To investigate the routes of U. virensinfection and analyze the infection effects on rice flower development, we 
performed semi-thin sectioning of smut balls. The cross- 
section showed the stigma with two branches were enclosed by hyphae (Figure 4(a)). High magnification 
showed hyphae could not breach the epidermal cells of 
the stigma (Figure 4(b)), even the stigma were densely 
covered by hyphse. In transverse section of smut balls, 
mycelia compactly packed the stigma and deformed the 
papillae. Hyphae entered the intercellular spaces of stig- 
ma via the thin-walled cells ofpapillae, they only breach- 
ed at the sites of the plumose part of the stigma, but 
could not reach the vascular bundles (Figure 4(c)).