root nodule A swelling on the roots of certain plants, especially those of the family Fabaceae (Leguminosae), that contains bacteria (notably Rhizobium) capable of fixing atmospheric nitrogen into ammonia, which is subsequently converted to nitrates and amino acids (see bacteroid; nitrogen fixation). Plants that possess root nodules increase soil fertility by increasing the nitrate content of the soil. The practice of crop rotation will normally include the cultivation of a leguminous species. Certain nonleguminous plants, such as alder (Alnus) and bayberry (Myrica), also form root nodules, although the bacteria are filamentous actinomycetes (Frankia), not rhizobia.

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.

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.

root nodule Small swelling in the roots of various plants, such as legumes, that contain nitrogen-fixing bacteria. See also nitrogen cycle; nitrogen fixation