WOOD DECAY IN SILICIFIED GYMNOSPERMS FROM ANTARCTICA

Gymnospermous secondary xylem degraded by wood-decaying fungi was examined in silicified fossils from Antarctica. Araucarioxylon-type wood from the Triassic and specimens of Vertebraria from the Permian demonstrate similar patterns of decay. Specimens are characterized by irregularly shaped areas lacking cells and are infected with branched, septate hyphae with clamp connections. The decay in these fossils is comparable in appearance to present-day rots caused by basidiomycetes. Two patterns of decay are evident: (1) A wall component, presumably lignin, is removed from the wall and middle lamella of infected tracheids, leaving a considerably thinner cellulose framework; and (2) the primary and secondary walls typically separate, and all wall layers are progressively reduced in thickness and eventually removed. The middle lamella, particularly where it is thickest in the corners between cells, persists longer than other layers. Indications of host response to fungal attack include the production of possible wall appositions. Evidence of wood decay occurs as early as the Upper Devonian, and all reported cases from the fossil record are similar to those found today.