The cambial zone in woody plants (i.e., the vascular cambium and its immediate derivatives) is known to contain high concentrations of the plant hormone auxin, specifically indole-3-acetic acid (IAA). This auxin is synthesized in young developing leaves, but the route that it takes to arrive in the cambial zone is not well understood. We used hybrid poplar lines (Populus tremula x alba) containing the auxin-responsive promoter DR5 (PtaDR5) to study auxin signaling during early stages of vascular development in a woody plant. In keeping with classical models of development, the earliest protoxylem differentiates within strands of auxin-responsive cells beneath the shoot apex. As conductive elements within the xylem differentiate, they appear to segregate this auxin-responsive tissue into two distinct regions: primary xylem parenchyma (PXP) to the inside, and a procambial strand to the outside. As more vascular bundles develop within the primary stem, the procambial strands unify into a continuous ring of auxin-responsive (and auxin-transporting) procambium, the precursor the cambium. Interestingly however, the strands of PXP continue to function in auxin signaling and transport as long as the leaves to which they are connected remain attached. These strands of PXP anastomose in complex ways within the stem that reflect phyllotactic patterning. They also appear to be capable of auxin biosynthesis, as indicated by YUCCA gene expression, but it is not clear what the ultimate fate or role of this auxin is.  Although PXP strands cross through the cambial zone along with each leaf trace as it enters the stem, there is no evidence that they contributes to the pool of auxin within the cambial zone. The observed auxin-responsive architecture will be discussed in the context of the transition from primary to secondary growth.