AbstractThe humidity response of individual twigs was determined in “clean” air and subsequently during exposure to SO2(0.3 to 0.05 ppm). Changes in regression were related to the effect of SO2on stomatal movement.In short‐term experiments, the response of the twig to SO2varied with the prevailing humidity of the air. During long‐term fumigations there were three distinct behavioural phases, a first with suppressed, a second with high, but erratic transpiration eventually followed by low rates and visible symptoms of