WATER-RELATION BETWEEN PLANT AND SOIL. 39 



ABSOLUTE TRANSPIRATION (T). 



The transpiration maximum (9.10) occurred with hour 11 of the first 

 series of observations and the low night rate was attained with hour 19, 

 continuing to hour 6 of the following day. This occurrence of the 

 February maximum of absolute transpiration two hours before that 

 of the evaporating power of the air is (according to Livingston and 

 Brown, 1912) evidence of pronounced incipient drying in the leaves. 

 That this condition was here pronounced during the hours 12 to 15 

 is clearly shown by the fact — noted in table 3 — that foUar wilting was 

 observed for these hours. The night average (for hours 19 to 6) is 

 0.69, and the maximum is 13.2 times this value. 



Turning to the March series, the transpiration maximum (7.70) 

 occurred with hour 13, the same hour as exhibited the maximum in the 

 evaporating power of the air. The failure of this transpiration maxi- 

 mum to occur earUer than that of evaporation, in spite of the fact that 

 wilting set in an hour earlier (hour 12) in the second series than in the 

 first, and lasted an hour longer (till hour 16), may be related to some 

 unexamined internal difference between the plants at the two times of 

 study, or possibly to the actual times at which the observations were 

 taken. No adequate data appear to be at hand for an investigation 

 of this discrepancy. The rapid decrease in rate, after the attainment 

 of the maximum in the second series, seems to denote more pronounced 

 incipient drying here. 



In this March series the low night rate of transpiration was attained 

 with hour 18, an hour earUer than in the February series, and the low 

 rate persisted till the sixth hour of the next day. The average night 

 rate is 0.55 and the maximum is 14.0 times this value. 



TRANSPIRING POWER (I-Hfi). 



The graph of transpiring power, or relative transpiration, for the 

 February series exhibits an unusual form for this kind of graph. Its 

 maximum (3.22) is attained with the eleventh hour, closely approached 

 with the tenth, and the fall to the low night value is remarkably uni- 

 form and gradual. The low night rate is attained with hour 19 and 

 maintained till hour 7, with imsatisfactory fluctuation related to 

 inaccuracies of weighing. The night average is 0.68 and the maximum 

 is 4.7 times this value. The early occurrence of the maximum, two 

 hoiirs before the maximum of evaporation, agrees with the similar 

 behavior of the graph of absolute transpiration for this February series, 

 seeming to indicate pronounced incipient drying early in this day. 



The relative transpiration graph for the March series of observations 

 shows a maximum (1.94) with hour 13, simultaneously with the maxi- 

 mum of the atmometric rate and with that of absolute transpiration. 

 The period of low night values begins with hour 17 and extends to hour 

 6 of the next day. The average night rate is 0.38 and the maxunum is 

 5.1 tunes this. This ratio value is similar to that given for the earlier 



