A special tumor growing in the roots of plants is formed by the establishment of nitrogen-fixing bacteria in parasitic tissues. Used to synthesize their own nitrogen-containing compounds.

There is a large amount of molecular nitrogen in the air, accounting for about 80% of the air composition. It is estimated that there are more than 4000 tons of molecular nitrogen in the whole atmosphere. However, most plants can only absorb bound nitrogen from soil to synthesize their own nitrogen-containing compounds, such as protein.

Nitrogen compounds in soil are not inherent in the soil itself, but gradually accumulated in the process of biological life activities, and a large part of them come from the biological nitrogen fixation of microorganisms. It is estimated that the total amount of biological nitrogen fixation on the earth's surface is about 100 tons every year, of which the amount of nitrogen fixation by leguminous rhizobia is about 55 villages, accounting for about 55% of the total amount of biological nitrogen fixation.

Rhizobia divide and multiply rapidly in cortical cells, and cortical cells also divide and grow rapidly due to the stimulation of nodule invasion, which expands the local volume of roots and forms tumor-like processes, which are nodules. Rhizobia absorb the water and nutrients they need for their life from nodule cells. It can fix free nitrogen and synthesize nitrogen-containing compounds, which are used by leguminous plants. The growth of leguminous plants and rhizobia obtained high yield due to nitrogen; At the same time, due to the shedding of nodules, roots or residual plants with nodules remain in the soil, which can increase soil fertility. Because of the nitrogen fixation of rhizobia, leguminous plants can fertilize fields.