AbstractThe air‐flow regime over aPopulusleaf was investigated using a constant temperature hot‐film sensor which could be glued on the abaxial surface. The leaf was exposed to a laminar or turbulent flow of air in a wind‐tunnel, while being free to undergo normal motion. In the laminar air‐flow the boundary layer remained laminar, despite the fluttering of the leaf, until a Reynolds number of 2.3 × 104was reached. When the air incident on the leaf was made turbulent, to resemble natural conditions, the boundary layer became turbulent at a Reynolds number of 0.4 × 104. The experiments suggest that the boundary layers over leaves are always turbulent in the nat