The Control of Vascular Branching in Coleus 1. The Side Bundle

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Abstract
Xylary branching at the proximal end of a differentiating side bundle was modified by surgical alteration of the surrounding leaf traces and manipulation of their auxin fluxes. Incisions through one corner trace, with the other pre-existing trace intact, resulting in xylem differentiation in the branch of the newly formed side bundle toward only the severed trace. Application of IAA to the cut trace allowed xylem branching of the new strand in both directions. With sufficient auxin imbalance created by increasing the concentration of the applied IAA, the new xylem strand branched away from the higher auxin source. Auxin relations were thus able to regulate the course of differentiation of vascular strands, but their role in regulating branching patterns in intact plants may be questioned. Xylem branched exclusively toward an incised trace only when the auxin flux of the incised trace was virtually eliminated. Phloem and procambium of the differentiating strand were unaffected by this treatment.