Is the orientation of the fibrillar helix in the main layer of cell walls constant or variable within a tree?

The study material consisted of samples from 270 Picea excelsa and 54 Abies alba trees with variable type of wood grain. No correlation was found between the orientation of the fibrils and the type of grain. In all the samples in which the fibrillar helix formed an angle with the cell axis greater than 10° (80% of the total) the helix was of Z type. In the other samples undulations commonly occured in the orientation of fibrils within a cell. These were in the form of local deviations to the left or right of the general trend to develop the Z type of spiral.