The outline of the vascular system in plants is not rigidly specified, but is strongly influenced by the auxin transport properties of the developing leaf primordium. When first node vegatative leaves develop in the presence of auxin transport inhibitors (untreated, left; increasing auxin transport inhibition to the right), pronounced vascular differentiation is observed near the leaf margin, while vascular differentiation is delayed and eventually incomplete towards the base of the leaf.
The insert shows an earlier stage in normal (left) and auxin transport inhibited (right) leaf primordium development. Note that vascular differentiation normally proceeds from the base of the leaf, but upon auxin transport inhibition is initiated and can be restricted to the margin. Adapted from Mattsson et al. 1999.