  We explore the possible role of elastic mismatch between epidermis and
mesophyll as a driving force for the development of leaf venation. The current
prevalent 'canalization' hypothesis for the formation of veins claims that the
transport of the hormone auxin out of the leaves triggers cell differentiation
to form veins. Although there is evidence that auxin plays a fundamental role
in vein formation, the simple canalization mechanism may not be enough to
explain some features observed in the vascular system of leaves, in particular,
the abundance of vein loops. We present a model based on the existence of
mechanical instabilities that leads very naturally to hierarchical patterns
with a large number of closed loops. When applied to the structure of high
order veins, the numerical results show the same qualitative features as actual
venation patterns and, furthermore, have the same statistical properties. We
argue that the agreement between actual and simulated patterns provides strong
evidence for the role of mechanical effects on venation development.
