ABSTRACTImages of chlorophyll fluorescence were used to demonstrate patchy stomatal closure at low humidities in leaves of well‐wateredXanthium strumariumplants. The pattern and extent of patchy stomatal closure were shown to be different for the two surfaces of amphistomatous leaves by taking images of leaves with CO2available to only one leaf was exposed to low humidity, patchiness was more extensive on that surface. Gas‐exchange experiments were also conducted to determine the apparent photosynthetic capacity of the mesophyll (photosynthesis rate at constantciwhen it was supplied with CO2through both surfaces or through each surface alone. These experiments showed declines in the apparent photosynthetic capacity of the mesophyll at low humidities that were consistent with patchy stomatal closure on one or both surfaces. The results suggest that patchy stomatal closure can be a factor in the steady‐state reponses of stomata to humidity. In amphistomatous leaves this is further complicated by the fact that patches on one epidermis may not coincide with those of the other su