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SUMMARY

Light and electron microscopy have been used to study the emergence through the spore wall and subsequent growth and development of the germ-tube of Gigaspora margarita. Spore-wall penetration by the germ-tube is unusual as two different mechanisms seem to be operating, one for the inner spore-wall layers and another for the outer layer. As the germ-tube develops a primary wall layer is formed, and this can be traced back to its origin as a layer continuous with the thickened region of the innermost spore-wall layer. In the maturing germ-tube a highly osmiophilic secondary wall is deposited and this may be the wall layer that persists when the fungus penetrates a host root and sets up a mycorrhizal association.

In common with other fungi that have been studied, the ultrastructural organization found in G. margarita can be categorized into approximately three zones: the apical zone, the subapical zone and the zone of vacuolation. Other features of the cytoplasmic organization are also discussed including the possible significance of bacteria-like organisms, membrane-bound crystals, osmiophilic granules and glycogen particles. Features of the culture-grown germ-tubes are also compared with those of host-associated stages of the fungus reported in the literature.