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SUMMARY A quantitative light and electron microscope study of developing and degenerating arbuscules of the mycorrhizal fungus Glomus fasciculatum (Thax. sensu Gerd.) Gerd. (2) V v ,(cy, c), the fraction of the host cell volume occupied by host cytoplasm (cy); (3) S v (p,c) the surface area‐to‐volume ratio of the host protoplast (p) to the whole host cell. Uninfected cortical cells of wheat had an S v ,(p,c) of 0.11 μm 2 μm −3 and those of oats 0.10 μm 2 μm −3 . As the fungus penetrates the cell wall, the protoplast invaginates, causing a decrease in protoplast volume and an increase in S t . In wheat this increase reached 1.17 μm 2 μm −3 and in oats, 1.04 μm 2 μm −3 . When the arbuscule is mature, the fungus occupies 35% of the cell in wheat with 20% as branches and 15% as trunk. In oats, the fungus occupies 36% of the cell, with 23 % as branches and 13 % as trunk. In wheat, host cell cytoplasm was initially 39 % and increased to 21.6% and in oats, from 3.6 to 28.8%. These values are similar to those observed in Zea mays . The rates at which the parameters V v (f,c) and S v (p,c) changed during arbuscule development and degeneration were similar. The amount of encasement and host cell cytoplasm showed the greatest variation among host species.
Alexander et al. (Tue,) studied this question.