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Statistical analysis of high-frequency variations in the radial dimensions of tracheids was carried out using precise measurements of five radial rows in each annual tree ring for several coniferous species: Larix gmelinii, L. leptolepis, L. sibirica, Picea abies, Pinus sylvestris, P. rigida, and P. densiflora. More than 25 tree rings with a varying number of cells and width were measured for each species. High-frequency variations in radial dimensions were found to have a regular (cyclic) characteristic and accumulated 5 to 11% of the total variability. The cyclic pattern was identified using Fourier analysis of time-related or successive series statistical procedure. The first order autocorrelations (-0.45 to -0.74) and the mean cycle of high-frequency variations (2.1-2.5) did not depend on the number of cells in the annual ring, i.e., they are determined by internal causes. The cyclic pattern of high-frequency variation in tracheid dimensions is used to interpret the seasonal mechanism of xylem formation. Specifically, (1) high-frequency variations in the tracheid radial size take place during the last asymmetrical division of the xylem mother cell before transition to the elongation zone, (2) the final tracheid radial size is mostly determined within the zone of division due to the last asymmetrical division, and (3) acceleration of cell cycle from the initial to the periphery of the cambial zone. The results obtained are also discussed with respect to the mechanisms of xylem differentiation.