Pisonia mycorrhizas found in Seychelles possessed transfer cells next to the fungal sheath and Hartig net. There were extensive wall-ingrowths on outer-tangential and radial walls of epidermal cells, and on parts of cortical cells, in contact with the mycorrhizal fungus. In general structure, having basidiomycete sheath, transfer cells, and Hartig net between epidermal cells only, the mycorrhizas resembled those from the Australian Great Barrier Reef; Seychelles material had poorer definition of the layers in the sheath and a better-developed Hartig net. Mycorrhizal development was best at sites with coral sand, the characteristic habitat for P. grandis. At the one site which had soil derived from granite, only one mycorrhiza was found, although many roots were examined and most of these had hyphae at the root surface. In the older mycorrhizas the transfer-cell walls changed from red-purple staining to blue with toluidine blue (pH 4-4), probably because of incorporation of phenolic material; the sheath showed damage and increased impregnation with dark-coloured phenolic material. From these changes it is suggested that the mycorrhizal association is likely to be functional for only a short distance back from the tip. Comparison of mycorrhizal and non-mycorrhizal roots suggested that the transfer cells were induced by the fungus, but that a complete sheath was not necessary for their formation. The transfer-cell wall-ingrowths were clearly distinguishable from ‘papillae’ formed in fungal invasion, by their morphology, staining properties, and appearance in the electron microscope. The unique structure of the Pisonia mycorrhiza puts it in a category of its own, and its discovery in Seychelles implies that the mycorrhizal habit is widespread in this species.
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