272 BOTANICAL GAZETTE [October 



The curves n fig. 8 were plotted from readings taken at sta- 

 tion D on September 2. One of the outstanding features of this 

 graph is the parallelism in all the curves. It will be observed 

 that the maxima of transpiration coincide with those of tempera- 

 ture, evaporation, and relative humidity; and that there is an 

 absence, or at least almost a complete absence, of an afternoon 

 saturation deficit. In both of these respects this graph differs 

 from the preceding one. The difference was found to be related 

 to the atmospheric conditions. The evaporating power of the air 



Fig. 8. — Graph plotted from data taken at station D on September 2, 1918; 

 parallelism of curves and absence of saturation deficit due to high relative humidity. 



was low throughout the day, never reaching 1 cc. per hour, while 

 ordinarily at station B it became 2 cc. or more per hour by 2 : 00 

 p.m.; the temperature was low and the humidity high. This 

 combination of factors in the field was always found to favor 

 parallelism of curves and an absence of a saturation deficit. 

 Under such conditions the maxima usually occur later in the day 

 than when the deficit is developed. 



At 7:30 a.m. there occurs a sudden fall in the transpiration 

 indices which resembles the deficit drop. This was due to a sud- 



