A method for determining the fibrillar angle in wood tracheids

SUMMARY The predominant direction of fibrils in isolated walls of wood tracheids can be determined from observation of the extinction position in the polarizing microscope. This method cannot be used directly to obtain the fibrillar direction for separate intact cells, because the light path must traverse the opposite walls of the cell, the fibrillar angles of which are similar but of opposite sense. This difficulty may be overcome by impregnating the cell lumens with mercury and using incident (epi‐) illumination with crossed polars. The incident light now passes through the upper wall of each cell, suffers reflection at the mercury surface and returns through the same wall. The extinction position gives the predominant fibrillar direction of the upper wall. The method is rapid, has been found suitable for tracheids in wood, wood pulp and paper, and should be of value for the study of other plant cells such as cotton.