Leaves often shrink significantly when soil water is limited. For gas exchange measurmements, the shrinkage can require correction for changing amounts of tissue in the apparatus. In sunflower plants (Helianthus annuus L.), a comparison was made between mathematically-corrected transpiration and clamping leaves at their original turgid size without mathematical correction. These methods should give the same result, but transpiration was substantially greater in the clamped leaves than in the shrunken and mathematically-corrected ones. Because the clamped leaves remained at their original turgid area, wounding was not a factor. If shrunken leaves were stretched to their original area, transpiration increased immediately and was traced to increased leaf conductance to water vapor and greater porosity for bulk air movement through the leaf, implicating the stomata. Releasing the leaf caused each of these properties to return to the tightened condition. When all the leaves were held at their original size during a soil water deficit, whole-plant water use was greater than when the leaves shrank naturally. It was concluded that shrinkage decreases the porosity of sunflower leaves. This natural tightening can be disrupted by stretching leaves during gas exchange measurements. However, stretching provides a useful means of changing leaf porosity for experimental purposes.