The growth of most plants depends on the presence of sufficient amounts of nitrogen contained in the soil. However, a family of plants, the legumes, is partially free of this constraint thanks to its ability to live in association with soil bacteria of the Rhizobium, genus, capable of fixing nitrogen from the air. When these bacteria come into contact with their host plant, they trigger in the roots the formation and development of organs, termed nodules, where they continue to live. This close relationship is symbiosis, which benefits both organisms involved: the plant supplies nutritive elements to the bacteria which in return pass on the nitrogen they have stored up.