70 



Deherain also arranged the following experiments showing the 

 effect of temperature. Some living leaves of wheat were kept within 

 a glass tube which lay in a water bath at a uniform temperature of 

 15° C. and the folloAving measurements taken : 



In full sunshine the transpiration was O.D39.gram of water per 

 hour i^er gram weight of leaf. 



In darkness the transpiration was 0.016 gram of water per hour 

 per gram weight of leaf. 



The water bath was then reduced to a temperature of 0° C, and the 

 temperature of the leaf within the tube must therefore have been at 

 the freezing point. In this condition the transpiration in full sun- 

 shine was 1,088 grams of w^ater per hour per gram weight of leaf. 



Thus leaves in sunshine in free air at 28° C. and leaves in the 

 air at 15° C, and again in the water bath at 0° C., give us the tran- 

 spiration under these conditions 0.882, 0.930, 1.088, respectively. It 

 is evident that this transpiration is not due to evaporation alone, else 

 it would be independent of sunshine and depend wholly on heat ; the 

 decided differences here shown must be attributed to the special 

 excitement of the cell by the solar radiation. 



Marie Davy gives for July 24 and 25, 1877, the following record 

 from a self-registering apparatus showing the diurnal periodicity of 

 the transpiration from the leaves of four plants of haricot beans 

 which were watered daily at 7 p. m. : 



Diurnal periodicity of transpiration. 



These same four plants showed the transpiration day by day, as 

 given in the first column of the following table (Marie Davy, 1880, 

 p. 239). The third and fourth columns, respectively, show^ the rela- 

 tion of this transpiration to the daily mean temperature and the daily 

 mean radiation, as shown by the conjugate thermometers. 



