THE VASCULAR CAMBIUM AND RADIAL GROWTH IN THUJA OCCIDENTALIS L

The vascular cambium, to use the term in the general sense as designation for the entire meristem producing secondary xylem and phloem, consists of the self-perpetuating uniseriate initiating layer and the derived tissue mother cells. In the dormant condition the cambium in the stems of mature trees is one to four, usually two to three, cells wide. The one to three tangential tiers of cells toward the xylem are made up of xylem mother cells, and the single tier next the phloem is composed of initial cells. On reactivation all cambial cells expand radially. Periclinal divisions soon follow, usually according to a sequence in which the xylem mother cells in the tier contiguous to the late wood are the first to divide and the initial cells last, although some variation occurs. The zone of periclinal division rapidly widens to 100-300 μ in vigorous open-grown trees, the center of activity during this vernal surge of growth clearly being among the dividing and redividing xylem mother cells. Thereafter, from mid-June to August, a continued decline in cell production follows as the zone of xylem mother cells decreases from many tangential rows to few. The initiating layer becomes relatively more important as the center of cell generation, and toward the end of the growing season the initial cells undergo those changes which modify the cellular pattern of the cambium and hence of the derived tissues. Phloem development begins later than that of xylem, and at the end of May the new phloem varies from none to four cells as compared with a xylemward production of 30-60 cells in vigorous trees. Phloem expansion continues at a more or less steady rate from June to the termination of cambial activity in August or early September. The annual phloem increments vary less in width than those of xylem.