Effects of relative humidity on leaf temperature of R. ferrugineum inside of two heat exposure chambers. In the heat exposure chambers of the Heat Tolerance Testing System twigs were exposed to a sub - lethal air temperature (+40°C) that was held constant A. in darkness and B. under full solar irradiation. Thin lines: leaf temperatures, bold lines: relative humidity, dashed lines: air temperature, solid rectangles: calculation periods for statistical comparisons. At high humidity (Rh > 90%) leaf temperatures moved markedly closer to air temperature due to a strongly reduced transpiration. C. Box plots of the mean leaf temperature as a function of each calculation period. Boxes: median, upper and lower quartile. Whiskers: maximum, minimum. Dotted boxes: in darkness (data from A.), white boxes: at full sunlight (data from B.). At high humidity (2, 4) mean leaf temperatures had significantly (P < 0.01) increased due to the restricted leaf transpiration, while leaf temperature variability was clearly reduced.