MITOTIC ACTIVITY IN THE CAMBIAL ZONE OF PINUS STROBUS

Mitotic activity in the cambial zone of 20-year-old Pinus strobus L. trees was determined quantitatively, using mitotic counts from serial tangential sections of sample pieces. Counts from nine cores 1.6 mm in diameter from each sample piece were averaged and expressed as the number of mitoses per core with the sampling error. Estimation of the number of cambial zone cells per core permitted calculation of mitotic indices. In the fourth internode from the ground the first mitoses were observed on April 15 and the last on September 15. The number of mitoses per sample piece was comparable throughout each internode at any one sampling, but the number was higher in internodes within the crown than in those below it. Mitotic index was not appreciably higher within the crown, but it decreased from 3.7 in spring to 2.0 in fall. An internode in the crown sampled May 11–13 showed afternoon peaks in mitotic activity, but an internode below the crown showed no peaks, nor did other internodes sampled later in the season.