In leaves, vascular strands of distinct hierarchical orders are organized in closed (i.e. connected) networks. Vascular networks are generated de novo during the development of each leaf primordium. Procambial cells, the precursors of all vascular cells, differentiate from preprocambial cells, which represent a molecularly defined subset of ground meristem cells in the leaf (1).
Mechanisms integrating aligned cell differentiation during vascular strand formation were probably not reinvented in the evolution of leaves, but recruited and revised by leaf-specific controls. In addition, species-specific cues are likely to be involved in leaf vascular patterning, as shown by the successful use of species-specific leaf vascular patterns as taxonomic diagnostic features (2). Whereas most dicot leaves show a ramified pattern of progressively branched veins, most monocot leaves have striate venation patterns in which major veins lie parallel along the proximodistal axis of the leaf and are connected transversely by minor transverse veins (3).
Auxin signals have a profound impact on vascular patterns of dicot leaves. Local auxin application induces the formation of a new vascular strand (4). Further, both defective auxin signal transduction and impaired auxin transport alter leaf vascular patterns in characteristic ways (5-8), and positions of leaf procambium formation are foreshadowed by sites of elevated auxin response (9).
The reduced auxin sensitivity of monocot leaves (10), and their highly reproducible distribution and arrangement of veins (3) suggest that vascular patterns in monocot leaves may be rigidly specified. Recent pharmacological and genetic evidence suggest, however, a role for auxin transport and signal transduction in vascular patterning of monocot leaves (11,12).

1. Scarpella E, Francis P, Berleth T. 2004. Stage-specific markers define early steps of procambium development in Arabidopsis leaves and correlate termination of vein formation with mesophyll differentiation. Development 131:3445-55.
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11. Scarpella E, Boot KJM, Rueb S, Meijer AH. 2002. The procambium specification gene Oshox1 promotes polar auxin transport capacity and reduces its sensitivity towards inhibition. Plant Physiol 130:1349-60.
12. Scarpella E, Rueb S, Meijer AH. 2003. The RADICLELESS1 gene is required for vascular pattern formation in rice. Development 130:645-58.