Legume root nodules. Light micrograph of sections through two root nodules from a broad bean plant (Vicia faber). These root nodules are full of nitrogen-fixing bacteria, and are characteristic of the family Leguminosae, which includes the legume vegetables such as broad beans and peas. Nitrogen-fixing is where the bacteria use nitrogen gas in the air to form nitrates, along with other nitrogen-containing chemicals. These are then used by the plant in its metabolism. This is a vital part of the nitrogen cycle, by which atmospheric nitrogen passes from the air to living organisms.