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Summary 1. The cells of yeasts and filamentous fungi are surrounded by a cell wall. The cytoplast is bounded by a plasmalemma and contains nuclei, mitochondria, food granules and vacuoles. 2. Cell walls of vegetative fungal cells usually possess a microfibrillar ‘skeleton’, architecturally similar to that of plants, but composed of chitin or hemicellulose. True cellulose predominates only in the Oomycetes. Spore walls are often differently constructed from those of vegetative cells of the same species. 3. The cell contents (plasmalemma, endoplasm and endoplasmic reticulum, nucleus, mitochondria, vacuoles and miscellaneous granular inclusions), as seen in sections and in centrifuged material, are in general similar to those of plants. Some minor differences are indicated. The structure of the flagella of motile cells of the Lower Fungi is fundamentally similar to that of the flagella of lower plants and animals. 4. The electron microscope has contributed to our knowledge of growth and development of fungal structures. It has extended previous knowledge of the process of budding in yeasts, of the development of haustoria and of the formation and germination of fungal spores. 5. Poor nutrition, anaerobic conditions and sublethal irradiation cause abnormalities in the structure of mitochondria. Toxic amounts of irradiation prevent normal nuclear division in budding yeast cells. 6. Electron microscope studies have aided taxonomy by demonstrating fundamental structural differences between Fungi and Actinomycetes and between certain subdivisions of the fungi.

SUMMARYConidia of Botrytis cinerea have a %layered cell wall, a thin electrondense outer layer and a thicker electron-transparent inner one. The cell contents comprise typical nuclei, mitochondria and endoplasm with a surrounding plasmalemma and a sparse endoplasmic reticulum. There are a few small peripheral vacuoles. On germination the outer spore wall ruptures and the emergent germ tube is surrounded by the elastic inner one and what appears to be a sheath of mucilage. A cross wall with a simple central pore is laid down a t the base of the germ tube a t a very early stage. As the germ tube elongates the contents of the spore flow into it, causing a large vacuole to form in the centre of the original spore. The numbers of nuclei and mitochondria suggest that divisions of these organelles take place during the early stages of germination. The mucilaginous sheath surrounding the germ tube appears to condense to give a thin electron-dense outer zone to the wall a t the base of the germ tube.

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