The old question, whether the stomata are the only mechanism by which the plants can regulate their transpiration rate or whether other physiological factors beside the stomata play a regulating role, was subjected to an experimental study. To this purpose experimental transpiration rates in still air as well as in wind were compared to those to be expected theoretically at the as same stomatal aperture, if the stomata really are the only factor regulating transpiration. The former were determined by weighing leaf discs of the test plant — Zebrina pendula — before and after a short time of transpiration, the latter by calculating in simplified models the successive resistances to water vapour diffusion in the leaf in their relation to the stomatal aperture. The external resistance only, which exists in still air and which is hardly amenable to an exact theoretical analysis, was derived from comparable evaporation experiments. Stomatal apertures were determined by direct measurement under the microscope. On comparison there appeared to be a close agreement between the experimental and theoretical values. To test the applicability of the principles used in the calculation an analysis along the same lines of thought was made of the experiments on evaporation through multiperforate septa by Sierp and Seybold (1929) and by Huber (1930). The agreement with their experimental results was satisfactory. The conclusion was drawn that under the conditions of the experiments the stomata, up to their maximal aperture, were the only physiological factor controlling transpiration.