Figure 7 . The mean diffusive resistance added by hairs to one leaf surface, calculated from observed differences in boundary layer conductance between hairy and bald patches (vertical axis), was unrelated to theoretical predictions based on the thickness of the hair layer (horizontal axis) (regression n.s. [p = 0.90]). Theoretical predictions assumed hairs entrain a layer of perfectly still air, and did not attempt to account for the density of hairs or the tortuosity of diffusion pathways through the hair layer. The grey line represents the 1:1 relationship.