Fibrillar angle variation in red pine determined by Senarmont compensation

Abstract

Fibrillar angles were determined in the secondary wall layers of three rings across the stem of a mature red pine. The Senarmont compensation method was adopted for this study since it gives results for all three wall layers. It involves the determination of birefringence in a series of sections cut at various angles to the long axis of the tracheids. Maxima and minima in the resultant plots of birefringence versus section angle correspond to fibrillar orientations. In the “early latewood” of each ring, the S2 layr was found to contain a single Z helix of small fibrillar angle and the S1 dual S and Z helices of much flatter orientation. Results for S3 were found to be so close to those for the corresponding S2 layers that there was some doubt as to their validity. In each layer a slight decrease in fibrillar angle was observed between rings 4 (juvenile wood) and 12 (transition zone); a sharper decrease was seen between ring 12 and the mature ring 20. This trend corresponds inversely to tracheid length differences for the three rings. Correlations of tracheid wall thickness and overall cross-sectional dimensions with the fibrillar angle variations were less readily apparent.