XYLARY UNION BETWEEN THE NEW SHOOT AND OLD STEM DURING TERMINAL BUD BREAK IN POPULUS DELTOIDES

Bursting terminal buds and subtending stems of Populus deltoides Bartr. ex Marsh. plants were examined at several stages to determine the pattern of xylary union between the 1st- and 2nd-yr growth increments. Metaxylem vessels differentiated first in traces serving the basalmost leaf in the bud. As successively younger leaves began growth, metaxylem vessels differentiated in their traces. The site of metaxylem reactivation was in the second or third internode beneath the bud (i.e., in the 1st-yr stem), and subsequent differentiation progressed bidirectionally in each set of traces as the leaves they served expanded. Traces leading either to abscised leaf positions on the 1st-yr stem or to bud-scale leaf positions were reactivated by the tangential “spread” of activity from adjacent traces serving expanding leaves. The new elements were all secondary xylem vessels, as were those of the basipetal trace components, although they were functionally continuous with the metaxylem vessels that differentiated acropetally. Xylem fibers were initiated in the same position and differentiated in the same sequence as vessels. However, fiber differentiation lagged behind that of vessels. Whereas vessel differentiation was associated with leaf expansion, fiber differentiation was associated with leaf maturation. As each leaf matured in sequence, the primary-secondary transition zone advanced acropetally to the bud base and then in the new shoot until it attained a positional relation with leaf maturation comparable to that of 1st-yr plants.