In both plants and animals vein networks play an essential role in transporting nutrients. In plants veins may also provide mechanical support. The mechanism by which vein patterns are formed in a developing leaf remains largely unresolved. According to the canalization hypothesis, a signal inducing vein differentiation is transported in a polar manner and is channeled into narrow strands. Since inhibition of auxin transport affects venation patterns, auxin is likely to be part of the signal involved. However, it is not clear whether the canalization hypothesis, initially formulated over 25 years ago, is compatible with recent experimental data. In this paper we focus on three aspects of this question, and show that: (i) canalization models can account for an acropetal development of the midvein if vein formation is sink-driven; (ii) canalization models are in agreement with venation patterns resulting from inhibited auxin transport and (iii) loops and discontinuous venation patterns can be obtained assuming proper spacing of discrete auxin sources.
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