Legumes

Legume species represent a wide variety of growth forms, ranging from annual plants to herbaceous perennials to woody shrubs, vines, and trees.

The leaves of legumes are typically arranged alternately on the stems and are commonly compound, meaning that each leaf is composed of several to many leaflets arranged along a central stalk. In some herbaceous, climbing species of legumes, some of the leaflets are modified into spirally winding, clinging organs known as tendrils.

The flowers of legumes are bilaterally symmetric and are generally arranged into groups known as inflorescences. The five petals are modified into distinctive structures. The top-most petal is called the banner or standard, the two lateral petals are called wings, and the bottom two are fused into a structure known as the keel, which encloses the ten stamens and single pistil of the flower. Legume flowers are usually brightly colored, and they contain nectar and are often scented. All of these are adaptations for attracting flying insects who are the pollinators of the flowers of legumes.

The fruits of legumes are dry or fleshy, multi-seeded structures known as legumes or pods. The fruits and seeds of some legume species are highly nutritious because of their large concentrations of protein. The seeds of some species, however, contain toxic alkaloids and can be poisonous.

About one-half of the species of legumes have bumpy nodules present on their roots which house symbiotic (or mutualistic) bacteria that have the ability to metabolically fix atmospheric nitrogen gas into ammonia which can be utilized by the plant as a nutrient for the synthesis of proteins. The bacteria responsible for nitrogen fixation in legumes are in the genus Rhizobium with separate strains or species associated with each species of symbiotic legume. The Rhizobium bacteria have the ability to synthesize a chemical known as nitrogenase which is an enzyme that can cleave the very strong triple bond of nitrogen gas (N₂) so that ammonia (NH₃) is produced. Because nitrogen gas is otherwise inert to biological reactions, while ammonia (as ammonium ion, NH ⁺ 4 ) is a chemical that plants can easily utilize in their nutrition, nitrogen fixation is an extremely useful function. Legumes that have symbiotic Rhizobium living in their root nodules may have important ecological advantages in competition with other types of plants, especially if they are growing in otherwise nitrogen-poor habitats.