Legumes refer to all members of the plant family Fabaceae, formerly called Leguminosae. Huge variety exists, as legumes can be either trees, shrubs, herbs or climbing vines. Some species are poisonous while others provide forage or grain food for both wildlife and humans. Legumes can be found growing naturally on all continents except Antarctica.
Regardless of plant species, a vast number of all legumes bear compound leaves--leaves comprised of leaflets that number from as few as two to hundreds. While others bear singular entire leaves or have a leaf that is trifoliate (having three lobes). The leaves are arranged alternately on the stems or branches. At the base of the leaf stem (petiole) is a swollen joint called a pulvinus, according to Kirsten Albrecht Llamas, author of "Tropical Flowering Plants."
Legumes produce any of three differently shaped flowers, and botanists further group legumes into subfamilies based on this flower morphology. Plants in Caesalpinioideae develop flowers that have one petal that looks like a lip, like in a royal poinciana. Those in Mimosoideae look like little puffballs with petals reduced and lots of hair-like stamens, such as a mimosa. Papilonoideae legumes produce blossoms that are "butterfly-like", with five petals--one is a banner, two form lateral wings and the the bottom two fuse to create a keel, such as those of sweet peas and beans.
Regardless of flower shape, legume plants produce a seed pod (the "legume") that splits open along both sides to reveal and release the seeds. Seed pods that split cross-wise between the seeds are known as "loments" and occur much less commonly among plants in this family.
A large number of legume plants convert atmospheric nitrogen into useful soil compounds. On the roots of legumes you may see small nodules that containing bacteria of the genus Rhizobium. These bacteria have a symbiotic relationship with the legume plants, fixing nitrogen for the plants to grow. According to the International Legume Database & Information Service this feature of legumes allows them to grow well in soils so nutrient-poor that many other plants fail. Interestingly, root nodules are quite widespread among plants in the subfamily Mimosoideae and Papilionoideae, but rarely in legume plants in the Caesalpinioideae.