The fine structure of actively growing hyphae ofColletotrichum graminicolawas investigated. The hyphae were found to secrete a mucilaginous sheath into which melanizing particles and fibrils were deposited. Classical ubiquitous mitochondria, multipored doublewalled nuclei, ribosomes, mesosome-like structures, lomosomes, vacuoles, and inclusions such as lipids, closely associated with vacuoles, especially noticeable during germination, as well as glycogen were present. During maturation, vacuoles appeared and enlarged. In addition, a few endoplasmic reticulum profiles were observed in actively growing hyphae. Mitochondrial profiles changed from spherical to elongate. A plasmalemma, perhaps double in nature, was present. Penetration of one hypha by another was also observed.The mature appressorium exhibited a thick three-layered cell wall. This thick wall appeared to play a part in poor fixation and embedding, resulting in a poor definition of such cellular organelles as ribosomes, nuclei, etc. Internally, appressoria, depending on the stage of development, exhibited many vacuoles and osmiophilic deposits indicative of lipid reserve material. A germ pore through which a germ tube and migrating organelles and cytoplasm emerged was also observed.
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