Cottonwood (Populus deltoides, Marsh.) leaves are amphistomatous and have an adaptation in their petiole which allows them to oscillate in wind. A possible function of these oscillations in enhancing gas exchange was studied.Cottonwood leaves were found to oscillate in the presence of wind velocities frequently encountered in nature. A pressure differential across the leaf was shown to result in bulk flow of air through that leaf. Oscillating a cottonwood leaf at frequencies found to occur in nature was found to increase the rate of O(2) flux through the leaf. The measured changes in boundary layer resistances during oscillations were found to be insufficient to account for the increased O(2) flux. Thus, the bulk flow of air through an oscillating cottonwood leaf results in a decreased total resistance which is typically 25% less than that of a still leaf.