June, 1919] Variations in Transpiration 499 



used for measuring the areas of the openings and a small 

 flexible centimeter rule for the peripheries. 



In this experiment as well as in the preliminary work which 

 was necessary to become familiar with the method, it was found 

 that 85% or more of the stomata examined were closed at night, 

 and as far as could be determined with the high power of the 

 microscope this closure was complete and prevented gases from 

 diffusing through them. In sunlight it was found that 90% 

 to 96% were open. This difference is due, probably, to the 

 fact that some of the stomata were fixed, or had ceased to open 

 and close. Those that were found closed were regarded as 

 zero in dimensions in calculating the area and periphery, and 

 the results are expressed as an average of the number observed 

 rather than the number found open. The results do not rep- 

 resent correctly the size of an average opening, because of their 

 elliptical shape and the manner in which they close, but more 

 nearly the average diffusion capacity of the stomatal area of the 

 leaf. This method of observing and calculating the average size 

 of the stomatal openings was followed in Experiment 1 and the 

 results are given on pages 492 and 494. 



These curves give the transpiration rates of mullein and 

 tobacco, size of stomata, leaf water deficit and other factors of 

 the environment. An inspection of these curves shows that 

 the maximum temperature during the day occurs about the 

 13th hour and the maximum saturation deficit about the 15th 

 hour. The maximum evaporation occurs about the 14th hour 

 because it depends mainly upon the temperature and saturation 

 deficit. The minimum of these factors occurs during the night. 



The day was clear and quiet and there was full sunlight 

 except for a short time, both in the morning and evening, when 

 misty clouds occurred near the horizon. The transpiration 

 curves began to rise about the sixth hour and rose rapidly 

 until the ninth hour when the rise became more gradual until 

 the maximum was reached at the 13th hour in tobacco and the 

 14th hour in mullein. This occurrence of the maximum rate of 

 water loss before the maximum evaporation shows that some 

 factor in the plant is operating to reduce water loss. The min- 

 imum of the transpiration curve occurs during the night. The 

 leaf-water deficit began to rise gradually about the eighth hour 

 and continued until a maximum was reached about the 15th or 

 16th hour when there was a decline to the usual amount by the 



