Polar auxin transport and patterning: grow with the flow.

Particularly in the fall, plant leaves demand attention as examples of the beauty of nature. Around that time, anyone can pick them up and have a close look at the variety in venation patterns. For a considerable time, biologists and mathematicians went one step farther, trying to find regularities in these patterns and rules behind them. Vein organization can be classified in several general patterns characteristic of a particular plant (Fig. 1A–D), suggesting that genetic input into the patterning system can be responsible for predictable gross changes in venation networks. However, close inspection reveals that there is considerable variation in the venation pattern in individual leaves of many plants. This reveals a large stochastic element in the mechanism by which veins are formed. In this issue of Genes & Development, Scarpella et al. (2006) provide new insights into how genetic and stochastic vein patterning processes might be related, but also connect the initiation of venation patterns to recent models of other auxin-dependent patterning processes in shoots and roots.