Bacterium‐like Organelles in the Vesicular‐arbuscular Mycorrhizal Fungus Glomus Caledonius

S ummary Bacterium‐like organelles (BLOs) were found by electron microscopy in the vesicular‐arbuscular (VA) mycorrhizal fungus Glomus caledonius. They were morphologically similar to those found by others in other V A mycorrhizal fungi, in the ectendomycorrhizal fungus Endogone flammicorona and in the Discomycete Scutellinia. They occurred free in the cytoplasm of Glomus caledonius reproductive spores and in the intercellular hyphae and thick arbuscule branches in infected roots. They were irregularly coccal with diameters of 0·3 ± 0·06 μm and had a conspicuous cell membrane and simple cell wall and appeared to divide by median constriction and separation. The fungal cytoplasm did not show any cytopathic effects in the vicinity of the BLOs. A competely different type of bacterium colonized and eroded the walls of reproductive spores of Glomus caledonius. These bacteria did not enter the fungal cytoplasm and had a thick complex wall and peripheral membranes. In contrast to BLOs, their cytoplasm was clearly differentiated into electron‐dense and transparent areas.