Fabaceae is the third largest family of angiosperms in the world. Combined, they have 650 genera and over 18,000 species. This family is divided into 3 sub-families called Papilionoideae, Mimosoideae, and Caesalpinioideae. These versatile plants can live on just about any continent. They are very useful economically with humans and also in the wild. They are most famous for their capabilities with nitrogen fixation.
There is much diversity within this Family. While most are woody perennial species, there are many that are herbaceous and even some that are aquatic, though none that are completely submerged in water. Each plant has 5 sepals and 5 petals. Usually 10 stamen, either fused together or in 2 groups, form cups at the base of the ovary.
The seeds range in characteristics depending on the specific plant. Some are small sizing up to only a few millimeters and others are larger and can grow to more than 30 cms. Some are single seeded while others have many. Many of them are also brightly colored while others are more dull. Seeds may or may not split open either, for example, the peanut does not.
Papilionoideae plants are mostly herbaceous and have simple leaves. They have ten stamen in each of the two clusters. Mimosoideae are made up of trees and shrubs that are bipinnately compound (double branching). They have leaves that are radial symmetrical to the flowers. Specific to them are the 10 or more stamen that grow beyond the petals. With mostly woody plants, the Caesalpinioideae sub-family have leaves that are also pinnately compounded. They hold 10 stamen to one cluster. 
These plants are classified as angiosperms, which means they are flowering plants that produce seeds enclosed within a flower containing ovaries. This protects the seed until the ovary can fully develop into a fruit, which is tissue surrounding the seed. In terms of this family of plants the legume is the fruit in which the seed is held. The actual legume is a single-chambered seedpod that is flattened with two sutures. 
The legume is formed by a superior ovary. It has two sides that are able to be split apart. Once these seed parts are detached from each other, they release seeds. These seeds are normally attached to one side of the legume, occassionally they may appear on both. Dispersal of these plants are due greatly to animals that eat it and then excrete the seeds in different land regions. They are also dispersed by water and wind.  The reproduction level may vary from plant to plant depending on its life span. Some legumes are annuals, living and dying within a year, while others are perennials, alive over 2 years.
Since legumes have such a diverse variety, species can be found all over the world. They range where ever the habitat is suitable, which can be on any continent aside from Antarctica. The trees of the family are found mostly in temperate and tropical regions. On the other hand, shrubs and some trees are found in desert and semiarid areas.  The need for nitrogen in their metabolism causes them to live in more arid to semi-arid conditions. 
Nitrogen fixation is part of what makes the Fabaceae family so important in its ecology. Most members are able to make nitrogenous compounds that can be used by other plants by converting atmospheric nitrogen. This is made possible by the symbiotic relationship the legume has with a bacteria. The roots pick-up a bacteria from the genera Rhizobium and form root nodules. These nodules appear as visible external bumps on the roots. As the legumes provide fixed nitrogen for plants surrounding it they also supply the bacteria with carbon produced through photosynthesis. This process increases the plants survival and gives it a higher competition rate in nitrogen poor areas.
Legumes play a huge role in today's economy - ranking second as the most important group of crops next to Paceae. Some of these food crops include beans, peas, peanuts, soybeans, and lentils. Beans are a good source of protein and vitamins. Soybeans are even considered one of the healthiest foods in the world because they are low in cholesterol and fat. They can lower cholesterol, blood pressure, increase bone density, and reduce the risk of some cancers if eaten in place of meat. When people don't eat food coming from animals, vegans or vegetarians, they look to legumes to get their nutrients. 
Not only are they used as food for humans, but they are also a major contributor to farm life. They can be used as animal feed or even green manure.  Farmers also take full advantage of their ability to produce nitrogen compounds for other plants. By rotating them with other plants they can improve the nutrients in the soil. This action saves farmers billions of dollars by not having to spend money on nitrogen fertilizers. 
They have many other purposes as well such as medicinal and insecticides. Peanuts can also be made into a variety of things including paint and varnish polish.
- Fabaceae Wikipedia.
- Fabaceae Page Ben Rebein. Washburn University.
- Legume Marshall R. Crosby, Ph.D. Microsoft Encarta Online Encyclopedia 2008.
- Leguminosae Wichard White. ILDIS.
- Fabaceae legumes Wojciechowski, Mahn, Jones. The Tree of Life Web Project.