Microfibril Angles Inside and Outside Crossfields of Norway Spruce Tracheids

Summary The major part of the wood cell wall consists of parallel-aligned cellulose fibrils. Locally, pits connecting adjacent cell walls disturb the fibril arrangement. The local fibril orientation around these mechanically weak points is crucial for the mechanical stability of the cell. In some softwood species like spruce, the pit apertures at junctions of tracheids and cross-running ray parenchyma cells are elongated and slit-like. The pit orientation has often been assumed to directly reflect the fibril orientation. In this paper we use X-ray microdiffraction to determine the local microfibril angle (tilt angle versus the cell axis, MFA) in single tracheid walls of Norway spruce in the vicinity of pit apertures. The results from microdiffraction are compared with the pit orientation observed under the light microscope.Whereas a good correlation was found in thick-walled latewood cells from the stem and compression wood, large discrepancies occurred for thin-walled earlywood cells. A simple mechanical model that could explain the different situation in earlywood and latewood is presented.