Shapes of river networks and leaves: are they statistically similar?

The structure of river networks is compared with the vein structure of leaves. The two structures are visually similar at the smaller scales. The statistics of branching and side branching are nearly identical. The branching structure of diffusion-limited aggregation clusters is also similar and can provide an explanation for the structure of river networks. The origin of the self-similar branching and side branching of the vein structure in leaves is not clear but it appears to be an optimal network in terms of transporting nutrients to all parts of the leaf with the least total resistance.