472 BOTANICAL GAZETTE [December 



from the control tanks. This might be expected from the fact 

 that the lily leaves floated on the water, forming a partial cover 

 and reducing the area of water surface exposed to the air. More- 

 over, lying in a plane only slightly above that of the water surface 

 itself, the lily leaves were constantly bathed by the very humid 

 atmosphere which is commonly present a short distance above 

 the water surface. It is also probable that the amounts assigned 

 to the taller-growing plants are somewhat lower than the actual, 

 since dew collected heavily on the cooler and broader leaf surfaces 

 during foggy nights and, running down the veins and petioles, 

 found its way into the tanks, thus making the observed measure- 

 ment too small. The water lily was the only plant found to 

 diminish the evaporation from a water surface. All other plants, 

 especially those with a large leaf and petiole surface exposed to 

 the air, increased the evaporation, in one case as much as three 

 times (for example, cat- tail, table V). 



(b) Periodicity of evaporation 



com 



y 



it will be noticed that a comparison of successive day and night 

 records for any one tank usually shows a period of high rate of 

 evaporation during the day, followed by a relatively low rate of 

 evaporation during the succeeding night. This phenomenon is 

 most conspicuous in the cat-tail, sweet flag, picker el- weed, and 

 others exposing a large transpiration surface to the air, and least 

 evident in the rushes and water lily. 



The amounts of evaporation from the various tanks given in 

 tables IV and V have been incorporated for the sake of convenience 

 and ready comparison in charts I-XIV, the standard of comparison 

 being the rate of evaporation in cubic centimeters per hour. Charts 

 I and VIII show the rate from the control tanks, and the mean 

 value has been taken to represent the rate of evaporation from a 

 free water surface. These values are plotted in the graphs of 



with a heavy broken line. The heavy 

 :e from the tanks containing emersed 



IX-XIV 



from 



from 



The vertical 



