Hypothesis on the role of cell wall structure in differential transverse shrinkage of wood

Cell wall thickness under dry and wet conditions and the cellulose fibril arrangement on transverse-fracture surfaces of the S2 layer of the tangential and radial walls of Scots pine tracheids were examined using SEM images and ESEM images, respectively. A hypothesis for explaining the anisotropic transverse shrinkage of wood was proposed based on the test results and statistical analysis. A simplified model of a tracheid's cross-section was drawn for calculating the differential transverse shrinkage. Results of this study indicate that the radial cell wall of Scots pine latewood is about 25 percent thicker than the tangential wall. Earlywood does not show such a difference. Furthermore, there is a significant tendency of a preferential orientation (i.e. packing density) of the fibrils on the tangential and radial walls in the overall tangential direction. Despite some phenomena of the cell wall structure not yet clearly understood, this might explain the anisotropic transverse shrinkage of wood to a major extent.