Field Emission SEM Studies on Softwood Tracheids and Hardwood Fibres – A Review of Activities at the EMPA Wood Laboratory

Abstract

Over the past ten years transverse fracture surfaces of tensionloaded softwoods, hardwoods and reaction wood have been studied using a high resolution FE SEM. Due to the fracture process the cell wall becomes loosened and its components can be distinctly observed (as opposed to smooth cross sections of cut wood). Softwood tracheids (compression wood, too) and hardwood fibres showed predominantly radial orientations(perpendicular to the other layers) of the cellulosic fibrils of the secondary wall layer 2 (S2). Similar structures were found on sections of different hardwoods degraded by white rot fungi. Contrary to that finding there is no discernible preferential orientation of the cellulose fibrils in the S2 or G layer of pronounced tension wood cells. In our opinion functional advantages of such structures in the strengthening tissue are quite clear. The sandwich-like structure is important for the buckling resistance of the tracheids and fibre cell walls and therefore also to the bending stiffness of the whole tree. Tension wood fibres of hardwood trees solely exposed to longitudinal tension do probably not need a compression stiffening, explaining why transverse fibril agglomerations are absent here. In many other studies a polylaminated concentric arrangement of the cellulose/ ligninhemicellulose matrix in the S2 layer is postulated and clearly documented. Taking our results into account, we conclude that concentric and/or radial arrangements (perpendicular to the CML) in the S2 of softwood tracheids and hardwood fibres coexist for good ontogenetic, physiological, and mechanical reasons.