May, 1919] Transpiration of Tobacco and Mullein 425 



The total water content of leaves for each hour of the day 

 from a similar plant was obtained by removing leaves at the 

 different hours and determining their water content by weighing 

 and drying to constant weight. 



A curve was then plotted for each of the following factors: 

 temperature, saturation deficit, evaporation, duration of sun- 

 shine, leaf water deficit, area of ^toma, periphery of stoma, 

 and the transpiration of the plant. The ordinates for each of 

 these curves were reduced to the same scale and the transpira- 

 tion curve was superimposed over each of the other curves. 

 This makes possible a comparison of the different limiting 

 factors controlling transpiration from muellin and tobacco. 



These records show that the differences between the night 

 and day rates of water loss from mullein and tobacco are 

 largely due to the diffusion of water vapor through the stomatal 

 pores. Transpiration from the leaf at night is cuticular and 

 the rate is controlled by the temperature and humidity of the 

 air. But the day rate is controlled by several factors operating 

 to increase or decrease the rate. When the stomata open, the 

 diffusion gradient (the difference between the intercellular 

 saturation and atmospheric saturation deficit) causes a sudden 

 rise in the rate of water loss. This rate continues until the 

 leaf water deficit decreases the diffusion gradient by increasing 

 the resistance of the mesophyll cells to water loss. After the 

 leaf water deficit reaches a certain point (about noon) there are 

 two factors, leaf water deficit and decreasing stomatal pores, 

 operating to reduce water loss and only one factor, diffusion 

 gradient, tending to increase it. This results in a rounded 

 curve. After the saturation deficit reaches a maximum there 

 are three factors, decreasing diffusion gradient, decreasing 

 stomatal pores, and leaf water deficit, operating to reduce 

 water loss with none of the factors opposing them. A rapid 

 decline in the rate of water loss from the plant follows and the 

 night rate is reached before the stomata are fully closed. 



While performing the experiments on the relation of hairy 

 coverings to the resistance of the leaf to water loss, a rhythm 

 in the transpiration curve was noticed in certain cases where 

 a plant was placed in the dark room and allowed to remain 

 in total darkness during the following day. This rhythm 

 was shown by a rise in the transpiration curve at the time the 

 stomata usually opens at daybreak. It reaches a maximum 



