Branching net-like structures are a trait common to most multicellular organisms. However, our knowledge is still poor when it comes to the genetic operations at work in pattern formation of complex network structures such as the vasculature of plants and animals. In order to initiate a causal analysis of venation pattern formation in dicotyledonous plant leaves, we have first studied its developmental profile in vegetative leaves of a wild-type strain of the model organism Arabidopsis thaliana. As landmarks of the complexity of the venation pattern, we have defined three main developmental parameters, which have been quantitatively followed in time: the ratios of (a) the length and (b) the number of branchpoints of the vein network with the surface of the lamina, which decrease in parallel as the leaf grows, only small differences existing between successive leaves, and (c) the number of hydathodes per leaf, which increases both during leaf expansion and from juvenile to adult rosette leaves. We next searched for natural variations in the first vegetative leaves of 266 ecotypes, finding only 2 which showed a venation pattern unequivocally different from that of the rest, Ba-1 and Ei-5, the latter displaying an extremely simple pattern that we have called Hemivenata. This phenotype, which is inherited as a monogenic recessive trait, is visible both in leaves and in cotyledons and seems to arise from a perturbation in an early acting patterning mechanism. Finally, we have screened for mutants with abnormal venation pattern but normally shaped leaves, concluding that such a phenotype is rare, since only one recessive mutation was obtained, extrahydathodes, characterized by the presence of an increased number of hydathodes per leaf.

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