Nodules on a legumious plant

Source of the photo: 
http://soils.usda.gov/sqi/concepts/soil_biology/images/Nods_on_beans_LR.jpg
Author of the description: 
KÖRINFO
Nodules

 

In the picture we can see the mutualistic association between nitrogen fixing bacteria and legumious plants. Nitrogen-fixing  Rhizobium bacteria colonized on the root hairs of clover plants. Nodules containing Rhizobium bacteria on the plant roots through the combined interaction between the plant and the bacterium. In the nodule the bacteria fix nitrogen, which they share with the plant. In exchange, the plant supplies the bacteria with a source of carbon and energy for growth.