Transmission electron micrograph of a sectioned nitrogen-fixing nodule on a pea root, Pisum sativum, following infection by the bacterium Rhizobium leguminosarum. Infection starts when a bacterium in the soil contacts a root hair. The root responds by producing an infection thread, inviting the bacterium to enter the root. Once inside, the bacterium divides repeatedly, enclosed within a network of plant cell membranes (purple).The plant develops a tumour-like growth on the root surface - a nodule. The bacteria lose their external wall, and differentiate to become pleiomorphic "bacteroids". Fed by the plant, the bacteroids are able to metabolise ("fix") atmospheric nitrogen, to produce ammonium salts. This benefits the plant; the relationship is one of symbiosis. Here, the bacteroids are granular dark purple; the plant nucleus (10 microns wide) is red; and the cell cytoplasm is green.