TRANSPIRATION OF SILPHIUM 315 



further evidence is given to substantiate the above conclusion. 

 The data for the evaporimeters used in a series of experiments, 

 herein mentioned and others, have been brought together in 

 table 3 and presented as further proof that the rate of evaporation 

 increases with an increase of height. 



Since in a series composed of as many experiments as this, 

 the evaporimeter at the greater height lost the greater amount 

 without a single exception, the natural conclusion is that evap- 

 oration increases with an increase in height. The impor- 

 tance of these variations in relation to height will be noted later. 



LABORATORY EXPERIMENTS 



The chief purpose in view in these experiments was the de- 

 termination of the effect of different wind velocities on the rate 

 of evaporation and transpiration. This particular problem was 

 undertaken in the laboratory rather than in the field because 

 in the laboratory by using an electric fan the experiment could 

 be kept more nearly under control than was possible in the field. 

 The fan was placed in .one end of an elongated box, and part of 

 the evaporimeters and leaves were placed side by side in the box 

 and part of them outside of the box. In selecting the leaves 

 for these experiments care was taken to get them from about 

 the same height, of about the same age, and of approximately 

 the same size. As it would be undesirable to give the details 

 for a whole series of experiments, the details of only one will 

 be given, with the final results of a few others for comparison. 



In these experiments Piche evaporimeters were used to deter- 

 mine the rate of evaporation. These instruments were very 

 convenient to handle, and trial tests showed that they gave very 

 accurate results if properly adjusted and used by the same 

 individual. In the experiments of July 8 (table 4) the first 

 evaporimeter and leaf were within the box at a distance of 60 cm. 

 from the fan; the second set was within the I o v at a distance of 95 

 cm. from the fan; and the third set outside the box at a distance of 

 140 cm. from the fan. The experiments were run twelve hours^ 

 beginnirg at 7.30 a.m. The instruments and leaves are numbered 

 in the order of their distance from the fan. The leaves were 



