Volume 14 Number 9 



The Plant World 



A Magazine of General Botany 

 SEPTEMBER, 1911 



A STUDY OF THE RKLATIOX BETWI'I-X SUMME^l 

 EVAPORATION L\TENSITV AXD CENTERS 

 OF PLANT DISTRIBUTION IN 

 ,THE UNITED STATES. 



Bi'RTON Edward Livingston. 



Since by far the major portion of the water absorbed by 

 ordinary gieen plants merely passes through the plant body 

 and is lost in transpiration, it naturally follows that the trans- 

 piration rate is itself the main condition \\hich determines 

 what may be an adequate rate of water supph' to such organisms. 

 But, as was first quantitatively shown in Publication 50 of the 

 Carnegie Institution (1906), the main condition which deter- 

 mines the intensity of transpiration from any plant is the 

 evaporating power of the air, and when an instrument became 

 a\ailable bv means ( f which it is possible rather readily to 

 measure and integrate this complicated climatic factor some- 

 what as it affects plant transpiration, it was suggested that com- 

 parative determinations of the various intensities of the evap- 

 orating power of the air which prevail in the different parts of 

 the United States in summer should furnish us with an approx- 

 imate measure of the demands of transpiration water which 

 the surrounding air makes during the season of greater plant 

 activity, upon the vegation of the different regions. In other 

 terms, it appears that the intensity of evaporation during the 

 growing season should offer a climatic criterion from which 

 might be derived a somewhat satisfactory correlation with the 

 general facts of plant distribution. This seems the more prom- 



