36 WATER-RELATION BETWEEN PLANT AND SOIL. 



In the February series of observations, as has been noted, the trans- 

 piring power appears to have attained its maximum only at a tune an 

 hour later than that marked by the maximum of evaporation. In the 

 March series, however, the maximum in calculated transpiring power 

 occurred an hour earlier than did that of the evaporating power of the 

 air. In the latter case incipient drying appears to have occurred. 



ENVIRONMENTAL ARIDITY (/XE). 



The two graphs of the value IE differ mainly in two respects: the 

 second series shows a much higher maximum (20.2, hour 14) than does 

 the first (11.54, hour 13), and the second shows a more rapid ascent 

 during the early hours of the forenoon. The period of low aridity 

 extends from hour 23 to hour 9 for the first series and from hour 1 to 

 hour 6 for the second. This last observation is to be taken in connec- 

 tion with the fact that the evaporating power of the air was seen to 

 exhibit a much longer period of low values, beginning with hour 20. 

 This tardy development of the night condition, in environmental arid- 

 ity, is no doubt due to the feature already emphasized, that the evening 

 fall in evaporating power and the nearly simultaneous and pronounced 

 fall in transpiring power (the latter probably caused mainly by closure 

 of stomata) do not immediately bring about a corresponding decrease 

 in the rate of absorption by the roots. Thus, the resistance to root 

 absorption remains relatively high for several hours after the aerial 

 desiccating conditions and the internal transpiring power have attained 

 the low values characteristic for the night. The downward slope of the 

 IE graph, from hour 20 to hour 1, is gentle and prolonged, since it is 

 affected by the lag in the fall of irrigator loss for the early night hours, 

 above mentioned. 



The average night value of this index is 1.31 for the 11 hours of the 

 first series and 1.58 for the 6 hours of the second. The corresponding 

 maxima are 8.8 and 12.8 times the respective night averages. 



DAILY MEAN CONDITIONS COMPARED. 



If we scrutinize the daily averages given at the base of table 1, it 

 appears that the evaporating power of the air was 1.39 tunes as great 

 for the March as for the February period. Similarly, the correspond- 

 ing ratio for the resistance offered by the soil to water absorption by 

 the roots was 1.04, and for the general environmental aridity it was 

 1.53. On the other hand, transpiring power and absolute transpira- 

 tion show greater average values for the earlier series, the ratios corre- 

 sponding to those just given being 0.68 and 0.92, respectively. It thus 

 seems to be indicated that the plants had lower transpiring power as 

 they became older (they were not very much larger at the second 

 period than at the first, but the percentage of young leaves was much 

 lower) and that this actually decreased the transpirational loss, although 



