Scanning electron microscopy (SEM) of Pinus sp. decayed by the brown-rot fungus Postia placenta confirmed the existence of extracellular membranous structures previously described by transmission electron microscopy (TEM). These structures appear to be an integral part of the hyphal sheath and assume a variety of forms including lamellar sheets, fibrils, and vesicles. These structures were observed (a) on the surface of hyphae, (b) extending from hyphae into the wood surface and covering the S3 layer, (c) embedded in the hyphal sheath and, (d) penetrating into the wood cell wall layers from S3 - S3. We conclude that penetration of the wood cell wall by the complex, multistructured hyphal sheath facilitates the decay process in an orderly and linear progression. The directionality of the decay process appears to alter the fiber orientation of individual wood cell wall layers. This model offers an alternative hypothesis to simple diffusion of wood degrading agents during wood decay by Postia placenta.

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