June, 1919] Variations in Tra^ispiration 505 



Experiment 2: The curves for this experiment show the 

 daily variation in transpiration of mullein and tobacco under 

 ordinary conditions,' with the environmental factors which 

 influence water loss from plants. The transpiration rates show a 

 rapid rise shortly after sunrise with high rates of water loss as 

 compared with the night rates during the period of sunlight. 



Experiment 3: This experiment shows the transpiration 

 curves for the same two plants in a dark-room. The temper- 

 ature, saturation deficit of the air, and evaporation were con- 

 stant in the dark-room. But in spite of these constant environ- 

 mental conditions the transpiration rates show a rise and fall 

 or rhythm during the day. This rise in the rate of water loss 

 begins about the time the stomata usually open or about the 

 fourth or fifth hour and reaches a maximum about the ninth 

 or tenth hour when there is a gradual decrease to the usual 

 night rate by the middle of the day. 



Experiment 4: In order to show that this rhythm was not 

 characteristic of a single plant, different plants were put in the 

 darkroom and their transpiration curves obtained. The curves 

 from three mullein plants and one tobacco plant show this 

 rhythm under conditions of constant temperature, humidity, 

 and evaporation and in total darkness, which began, reached a 

 maximum, and declined to the usual night rate similar to the 

 curves of Experiment 3. 



Experiment 5: The plants used in Experiment 4 were left 

 in the dark-room and the curves for the second day under con- 

 stant environmental conditions were obtained. There are no 

 indications of a rhythmic rise and fall in the transpiration 

 rates in this experiment. 



Experiment 6: These curves show the hourly rates of 

 transpiration and the related environmental factors in the 

 greenhouse for three different plants, mullein, tobacco, and 

 moth mullein (Verbascum blattaria). These plants have similar 

 transpiration curves under these conditions. These mullein 

 and tobacco plants were not used in any of the preceding 

 experiments. 



Experiment 7 : These curves are from the same three plants 

 that were used in Experiment 6, but they were obtained under 

 constant environmental conditions in the dark-room. The 

 curves for the rate of water loss from mullein and tobacco show 



