ELECTRON MICROSCOPY OF UREDOSPORES OF MELAMPSORA LINI AND OF RUST-INFECTED FLAX

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Abstract
Thin sections of germinated and ungerminated uredospores of the flax rust fungus (Melampsora lini (Pers.) Lev.) and rust-infected cotyledons of flax (Linum usitatissimum L., variety Bison) were examined in a Philips 100B electron microscope. The line structure of the mature uredospores, the formation of germ tubes including the development of cross walls, and of the intercellular mycelium, haustoria, sporogenous hyphae, and developing uredospores are all briefly described. Germinated and ungerminated uredospores were fixed in KMnO4 followed by OsO4 at room temperature. Infected tissue was fixed in KMnO4 at room temperature or in glutaraldehyde at 4 °C followed by OsO4 at 4 °C. Nucleoli were not observed in the nuclei of mature uredospores or germ tubes but were present in the intercellular mycelium, sporogenous hyphae, and developing uredospores and were particularly prominent in the two haustorial nuclei. The results are discussed and attention is drawn to the need for further work on the nuclei and nucleoli in germinating uredospores under conditions conducive to the development of infection structures.

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