Nodulation begins when rhizobia attach themselves to epidermal cells. Epidermal cells with immature or as yet unformed root hairs are the usual sites for bacterial penetration (Bhuvaneswari et al.1980). Prior to attachment, communication between the two symbiotic partners, soybean plant and B. japonicum bacteria, is required and a certain minimum period of contact is needed. Infected hairs are always shorter than mature intact hairs, due to marked curling upon infection. At the point of infection, the root hair wall forms a depression that invaginates deeply, forming an infection thread lined by a continuation of the root hair cell wall and membrane. Infection threads may branch within a root hair (Turgeon and Bauer 1982). The infection thread, with its included dividing bacteria, grows 60 to 70 um to the base to the root hair cell. The cortex adjacent to infected root hairs becomes meristematic and produces a wedge-shaped area of dividing cells even before any infection threads enter (Turgeon and Bauer 1982). These mitoses increase cell number in the cortical layer, which then becomes the main area of infected cells ( Newcomb et al. 1979). The combination of multiple threads and branching of threads in the cortex results in penetration of many, but not all, of these cells. The peripheral uninfected area becomes the nodule cortex, which includes a scleroid layer and several vascular bundles.At some time during or following mitotic activity, rhizobia are released into cortical cells through thin areas on the tips of the infection threads. Bradyrhizobia are called bacteroids after their release into the host cell. Their cell walls have been considerably modified or, in the case of peanut entirely removed (Werner and Mörschel 1978). Mitosis in infected cortical cells ceases about 14 days after infection. Subsequent increases in the volume of infection tissue are due entirely to cell enlargement. As the nodule matures, oxygen-binding leghaemoglobin develops gradually in the host tissue and the nodule becomes pink, remaining so until it begins to senesce. As leghaemoglobin forms, bacteria cease dividing, and dinitrogen fixation commences (Lersten and Carlson 1987).