Desmodium Illinoense Dalea purpurea
L: Illinois tick trefoil R: Purple prairie clover
Legumes are flowering plants (angiosperms) in the bean family (Fabaceae) that play a crucial role in transforming elemental nitrogen into a form that is usable by plants and animals. Nitrogen is essential for all living organisms. It is a component of proteins and of DNA. The atmosphere is 78% nitrogen but most organisms cannot use nitrogen directly from the atmosphere.

Nodules on the roots of legumes contain bacteria in the genus Rhizobium that ‘fix’ nitrogen into a form that other organisms may use directly, or that may convert to nitrate, the form most plants prefer. Farmers often rotate crops that deplete the soil of nitrogen with legumes which restore nitrogen to the soil.

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http://exhibits.museum.state.il.us/exhibits/midewin/legumes.html, Last modified September 1st 2011, 03:13PM.