root nodule

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root nodule (actinorrhiza) A small, gall-like growth on the roots of certain types of plant, especially leguminous plants. The nodules develop as a result of infection of the roots by bacteria (Rhizobium or Bradyrhizobium species in the case of legumes); the bacteria then inhabit the root nodules and benefit the plant by fixing atmospheric nitrogen, much of which becomes available to the plant.

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