Development of the Xylem Ring in Stems of Young Red Pine Trees

Two annual sheaths of xylem were studied in 25-year-old Pinus resinosa Ait. growing under three experimental conditions (irrigation, imposed drought, and removal of buds) supposed to influence xylem development. Graphic reconstructions of xylem development were prepared from transverse microtome sections of tissue collected weekly at two stem positions, in the lower bole and within the base of the live crown. Several zones of maturing and mature xylem derivatives were measured for widths and numbers of cells produced weekly: (1) the zone of expanding derivatives prior to initiation of secondary wall thickening, (2) the zone of active assimilation of secondary wall material, and (3) the zones of mature tracheids in earlywood, in Mork's latewood, and in radially flattened latewood. Flattened and thick-walled latewood tracheids appeared in the drought trees earlier than in the irrigated or bud-clipped trees. There was no difference in xylem development between the irrigated and the bud-clipped trees. Transition from earlywood to latewood appeared to be related to a midseason interaction between the rate of cambial derivative division and enlargement, and the width of the region of tracheids containing cytoplasm. The rate of cambial division determines how rapidly the phloem is displaced away from a given derivative, whereas the width of the cytoplasmic layer indicates how far removed from the phloem cell wall synthesis can occur. The rate of cambial division is probably regulated by growth substances but not limited to those produced in expanding new shoots. The width of the cytoplasmic zone is probably controlled by current photosynthesis and phloem transport. Both processes are affected by environmental conditions, especially moisture, and both are ultimately limited by intrinsic patterns of growth.