WATER-RELATION BETWEEN PLANT AND SOIL. 35 



are 0.65 and 0.32. The maximum for the February series is 12.5 times 

 and that for the March series is 25.0 times the corresponding night 

 average. The great discrepancies between these ratios, on the one 

 hand, and the corresponding ones for evaporation and irrigator loss 

 on the other, are apparently related to alterations in transpiring power 

 brought about by changes within the plant. These two maxima are 

 followed by approximately parallel declines. 



In the earlier series of measurements, the occurrence of the maxi- 

 mum transpiration rate an hour later than the occurrence of the maxi- 

 mum in the evaporating power of the air may be related to the fact 

 that the position and surroundings of the atmometer could not be 

 actually identical with those of any single culture; the evaporating 

 power measured may have been somewhat lower than that really 

 effective about the group of plants in question. Or, it may be due to 

 the particular times at which observations were taken. At any rate, 

 the failure of the transpiration rate (and also that of relative transpira- 

 tion, as will be seen later) to exhibit a maximum earlier in the day than 

 the maximum of evaporating power of the air makes it appear that 

 transpirational water loss was not retarded by incipient drying in this 

 earlier series. In the later series transpiration attains its maximum 

 (hour 12) an hour earlier than does evaporation. The latter seems to 

 be the typical condition of affairs (Livingston and Brown, 1912, and 

 Edith B. Shreve, 1914) with plants taxed by environmental aridity. 

 The points just brought out constitute additional evidence that the 

 effective aridity of the surroundings was more intense during the fore- 

 noon hours of the March experiment than during the hours of forenoon 

 and early afternoon of the February series. 



TRANSPIRING POWER (T^E). 



This involves the rate of absolute transpiration, and its graphs there- 

 fore show characters somewhat similar to those just considered. The 

 maximum of the earlier graph (3.25) occurs with hour 14, the same 

 hour as was seen to show the transpiration maximum. In the later 

 graph the maximum (2.51) occurs with hour 12, which also agrees with 

 the occurrence of the corresponding maximum in absolute transpiration. 



The periods from hour 18 to hour 7 in the first series and to hour 6 

 in the second (the night hours) were characterized by very low magni- 

 tudes of the relative transpiration index. The average night value is 

 0.61 for the February series and 0.27 for the March series, and the 

 corresponding maxima are 5.3 and 9.3 times their respective night 

 averages. It thus appears that the internal variation in transpiring 

 power, from night to day, is considerably greater than the correspond- 

 ing environmental variation either in evaporating power of the air or 

 absorbing power of the soil. In this connection it should be remem- 

 bered that both these environmental variations were relatively small 

 in our experiments. 



