The Initiation and Development of Secondary Xylem in Axillary Branches of Populus deltoides

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Abstract
The initiation of secondary xylem in elongating axillary branches of Populus deltoides Bartr. ex Marsh. is independent of that in the main stem. Although secondary xylem differentiates acropetally in the main stem, it does not differentiate from the stem into the axillary branch. Secondary xylem is usually initiated in internode 4 (occasionally 3) of the axillary branch, and from this site it develops both acropetally in the elongating branch and basipetally toward the main stem. Secondary vessel differentiation always precedes fibre differentiation. Although secondary xylem differentiates in internodes that have ceased elongation, it differentiates first in traces of the vascular cylinder serving rapidly expanding and maturing foliage leaves. As younger leaves on the branch expand and mature, secondary xylem differentiates in their traces eventually producing a complete secondary vascular cylinder. Scale leaves do not initiate secondary xylem independently in their traces; they are activated by adjacent traces in the vascular cylinder serving foliage leaves. Once established, the primary-secondary vascular transition zone advances acropetally in a branch just as it does in the main stem.

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