| Abstract: | The duration and daily rate of radial growth and secondary-wallformation of all consecutive, radially-formed tracheids throughoutthe season were investigated in stems of adult trees of Pinussilvestris L. It was found that (1) the variation in radial diameter of tracheidswas probably dependent on seasonal changes in the rate of growthduring the phase of radial enlargement, (2) the daily rate ofcell-wall formation determined the final cell-wall thicknessof tracheids only at the beginning and the end of the season,(3) both rates were affected by temperature, (4) seasonal changesin cell-wall thickness were dependent mostly upon seasonal changesin the duration of the maturation period, (5) the changes inthe duration of the maturation period which brought about transitionfrom early to late wood were determined mostly by the delayin onset of autolysis of cytoplasm which terminates the phaseof tracheid maturation. This process, unlike the xylem productionfrom cambium and the termination of radial enlargemnt, was foundnot to be affected by the seasonal variation of temperature. An attempt to correlate these processes with the activity ofnatural auxin extracted from the cambial region gave negativeresults. On the basis of the results obtained, auxin and environmentalfactors such as precipitation and temperature seem not to bespecific for xylem differentiation. They may seriously affectwood differentiation if they become limiting or exceed the limitof tolerance, but probably they do not determine differentiationof the annual ring of conifers into early and late wood. |
