June, 1919] Variations in Transpiration 503 



The following conclusions concerning the limiting factors 

 which control water loss from mullein and tobacco are based on 

 Experiment 1. The diagrams on page 508 show the transpiration 

 curves and the operation of these limiting factors. At night 

 transpiration is limited to cuticular evaporation because the 

 stomata are closed and the rate of water loss is determined by 

 the temperature of the leaf and the humidity of the air, which 

 were in these experiments nearly constant during the night. 

 The process of water loss at night is simply and easily explained, 

 but in the daytime the stomata open and there are changes in 

 temperature and humidity due to sunlight which make the 

 explanation of the process more difficult. The rate of water 

 loss in the daytime is affected by the diffusion gradient of the 

 stomatal pores although the cuticular transpiration still continues 

 and is slightly increased by the rise in temperature and satur- 

 ation deficit of the air. The increase in the rate of water loss 

 during the morning hours is brought about by the increase in size 

 of the stomatal pores, beginning about sunrise and the probable 

 increase in diffusion gradient through the stomatal pores, result- 

 ing from the increased saturation deficit of the air. At about 

 noon, there are two factors, leaf water deficit and decrease in 

 stomatal pores, operating to diminish water loss from the leaf, 

 and only one factor, diffusion gradient, tending to increase 

 it. This results in a rounded curve and finally a steady 

 decrease. The diffusion gradient becomes an additional factor 

 tending to decrease water loss after the saturation deficit 

 reaches a maximum and there are then three factors all oper- 

 ating in the same direction which cause the rate to quickly 

 decline to the night level. Thus the water loss is reduced to 

 the night rate, even though the stomata are not fully closed. 



The stomata open normally every morning in sunlight and 

 the normal transpiration curve shows a sudden rise at that 

 time. The fact that the stomata open slightly but fail to open 

 fully or to remain open if the plant is in continuous darkness in 

 the morning has been inferred by Curtis and Darwin, who 

 found a rhythm in the transpiration curve in darkness and 

 concludedthatit was caused by stomatal movements, but Lloyd, 

 although he showed that an induced rhythm of the transpiration 

 curve occurs in ocotillo, could find no corresponding increase in 

 the size of the stomata. A number of experiments were per- 

 formed which show this rhythm of the transpiration curve in 

 mullein and tobacco. 



