Abstract: There were two forms of rhizobial bacteria present in infected host cells of nodules. One was bacteroids which were enclosed in peribacteroid membrane originated from the infected host cells. The other was rhizobia as vegetative cells. The infected host cells were occupied by most of the bacteroids and a certain number of the vegetative cells respectively. With the nodule senescence, there were two kinds of fate of the bacteria: The bacteroids degenerated togather with the infected host cells at the same time and further disintegrated completely, so it is not possible that the disintegrated bacteroids could be returned into soil to revive: the vegetative cells did not disintegrate and die when the infected host cells senesced, eventually could be turned back into soil. The vegetative cells may play an important role, on the one hand, in cycle between legume and soil, on the other hand, maintain rhizobia in natural balance of population ecosystem.

Key words: Legume, Nodule, Rhizobium, Nodule senescence, Ecological balance