SUMMARYAn ultrastructural study was made of the vesicular-arbuscular mycorrhiza formed by Endogone (Glomus) mosseae with roots of Allium cepa. The fungal arbuscules consist of highly branched hyphae that have a characteristic reticulate vacuolation. The host plasmalemma surrounds the individual branches of the arbuscule. Around the penetration point and the trunk of the arbuscule a collar of wall material lies between the host plasmalemma and the fungal wall. This layer is absent from the branches. When the arbuscule senesces, the branches collapse and the empty fungal walls aggregate into a clump. Within the clump the fungal walls appear to fragment and disintegrate. The host plasmalemma at this stage surrounds the clump as a whole. When an entire infection becomes senescent, a thick layer of wall material apparently derived from the host encases all intracellular fungal structures.

Ectotrophic mycorrhizas have been studied intensively and their value to some tree species is well known. Endotrophic mycorrhizas have received less attention and their potential value to plants has only been fully substantiated within the last 12 years. Responses of the host to mycorrhizal infection seem always to be associated with improved phosphorus nutrition of the plant. The mechanism whereby this occurs is considered. There is evidence that the ultimate limitation on phosphorus uptake by a simple cylindrical root is the diffusion impedance in the soil around it, and that widely spreading hyphae effectively short‐circuit this impedance. Some data on hyphal length, and estimated net flux of phosphorus through the hyphae are given, with a discussion of possible mechanisms driving this flux.

SUMMARYFour vesicular-arbuscular endophytes were inoculated into onions cultured in soil in a growth chamber. Dry weight, root length, infected root length, phosphorus content and quantity of external mycelium were measured at intervals. Three endophytes produced similar hyphal inflows, growth increments and external mycelium. One endophyte produced no growth increase in the host, and had little external mycelium and slow increase in percentage infection.

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Summary X‐ray microanalysis has confirmed, and measured, the phosphorus content (0·03 g cm‐3) of cytochemically‐demonstrated polyphosphate granules in onion mycorrhizas. The volume occupied by the granules measured from stereoscopic high‐voltage electron micrographs of thick sections was 0·8 % of the fungal volume, and cytoplasmic streaming rates of the extra‐matrical mycelium in soil measured in the light microscope were around 12·6 cm h‐1. From these data a flux rate of about 2·7 × 10‐8 mol P cm‐2 s‐1 has been calculated, which fulfils the hypothesis that phosphorus is translocated in the fungus by means of the transport of polyphosphate granules by cytoplasmic streaming.