SOIL MOISTURE AND TO EVAPORATION. 49 



ment. This fact is shown also by the uniformity of the ratio curve 

 itself, and it makes possible the use of this method for determining the 

 average rate of relative transpiration. 



The physiological periods cut off by the average line are seen to be 

 fairly regular. They do not, however, as has been already noted, 

 coincide with the periods of solar day and night, but terminate in the 

 vicinity of the hours 3 a.m. and 3 p.m. The period of high rate of 

 relative transpiration falls mainly in the day and that of low rate 

 mainly in the night. 



The average ratio for each of the partial periods just described was 

 determined in the same manner as was that for the whole period of 

 the experiment. These ratios are given in Table VIII and are shown 

 on the ratio curve by horizontal lines extending within the limits of 

 the time period which they represent, and having the average ratio 

 for constant ordinate. In the last line of the table are given second 

 averages of the three night periods and of the two complete day periods. 

 Inspection of these data makes it evident that in this case relative trans- 

 piration was, in round numbers, three times as great for the day 

 periods as for those of the night. 



TABLE V 'III. Average Ratio Experiment I. 



A comparison of the rate of evaporation from a free water surface 

 with the transpiration rate from an equal leaf surface was long ago 

 made by Unger (1861), who even went so far as to determine the 

 ratio between the two daily rates, showing that this ratio for Digitalis 

 purpurea varied in value, under different weather conditions, from 1 : 7 

 to 5:7. This writer observed the existence of a daily periodicity of 

 absolute transpiration and called attention to the fact that the varia- 

 tions in the rate of transpiration do not follow exactly the variations in 

 the evaporation rate. He regarded transpiration (p. 368) , as ' 'ein physi- 

 kalischer durch die Beschaffenheit der Pflanze modificirter Process. " 



The only other experimenter who has studied the ratio of the trans- 

 piration rate to that of evaporation is Masure (1880), who obtained, 



