Ultrastructural cell wall modifications in secondary xylem of American elm surviving the acute stage of Dutch elm disease: fibres

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Abstract
Various ultrastructural cell wall modifications of fibers were observed in artificially or naturally infected American elm (U. americana L.) trees surviving the acute stage of the Dutch elm disease caused by Ceratocystis ulmi (Buism.) C. Moreau. Bands of fibers with gelatinous (SG) layers characteristic of tension wood are frequent in or near invaded tissues; similar layers are also present sometimes in cells identifiable as parenchyma. The SG layer is often different from that observed in healthy trees. Masses or bands of dense material are present which often extend perpendicular as tubular structures to the plasmalemma location. Also, orientation of fibrils in such layers may be disordinate. One or more additional lignified-like layers alternating with additional SG-like or other opaque layers may occur inside the 1st SG layer. The innermost layer in these cases is often lamellate. Small intracellular locules delimited by 1 or 2 wall layers also occur in other cells. The possible significance of these observations in host-parasite relationships of the disease is briefly discussed.