| Summary: | 1. The septal structure of <i>C. cinereus</i> and <i>N. crassa</i> was examined via fluorescence microscopy and transmission electron microscopy. The septa of these organisms were shown to contain two plates of chitin, not one, as had been previously believed.2. The effect of the fluorochrome Calcofluor White on chitin synthesis <i>in vivo</i> and <i>in vitro</i> was investigated. Growth of <i>C. cinereus</i> in media seeded with the dye caused abnormal deposition of chitin at points of growth i.e. tips, septa, hook cells and clamps. However, high concentrations of the dye were required to have an effect on the chitin content and growth rate of the mycelium. X-ray crystallography showed that chitin synthesized in the presence of the fluorochrome was less crystalline than chitin synthesized when the dye was absent. It would appear, therefore, that Calcofluor binds to nascent chitin inhibiting its crystallization into microfibrils. It was concluded that chitin synthesis is a two-step process involving polymerization and subsequent crystallization.3. A range of fungi and growth forms were probed for the presence of actin using the actin-specific fluorochrome rhodamine phalloidin. Only <i>U. phaseoli</i> germlings and <i>C. albicans</i> yeast cells appeared to contain the protein. It is suggested that the failure to visualize actin in the other organisms examined is linked to their cell wall structure which prevented penetration of the rhodamine phalloidin to sites of actin localization.4. In the light of these results, mechanisms of hyphal growth are discussed and a model for the polarity of apical extension is presented.
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