Leonardo's rule, self-similarity and wind-induced stresses in trees
  Examining botanical trees, Leonardo da Vinci noted that the total cross-section of branches is conserved across branching nodes. In this Letter, it is proposed that this rule is a consequence of the tree skeleton having a self-similar structure and the branch diameters being adjusted to resist wind-induced loads. 
Christophe Eloy
physics.bio-ph physics.flu-dyn