Abstract:Frankia hyphae penetrate root ha rs and invade root cortical cells of Coriaria nepalensis Wall., leading to nodulation. Owing to the stimulation caused by infection, a part of the cortical cells in the infected root divide and hypertrophy, resulting in the formation of a prenodulein which the nodule primordium develops. The primordium divides and differentiates, forming a primary nodule lobe. The apical meristem of a lobe hranchs dichotomously and the branchings develop with the infection of new host cells by the endophytes, giving rise to a coralloid root nodule consisting of many dichotomic branchings. In cross section, the endophyte-infected tissue zone in a lobe is a horse-shoeshaped compactmass, incompletely surrounding the acentral stele. In longitudinal section, the apical part of a mature lobe is the meristem, followed successively by the non-infected tissue, the newly infected tissue, the mature infected tissue and the senescent infected tissue. The lobal stele is surrounded by several layers of cortical cells containing starch granules.