Jan., 1920] Resistance of Leaves to Transpiration 67 



of the forenoon. The hourly ratios of the water loss from the 

 two plants vary from 0.60 to 1.40 and the ratio for the total 

 daily water loss is 0.81. 



Experiment 2. This experiment was performed under con- 

 ditions similar to experiment 1, except that an electric fan was 

 used to produce wind, at three hour intervals. The tabulated 

 results (page 76) and curves (page 64) show that under these 

 conditions the temperature and saturation deficit are constant 

 throughout the day while the results for water loss are deter- 

 mined by the wind velocity. The evaporation is proportional 

 to the wind velocity, but the transpiration does not show 

 these results, although the water loss of each plant is increased 

 by the wind. In tobacco the highest rate of water loss is during 

 the time when the wind velocity is least and the highest wind 

 velocity produces the smallest increase in the rates of both 

 plants. These irregularities are due to the fact that there is a 

 rhythm in the curve at the time of the second interval as shown 

 in experiment 1. The hourly ratios show less variation under 

 the conditions of this experiment than in experiment 1, and 

 the total daily ratio of water loss is considerably decreased; 0.59. 



Experiment 3. The tabulated hourly rates of this experi- 

 ment, which was performed in the Greenhouse in still air, 

 are given on page 77 and a set of curves to correspond to 

 this table on page 64. The variations in temperature and 

 saturation deficit in this experiment are due to sunlight which 

 increases them. The rates of evaporation correspond to the 

 increased temperature and saturation deficit. There is a 

 greater variation between the maximum and minimum rates of 

 transpiration than in either of the two preceding experiments, 

 due to the sunlight. This experiment is taken as a standard for 

 comparison with the other experiments because the conditions 

 are those to which plants are usually exposed. The total daily 

 ratio for this experiment is 0.69. 



Experiment 4. This experiment was similar to experiment 

 3, except an electric fan was used to produce wind, at three 

 hour intervals as in experiment 2. The results are given on 

 pages 77 and 64. The variations in temperature and satura- 

 tion deficit occur according to the periods of day and night. 

 The evaporation is proportional to the wind velocity, taking 

 into consideration the changes in temperature and saturation 



