To shield the entire leaf except circular areas of 0.8 mm, thin polyethylene plastic foil (20 μm) was punched with a 0.8-mm syringe needle, which was ground squarely and sharpened. The correct size of the holes was confirmed microscopically after attachment to the leaf. Very thin double-sided adhesive tape (Pritt permanent) was used to attach the foil to the leaf, allowing only the epidermis inside the circular holes to transpire. In some cases, the foil was not attached firmly to the epidermis at the edge of the punched hole, leaving a gap between leaf and foil. These stomata were excluded from the experiments. Up to six holes with a distance of at least 10 mm were punched into a foil in one experiment. The foil was at first attached provisionally in its final position to select stomata located inside the holes. The foil was then removed to observe a control response to a decrease in air humidity of selected stomata. Thereafter, the foil was attached firmly in the same position as before to measure the humidity response of the sample of stomata on the next day. In some experiments, the foil was carefully removed afterward and another control response was measured on the next day to test for permanent damage due to experimental treatment.