Nodule Formation. Nodule formation involves a sequence of multiple interactions between Rhizobium and roots of the host plant. Principal stages in the nodule formation are summaried as follows: Rhizobia multiply and colonise the surroundings of roots and gets attached to epidermal and root hair cells. The root hairs curl and the bacteria invade the root hair. An infection thread is produced carrying the bacteria into the cells which leads to the differentiation of specialised nitrogen fixing cells. The nodule thus formed, establish a direct vascular connection with the host for exchange of nutrients. Development of root nodules in soyabean:(a) Rhizobium bacteria contact a susceptible root hair divide near it. (b) upon successful infection of the root hair cause it to curl. (c) Infected thread carries the bacteria to the inner cortex. The bacteria germ modified into rod-shaped bacteriods and cause inner cortical and pericycle cells to divide. Division and growth of cortical and pericycle cells lead to nodule formation. (d) A mature nodule complete with vascular tissues continuous with those of the root.