The duration and daily rate of radial growth and secondary-wall formation of all consecutive, radially-formed tracheids throughout the season were investigated in stems of adult trees of Pinus silvestris L. It was found that (1) the variation in radial diameter of tracheids was probably dependent on seasonal changes in the rate of growth during the phase of radial enlargement, (2) the daily rate of cell-wall formation determined the final cell-wall thickness of tracheids only at the beginning and the end of the season, (3) both rates were affected by temperature, (4) seasonal changes in cell-wall thickness were dependent mostly upon seasonal changes in the duration of the maturation period, (5) the changes in the duration of the maturation period which brought about transition from early to late wood were determined mostly by the delay in onset of autolysis of cytoplasm which terminates the phase of tracheid maturation. This process, unlike the xylem production from cambium and the termination of radial enlargemnt, was found not to be affected by the seasonal variation of temperature. An attempt to correlate these processes with the activity of natural auxin extracted from the cambial region gave negative results. On the basis of the results obtained, auxin and environmental factors such as precipitation and temperature seem not to be specific for xylem differentiation. They may seriously affect wood differentiation if they become limiting or exceed the limit of tolerance, but probably they do not determine differentiation of the annual ring of conifers into early and late wood.