AXILLARY BUD DEVELOPMENT IN POPULUS DELTOIDES. II. LATE ONTOGENY AND VASCULARIZATION

A nearly mature axillary bud of Populus deltoides was embedded in epoxy and serially sectioned at 6 μm. Sectioning extended from the cataphyll tips to a level in the subtending internode about 6 mm below the bud base. Vascular development was followed through the serial microsections and the vascular system was mapped in its entirety from initiation of the original bud traces to termination of the last recognizable leaf trace beneath the bud apex. Each vascular trace was identified as to its origin, its termination within a foliar organ, and its relation to other traces comprising the bud vascular cylinder. Analysis of these data confirmed the procambial patterns found in Part I of this study. Two original bud traces that diverged from the central trace of the axillant leaf gave rise to two pairs of scale traces in quick succession, and these scale traces become the progenitors of all subsequent vascular traces that were perpetuated within the bud. Just before the bud vascular system separated from that of the stem, a third pair of scale traces diverged from the original bud traces; the latter then receded toward the stem to eventually merge with its vasculature. The third pair of scale traces produced a horizontal vascular connection between stem and bud before terminating in the adaxial cataphyll. The vascular system at first conformed with a ½ vascular phyllotaxy when the original bud traces were initiated, progressed through a ⅓ vascular phyllotaxy in the scale trace system, and terminated at the time of sampling with a ⅖ vascular phyllotaxy in the foliage leaf primordia.