TRANSPIRATION UNDER CONTROLLED CONDITIONS. 



61 



the box was turned as the day advanced, so that no shadow ever fell on 

 plant or atmometer. The rapid evaporation caused an unavoidable lower- 

 ing of temperature within the box. The polymeter had to be so placed that 

 the humidity readings are probably a little too low. 



/Apr. 11 

 E 



Base for 



Apr. 11 E 



Apr. 11 T 



Apr. 4 



Apr. 4 

 T 



Base for T & E Apr. 11 



Base for T & E Apr. 4 



7 8 9 10 11 12 1 2 3 4 5 6 



FIG. 27. Graphs for transpiration of plant No. 7, under 

 two conditions of evaporation rate. Exp. XXII. 



These results also appear in table 24. They show that the evaporation 

 conditions for No. 6 were greatly reduced and that the actual transpiration 

 was greatly cut down. The maxima for both relative and actual transpira- 

 tion are coincident with the evaporation maximum. Thus, without doubt, 

 the drop and rise do not occur under the conditions of lessened evaporation, 

 but do occur in this same plant under more strenuous evaporation conditions. 

 In the case of No. 7, the evaporation conditions were more severe than 

 on April 4, but the difference is by no means so marked as was the case with 

 No. 6. The actual transpiration amounts are higher than before, showing 

 that the plant was capable of giving off more water than on April 4, but the 

 relative transpiration is on the whole lower. Furthermore, a flattening of 

 the curve of actual transpiration occurs between 11 and 1 o'clock, and the 

 relative curve shows a corresponding dip and rise. Evidently the limit of 



