200 



BOTAXICAL GAZETTE 



[SEPTEMBER 



of a thermostat. Throughout this experiment the air stream was 

 constant at 70.32 liters per hour, and the dewpoint of the exterior 



of temperature was 17 . 8-36 . i°. 



range 



air was 4.8°C. The 

 The range in the value of / 



t z was 18.3 



The lines ot and ot 1 



give the values of / and t L respectively. The line oy gives the cal- 

 culated value of y where z [formula y = z (t — Q] equals 7.45 per 

 cent. The dots show the agreement of the experimental with the 

 calculated value of y. 



7 7 // I J /$" /7 II z t ll 2.4- 2.7 2? ;/ '3 3S-J7 *C 



Fig. s 



seems 



of the magnitude here given, the plant acts essentially as a physical 

 evaporating surface. On the other hand, when the relative 

 humidity of the external air is varied, there appears to be a very 

 decided response on the part of the plant, as is shown in the follow- 

 ing experiment. 



from 



highly humid atmosphere of the greenhouse and brought to the 

 much drier atmosphere of the laboratory, where the light also was 



greenhouse 



stems 



this 



were sealed into the evaporation cylinder as described (of course 



