'913.] OF THE UNITED STATES. 267 



The results of these studies have been pubUshed"* and furnish, for 

 fifteen weeks only, the second chart of evaporation which has ever 

 been prepared for this country. It is interesting to note that a fifth 

 of a century elapsed between these two studies, and tliat nothing 

 further has yet been attempted. 



Judging from the results already obtained, it appears that the 

 simple measurement and automatic summation of the evaporating 

 power of the air for the various climatic areas furnishes as satis- 

 factory a measure of the water-extracting power of the environment 

 as the student of plant relations can hope for from a single condition, 

 and the future development of this branch of science will depend 

 largely upon whether or not comparative evaporation records may 

 become available. 



Treatment of observational data. 



The frostlcss season. — In the preceding paragraphs have been 

 considered the most requisite methods for obtaining climatic obser- 

 vations. \\'e shall now turn our attention to the application of these 

 observations after they have been obtained. It is the custom of 

 meteorologists to derive from the actual observations, daily means, 

 monthly means and annual means, and to give most attention to the 

 latter. Now% for the purposes of vegetational-climatic investiga- 

 tions, it appears that none of these means offers much assistance. 

 In the determination of plant activities, at least in the majority of 

 cases, the controlling climatic factors are primarily effective only 

 during the growing season, and I am convinced that this season 

 should form the basis of a large part of the manipulation of climatic 

 records, which which we are here interested. As an approximation 

 of the vegetational growing season, for general use throughout our 

 country, it seems most promising to adopt the length of the frostless 

 season, the number of days intervening between the average dates 

 of the last killing frost in spring and the first in autumn. That other 

 duration factors will be required in many cases is not to be doubted, 



* Livingston, B. E., " A Studj' of the Relation between Summer Evapora- 

 tion Intensity and Centers of Plant Distribution in the United States," Plant 

 World, 14: 205-22. loii. 



