TRANSPIRATION OF SILPHIUM 327 



DISCUSSION 



The experiments carried on in the field indicate that the 

 greatest amount of evaporation usually occurs between 1.00 and 

 3.00 p.m. This period is also the one when temperature is usually 

 the highest and relative humidity the lowest. On a clear day 

 the rate of evaporation usually rose more or less regularly until 

 a maximum was reached after which there was a regular but more 

 rapid decrease in the amount of loss until evening or the end of 

 the experiment. The experiments in the laboratory brought out 

 the fact that the rate of evaporation varied in an almost direct 

 ratio with an increase in wind velocity. 



By comparing the tables and graphs for evaporation with those 

 for transpiration, it is seen that transpiration usually reached 

 a maximum from one to three hours before the time of greatest 

 evaporation. 



The data on the rate of evaporation at different heights show 

 very conclusively that the rate of evaporation increases very 

 much with an increase in height. This increase was due more 

 to the influence of increased wind velocity than to any other 

 factor. Since it was shown in the laboratory experiments that 

 the rate of transpiration increases with an increase of wind veloc- 

 ity within certain limits, and in view of the fact that trans- 

 piration, up to a certain point, increases with an increase in the 

 rate of evaporation, it becomes apparent that the upper portion 

 of a tall plant is exposed to very unfavorable conditions. How 

 a plant meets these conditions by a reduction of its leaf area in 

 the upper portion of the stem is well illustrated by Silphium 

 laciniatum. In one case two leaves were taken from the same 

 stem, one at a height of 12 cm. and the other at a height of 93 cm. 

 The one taken from the lower position had an area of 304 sq. 

 cm., while the one from the upper position had an area of only 

 63 sq. cm. In another instance a leaf taken from a height of 

 18 cm. had an area of 379 sq. cm. while another leaf from the same 

 plant taken from a height of 100 cm. had an area of 65 sq. cm. 



