Abstract:Bacteroids, in numbers of two, three or more, in the legume nodules of Sesbania cannabina (Retz.) Pers., Glycine max (L.) Merr. and Pisum sativum L. were enclosed in each peribacteroid membrane (PBM). In view that PBM was formed synchronously with the bacteroid reproduction, the origin as well as the expansion of PBM ought to be in large scale in order to meet the need of the physiologic process of symbiotic nature. It was observed that in the same nodule sample there were kinds of modes of the PBM expansion, such as chimeric fusion between, or among the PBMs in which a protrusion of one PBM fit into a depression of another PBM, fusions of the PBM with endoplasmic reticulum (ER) and its vesicles, with small vacuole membrane, etc. Cytochemical study indicated that PBM, plasma membrane, ER membrane, etc. possessed the same type of ATPase, and the PBM was structurally similar to the plasma membrane. Therefore, the PBM appears to be a mosaic membrane possessing features of the plasma membrane, ER membrane and vacuole membrane, etc. In final, the physiological significance of the diversity in the origin and expansion modes of the PBM was discussed.