Origin and diversity of leaf vein patterns.

We have all felt at the touch the vein stripes in a grass leaf or have seen against the sun the vein mesh in a fallen leaf. Those veins distribute water to and collect sugars from all leaf areas. How vein networks evolved and develop is therefore a key question in biology. Here we address that question for vascular plants, in whose leaves veins are thought to be arranged in either grids or webs. We will see that those are just two instances of the natural diversity of vein patterns, which evolved independently multiple times. We will also see that diversity may result not from different vein patterning mechanisms but from different leaf growth patterns acting on a single underlying mechanism. Vein patterning mechanisms have been inferred from the patterns with which vascular precursor cells appear in the leaf. Because vascular tissues predated leaves, vein precursor cells should share defining features across all vascular plants. We will see that the identification of vein precursor cells is instead arbitrary, depending on which features are emphasized. Identifying features that are not only shared by vein precursor cells across all vascular plants but that are also required for vein formation will be essential to understand how veins are patterned.