The Ultra-structural characteristics of the vacuoles in the infected cells of Onobrychis viciifolia root nodules were studied by transmission electron microscopy. The results showed that the vacuoles of the infected cells contained a small amount of fiber-shaped inclusions at the early developmental stage. While the infected cells developed, the vacuoles increased in size and contained more and more fiber-like and membranous substances. After the central vacuoles appeared, the fiber-like substances declined...