To the cook, a legume is a pea or bean, used to provide protein in dishes, but to a botanist or gardener, legumes serve a very different purpose. The legume family is one of the largest families of flowering plants, containing a diverse array of trees, shrubs and herbaceous plants. What legumes have in common is a unique relationship with soil bacteria that allows them to "fix" atmospheric nitrogen and makes them important agricultural and garden plants.
The legume family is highly diverse but shares several traits that aid in identification. Leaves tend to have a slight swelling at the base, where the leaf joins the stem. Flowers consist of five separate petals, arranged as a "banner" and "wings." The banner is the large petal at the top of the flower, and the wings are two smaller petals beneath. The remaining two petals enclose the flower's reproductive structures. One of the most important traits of legumes is the production of seeds inside of a pod.
The fossil record indicates that legumes were established and well diversified by the early Tertiary Period, 65 million years ago. This time period is often called the Age of Mammals but, as pointed out on the University of California-Berkeley's website Introduction to the Cenozoic, it could be just as easily called the Age of Flowering Plants. The legume family represents just one plant family that expanded rapidly during that time.
Nitrogen is an essential nutrient for plants and forms the bulk of the Earth's atmosphere but exists in a stable form that can't be broken down and used by plants. One of the defining traits of legumes is their ability to "fix" atmospheric nitrogen, converting stable molecules into compounds that plants can use. Nitrogen fixation occurs with the help of rhizobia bacteria that take up residence in the roots of legumes, seeking shelter in exchange for the nitrogen they provide the legumes.
The ability of legumes to fix nitrogen has important consequences for farmers and gardeners. Plants draw nitrogen constantly from the soil and, unless it is replenished, will eventually deplete their supply of it. While legumes mostly take up the nitrogen fixed in their roots, small amounts replenish the soil as well, and because the plants themselves are rich in nitrogen, turning legumes into the soil at the end of the season provides nitrogen for future crops. For this reason, legumes are incorporated into crop rotations and planted as "green manures," crops planted during the winter or fallow season that restore soil fertility.
Legumes can be found in both temperate and tropical areas, although they grow primarily in arid habitats or regions with a dry season. According to the Tree of Life website, the ability to fix nitrogen likely explains the ability of legumes to withstand the challenges of arid climates.