require nitrogenous fertilizer, and even enrich the nitrogen content of soil.

Characteristics The family is characterized by the fruit, technically known as a legume, which develops from a single carpel, and when ripe opens along both margins to release the seeds; sometimes, as in gorse, the seeds are thrown out violently by sudden contraction of the sides of the legume, or pod. In some cases the fruit breaks transversely, as in horseshoe vetch, or does not open at all, as in clover.

The leaves of the Leguminosae bear stipules (outgrowths at the base of the leafstalk) and are often compound, usually pinnately compound, sometimes with tendrils at the leaf tips.

Another distinctive feature of the family is the presence of root nodules containing rhizobium bacteria, which fix atmospheric nitrogen, converting it into forms useful to the plant; see nitrogen fixation.

Subdivisions The Leguminosae are generally divided into three subfamilies, which are sometimes raised to family rank. The Mimosoideae generally have small, radially symmetrical flowers, often with many stamens; the best-known genus is Acacia. In the Caesalpinioideae the stamens are usually few in number, and the five petals are differentiated to some extent into an upper standard, two lateral wings, and two lower keel petals, which overlap the outside of the wings. The Caesalpinioideae are mainly tropical woody plants, though some species of Cassia and the Judas tree Cercis siliquastrum are hardy cultivated ornamentals in temperate regions. In the Papilionoideae, the standard, wings, and keel are clearly differentiated, the wings overlapping the outside of the keel petals; there are ten stamens, nearly always joined together, either all ten together or nine joined and one separate.