iQ2o] FREEMAN— EVAPORATION AND TRANSPIRATION 203 



outside, but slowly fell throughout periods 0, b, and c y reaching its 

 lowest point in the later period of dry air and high wind move- 

 ment when it was 2 . 2 below that of the outside air. When in 

 period d slow wind movement was used, the cylinder temperature 

 rose o.6°. In period e, when normal air was used t the cylinder 

 temperature rose 0.2 . In/, when fast wind was again used, the 

 temperature dropped 0.2 . When high humidity air was used in 

 periods g, h, and i, and the transpiration was very low, the tem- 

 perature rose to within o . i° of that of the external air, but during 

 period 7, when normal air was used and the evaporation increased, 

 the cylinder temperature steadily fell, until at the end of the 

 experiment it was 1 . 2 below that of the exterior air. We thus 

 have a definitely measurable cooling effect of evaporating leaves 

 upon the air which passes over them. 



In order to study the physiological response of the plant foliage 

 as an evaporating surface, the column giving the dewpoint inside 

 (the dewpoint of the air after it has passed over the plant) may be 

 compared with the next column, which shows the calculated 

 values, considering the leaves as physical evaporating surfaces 

 and using the constants already given. To facilitate this com- 

 parison the last column is added to give the ratio of the found to 

 the calculated dewpoint. 



It is estimated that the time required to bring the plant from 

 the greenhouse and instal it in the transpiration apparatus and 

 obtain the first reading was about half an hour. Suppose the 

 evaporating condition at this time to be 104 (see last column of 

 table). By the end of period 0, with the plant exposed to the 

 drier laboratory air (23 minutes), this had dropped to 92. During 

 periods b y c y and d (46 minutes) the plant was exposed to air which 

 had passed over phosphorous pentoxide. Although the comparison 

 of the calculated with the found results cannot be made here, it 

 may be observed that the dewpoint during period d was lower 

 than during period b, which indicates a still further restriction of 

 evaporation. When the plant was put back on normal air in 

 period e, it was found that this restricting influence had cut down 

 the found dewpoint to 70 per cent of the calculated value, and 

 that this was maintained throughout the 2^ minutes of periods e 





