The mechanism of spiral grain formation in trees

Spiral grain in trees is formed during the process of cell division and maturation within the vascular cambium (sensu lato). Much effort in the past half century has been put into elucidating the mechanism(s) involved. The most accepted view is that the dominant factor involves the pseudotransverse cell divisions during anticlinal (multiplicative) division and subsequent intrusive growth of the daughter cells, the slant of the partition (S or Z) and the direction of the intrusion being statistically biased to the left or to the right. A strong correlation is known to exist between slant direction and its frequency of occurrence on the one hand and the rate of change of spiral grain angle on the other hand. It is thus enticing to see the dominance of the orientation of the slant of the partition during pseudotransverse anticlinal cell division as the causative factor in the formation of spiral grain; the visual evidence seems clear. However, it will be shown that there is a constant, incessant tendency of all maturing cells (which are predominantly the result of periclinal divisions) to change their orientation in a given direction; this tendency will be modulated as to magnitude and direction by certain prevailing physical parameters of the system during the period of maturation. Thus it is concluded that neither the slant of pseudotransverse divisions nor other "isolated events" (imperfect periclinal division, biased intrusive growth) are causative, but that they rather result from the fact that there is a radial gradient of the inclination angle (in the tangential plane) of fusiform cells, i.e. from the general tendency of a maturing cell to take on a preferred inclination with respect to the cell which immediately preceded it in its file. Growth stress patterns in trees have also been extensively investigated in the past half century. It is shown that the development of these stresses and the formation of spiral grain are just two aspects of the same process occurring throughout the cambial zone during cell maturation. Models are presented to justify this claim which accord with reported patterns of growth stress and spiral grain in conifers.