The role of acrotony in reproductive development in Picea.

The expression of acrotony (i.e., the increasing size of comparable lateral shoots toward the apex of the main shoot) was similar among first-order shoots borne along previous-year leaders of young Picea glauca (Moench) Voss, Picea mariana (Mill.) B.S.P. and Picea rubens Sarg. trees. The position and nature (i.e., whether cones, second-order shoots or non-flushed buds) of lateral axes borne along the first-order shoots were investigated. Seed cones occurred in proximal to distal positions, but not in terminal positions, on upper shoots, in medial to terminal positions on middle shoots, and in distal to terminal positions on lower shoots. Most non-flushed buds occurred in proximal positions on middle and lower shoots. In P. rubens, rate, duration and amount of elongation of first-order shoots increased acropetally. Thus lower shoots stopped elongating before upper shoots. Evidence of bud differentiation in P. rubens, as indicated by the presence of initiating leaf primordia, was seen first in sections of terminal buds of upper shoots. Differentiation of buds then proceeded basipetally along shoots and among shoots down the crown. Differentiation of buds in positions where cones are commonly borne became evident soon after shoot elongation was completed.