The interface between Erysiphe pisi and its nonhost, barley, was investigated by transmission electron microscopy. Coleoptiles of barley inoculated with the fungus were fixed at predetermined time intervals: (1) before penetration; (2) 30min after initiation of coleoptile cytoplasmic aggregation; (3) 15-16hr after cytoplasmic aggregation. When appressoria of the fungus matured on the surface of coleoptiles, the appressoria cell wall was thin and flat where the appressorium was in contact with the coleoptile surface. At this stage of development, no noteworthy changes occurred in coleoptile cells below appressoria. In specimens fixed 30min after the initiation of cytoplasmic aggregation, short penetration pegs from appressoria, breached the coleoptile cell wall and were surrounded by and embedded with papillae. The papillae at this stage were mostly electron-dense with some heterogeneous, electron-lucent regions. A large number of spherosomes appeared around the margin of papillae in cytoplasmic aggregates. Some of spherosomes were attached to the margin of papillae and probably allowed their contents to be deposited into the papillae. In specimens fixed 15-16hr after the initiation of cytoplasmic aggregation, prominent papillae were always found below appressoria which apparently had attempted to penetrate the cell wall of coleoptile. These papillae consisted of multiple, alternately electron-dense and -lucent layers. This suggests that osmiophilic and non-osmiophilic materials were deposited alternately during papilla development which occurred in a period of 15-16hr after the initiation of cytoplasmic aggregation. Thus, the gradual development of papillae probably depends on such a continuous deposition of materials.
