Auxin flows from margin inwards to create leaf veins.
BERLETH/CSH
Models for leaf vein patterning have been based on either auxin flows or reaction–diffusion systems. The flow models had a hard time explaining how loops would arise. But “all the modeling so far has been based on the final pattern,” says Berleth. “Now flow can be reconciled with closed networks, although other types of cellular interactions may also play a role.”
His team used the auxin efflux protein AtPIN1 as an early marker of vein formation. Expression was found at a series of “convergence points” at the leaf margin, from where it led down paths that foreshadowed first the leaf's main central vein and then other major veins connected to the main vein.
Auxin is presumed to flow from leaf margin to the interior, consistent with AtPIN1's polarization to the parts of the cells closest to the main vein. Loops could also form, however, because certain cells had bipolar AtPIN1, allowing auxin flow to the main vein via two alternate routes. Auxin turns on AtPIN1 expression, thus refining and reinforcing the pattern.
Interfering with auxin flow disturbs vein formation and spacing. Vein placement seems to depend on the positions of convergence points, but those may be defined both by auxin's positive feedback on its own flow and on undefined genetic circuits. 
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