root nodule

Clearly visible swelling that develops in the roots of members of the bean family, the Leguminosae. The cells inside this tumourous growth have been invaded by the bacteria Rhizobium, a soil microbe capable of converting gaseous nitrogen into nitrate. The nodule is therefore an association between a plant and a bacterium, with both partners benefiting. The plant obtains nitrogen compounds while the bacterium obtains nutrition and shelter.

One of the main ways nitrogen in the atmosphere is cycled back into living things is by nitrogen fixation by bacteria. The economic value of the process is so great that research has been carried out into the possibility of stimulating the formation of root nodules in crops such as wheat, which do not normally form and association with rhizobium.