Abstract: Aided by the techniques of ultrathin section and electron microscopy, the changes of cell ultrastructure of seed germination of Bletilla striata was investigated. It was found that the seeds of B. striata were germinated easily among Orchidaceae, the large thin-wall cells of embryo contained a great amount of starch and lipid materials which were nutrients of seed germination. The differentiation and growth of the nutritive organ of B. striata would be accelerated after fungus infection. Fungus could penetrate seed coat in cell interstice, invaded， embryo through suspensor, and interspersed in several layers of embryonic ceils. Cytoplasm and organelles of embryo would disappear after the cells infected by fungus, and then endocytic plasmalemma produces some sacs enclosing and digesting hyphae. The hyphae with its tip inflation, relying on expansion-pressure of its cytoplasm, could penetrate cell wall of embryo. In addition, after invasion hyphae wall was significantly thickened by the deposition of the embryonic cytoplasm. In the end, the hyphae would be wholly collapsed and its degradated materials were used as nutrient for germination. The cells without penetration' by hyphae show vigrous metabolism activity in protocorms.

Key words: Bletilla striata, Fungus infection, Seed germination, Ultrastructure