The extent to which our results can be explained by assuming that the leaf is a purely physical body, showing virtually no physiological changes in response to changing environment, is rather impressive. Two results seemed, at first glance, to be exceptions to this conclusion: the temperature of irradiated leaves rose less above air temperature when the air temperature was highest; and transpiration of irradiated leaves decreased steeply with increasing wind velocity. The analysis of these results indicated, however, that even these reactions were primarily physical. Physiological adaptations became apparent only at unnaturally high radiation intensities and at high wind velocities.
The correlation between leaf-air-temperature difference and transpiration can also be explained on a purely physical basis. In the one exception (Fig. 5) more than one environmental variable changed, and the failure of correlation might be expected on theoretical grounds.
The relatively minor importance of convection as a mechanism of heat transfer as revealed by calculation of the energy budget (Table 4) is also of interest, as is the strong response of plant temperature to light quality (Fig. 7).
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