GROWTH AND MORPHOGENESIS IN THE CANADIAN FOREST SPECIES: I. THE CONTROLS OF CAMBIAL AND APICAL ACTIVITY IN PINUS RESINOSA AIT.

The anomalous complexity of the annual rings of young trees which generally disqualifies them from use in growth studies is, in P. resinosa, found to arise from a remarkably thorough organization of ring width and therefore of cambial activity in the tree under the influence of intrinsic determinants. The pattern is manifest when the widths of the internodal wood rings of a single year are followed in sequence from internode to internode down the tree from the apex. A similarly patterned view of the rings is obtained when the ring widths are traced in the ring sequence, conventional for growth studies, that passes from ring to ring in a given internode. The controlling intrinsic factors are held to be nutritional gradients in the axis inferred from the distribution of foliage and light along the axis of trees growing in the forest and in the open.In both types of sequence the pattern obscures the variations induced by random extrinsic factors and severely limits the value of these sequences for examining the effect of such factors. This disability can be avoided by the use of a third sequence of ring widths in which each term is the width of a ring which was laid down in an internode different but of the same age at the time of ring formation as the others in the sequence. Such sequences have never been used in growth studies. Yet they are found to be unpatterned and the effect of the fluctuating extrinsic factors can be examined effectively in them and in them alone.The complex relation between the responses of the cambium thus determined and those of the apical growing point to the random extrinsic factors is found to derive from the discontinuity of terminal growth introduced by the winter pause between bud formation and axial extension. These two stages of terminal growth are influenced by the extrinsic factors of the two different years. The effect on the cambium is simpler than this but is determinably related to that on the apical growing point.The results afford the ground for a first advance toward the removal of the disqualification of the use of young trees in studies of growth and of its factorial control.