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. Legumes attract rhizobia by releasing flavonoids from their roots into the soil. These also trigger synthesis by the bacteria of Nod factors, which cause certain cells in the root to divide and form a primary nodule meristem, which gives rise to the root nodule. Certain nonleguminous plants, such as alder (Alnus) and bayberry (Myrica), also form root nodules, although the bacteria are filamentous actinomycetes (Frankia), not rhizobia.