Leaf venation architecture varies enormously among modern plant species and across the fossil record.  Recent work has shown that this diversity in venation has consequences for leaf and whole-plant hydraulic capacity, for leaf biomechanical support, and for leaf tolerance of vein damage.  However, little work so far has focused on determining if the leaf venation architecture follows general design principles.  We asked whether venation traits are generally linked with leaf size and shape, which are evolutionary labile traits, extremely variable across species.  We chemically cleared leaves of (1) diverse rainforest tree species from Barro Colorado Island, Panama, and of (2) maple species of North America, Europe and Asia (Acer, Sapindaceae) growing in the Arnold Arboretum, Massachusetts.  We quantified aspects of leaf venation architecture, including densities (length/area) of veins of each order, and branching angles.  We determined trait variation with leaf size, within and across species, and tested the data against scaling models based on simple geometric and/or developmental constraints.  

Results/Conclusions

We found novel scaling relationships, including a general negative scaling between major vein densities and leaf size, both within and across species.  On the other hand, minor vein density was independent of leaf size and shape.  These findings are consistent with the major and minor venation systems developing for the most part independently, and having the potential to evolve independently.  Additionally, our finding of a greater major vein density in smaller leaves, points to a tendency to have stronger tolerance of vein damage or blockage.  Further, since minor vein density is independent of leaf size, hydraulic capacity and gas exchange would be unconstrained by leaf size and shape.   Our findings have additional applications to paleobotany: these new scaling relationships may allow predictions of intact leaf size and shape from fossil leaf fragments, given extracted information of their venation characters.