202 BOTANICAL GAZETTE [September 



without removing them from their own roots growing in the pot), 

 a current of air was drawn over them, and successive readings 

 made, as shown in table IV. 



Since the area of leaves was not changed in table IV, the 



t—t t 



simpler form of the formula y = — ; — t~\z could be used; c was 



taken as 0.25 and n as 0.93. This high value of n is in keeping 

 with the fact already mentioned, that when the evaporating area 

 is large in comparison with its environment, the value of n will be 

 large. Sixty-two leaves, having an area of approximately 200 

 sq. cm., were on the stems included within the cylinder. Thus it 

 may be noted that the value of y for the slower wind movement 

 averaged 80 per cent of the value of t — t z . In this experiment 

 both the wind movement and the humidity of the air passed over 

 the plant were varied. The latter was accomplished by (1) using 

 the normal air of the room, (2) first passing the air through tubes 

 of phosphorous pentoxide and then over the plant, and (3) passing 

 over the plant a controlled mixture of saturated and normal air. 

 It is not assumed that the air passed over the phosphorous pen- 

 toxide was absolutely dry, but its dewpoint was so low that it could 

 not be detected by the ether-cooled mirror, which could be easily 

 read down to —6 or — 7 C. 



For the purpose of discussion the experiment may be divided 

 into eight periods, as follows: 



(a) 1 144-2:07; normal air, slow wind 



(b) 2:07-2:32, dry air, slow wind 



(c) 2:32-2:43, dry air, fast wind 



(d) 2 : 43-2 : 53 , dry air, slow wind 



(e) 2 : 53-2 : 59, normal air, slow wind 

 (/) 2 : 59-3 : 16, normal air, fast wind 

 (g) 3 : J 6~3 :45, moist air, slow wind 

 ih) 3 : 45~3 : 55, moist air, fast wind 

 (*) 3 : 55~4'-05, moist air, slow wind 

 (J) 4:05-4:29, normal air, slow wind 



First compare the column showing the outside temperature 

 with that exhibiting the temperature within the cylinder and 

 measured by a thermometer having its bulb among the leaves. 

 The temperature within the cylinder began o.3°C. above that 



; 



