492 BOTANICAL GAZETTE [December 



intimate 



The effect of wind was shown clearly on September 8 (charts VIII- 

 XIV), where relative humidity and temperature did not vary 

 much, but the miles of wind were greater during the day than 

 during the night; the rate of transpiration was greater during the 

 period of greater wind. That wind is a minor factor is shown by 

 the fact that the periods of highest wind velocity are not in all 

 cases periods of highest transpiration rates. 



Whether transpiration from plant surfaces is due to one factor 

 more than to another, we are not prepared to say from an examina- 

 tion of the data at hand. Indeed, considering the 

 tion existing between temperature and humidity, it would be 

 impossible to determine the exact influence of the one without being 

 able to maintain the other constant, a state of rare occurrence in 

 nature. And, after all, we are not greatly concerned with these 

 various factors individually, since not one of them of itself is capable 

 of exerting its greatest influence without the accompanying influ- 

 ence of the others. Whether the factors which influence plant 

 transpiration are identical with those which influence evaporation 

 from water surfaces, it is clearly brought out in these experiments 

 that the effects produced on the one and the effects produced on the 

 other are profoundly different in their intensities, and we are led 

 in consequence to the opinion that plant transpiration is not a 

 strictly physical process, but a physiological process as well. The 



men 



confirm the same 



Summary 

 Emersed water plants transpire large amounts 



With one exceDtion (water lilvV the evaporation 



y 



from a water surface occupied by emersed water plants is much 

 greater than that which takes place from a free water surface of the 

 same area and subjected to the same external conditions. 



3. The amount of evaporation from a water surface on which 

 water lilies are growing is less than that which takes place from a 

 free water surface of the same area and subjected to the same 

 external conditions. 



