Formation of spiral grain in the wood of Pinus sylvestris L

In cambium producing wood of increasing spirality, the angle of inclination of fusiform cells to the stem axis is augmented with time. Frequently, events such as pseudotransverse anticlinal divisions and intrusive growth between radial walls do not explain the relatively large rate of change in the angle. The present analysis of the cambium of Pinus sylvestris L. indicates that the large rate of change in the angle of a cell inclination is caused by oriented intrusive growth of initial cells in certain files, entering the space between the tangential walls of neighbouring files. Such intrusive growth of the initial cell ends is caused by a deflection of the ends of neighbouring initial cells in the radial direction. A periclinal division plane does not reach the deflected end. This results in two derivatives, unequal in size, the shorter of which remains an initial cell, whereas the longer becomes a xylem or phloem mother cell. When these events are intensive, they cause a rapid change of cell inclination along the stem axis. Such a rapid change can take place even without oblique anticlinal divisions.[PUBLICATION ABSTRACT]