204 BOTANICAL GAZETTE [September 



and /. When, however, a stream of very moist air (relative 

 humidity 79-82 per cent) was used, the plant again responded, 

 and within 20 minutes (reading at 3:36 p.m.) had reached an 

 evaporation rate of 95 per cent of the calculated value. This 

 increase continued until at 4:04 (28 minutes later) it was 101. 

 When the shift was again made to normal air (at 4:05) there was 

 exhibited at 4 : 09 (4 minutes later) a special emphasis of this high 

 transpiring condition of the plant, since it gave a reading of 121 per 

 cent of the calculated value. The effect of the drier normal air 

 soon became apparent in the steady fall of this ratio, which at 

 4: 29, or 24 minutes after removal of the moist air, had reached a 

 value of 104, which was identical with its value at the beginning 

 of the experiment, when it had likewise been removed from the 

 saturated air of the greenhouse some 25 or 30 minutes previously. 

 The response of the plant to the humidity of the air is strongly 

 indicated here. This response is rapid but not immediate. In 

 looking over a number of experiments similar to the one described, 

 it appears that when the humidity is suddenly changed but remains 

 constant thereafter, complete adjustment is usually attained in 

 30-35 minutes. 



It may be recalled that in the experiment involving changes in 

 temperature only (fig. 5), the j^ant leaf acted approximately as a 

 physical evaporating surface. This was true in spite of the fact 

 that the change in the temperature of the air passing over the 

 plant also involved a considerable change in its relative humid- 

 ity. Its actual dewpoint, that is, the water contained per liter, 

 remained constant. At the higher temperatures the actual 

 dewpoint of the air after passing over the plant was always 

 higher than at the lower temperatures; that is, the leaves were 

 evaporating more into an air of higher dewpoint than at the 

 lower temperatures. This fact approximately offsets the effect of 

 the greater water demand made upon the plant by the greater 

 transpiration at the higher temperature, and gives a result which 

 is very close to the behavior of a physical surface. When, how- 

 ever, the water content, that is, the dewpoint of the incoming air, 

 is increased (other conditions remaining the same), the evapora- 

 tion, or the water demand upon the plant, is strongly reduced. 



