8 THE PLANT WORLD. 



week of Its period of operation corresponded approximately 

 to the last week of the first mountain period. A general idea 

 of the relation of the evaporating power of the air at these 

 four stations can be obtained by comparing the weekly rates 

 of loss from the mountain instruments with the evaporation 

 from the Tucson instrument for the first week after its in- 

 stallation. The corrected results are given in Table IV. The 

 rates are in cubic centimeters and of course are to be consid- 

 ered simply as numbers representing the relative evaporation 

 rates at the four stations at this time of the year. In the 

 third column of the table are given the relative numbers, the 

 rate at Tucson being considered as unity. 



TABLE IV. 



Altitude. Weekly rate, cc. Relative rate. 

 2412 feet. 289 1. 00 



6000 feet. 238 0.82 



7500 feet. 147 0.51 



8000 feet. 133 0.46 



From these data it appears that, during the last week in 

 May the evaporating power of the air at the four stations 

 considerably decreased toward the higher altitudes. The week 

 which these records are taken to represent marked the first 

 vigorous spring growth in the highest altitude given. The 

 average weekly rates for the period from May 13 to June 3, 

 at Orono, Maine, Burlington, Vt., and Raleigh, N. C, were 

 123 cc, 1 12 cc, and 126 cc, respectively, by instruments ex- 

 actly similar to those here used. The vegetation about the 

 highest instrument in the Santa Catalina Mountains possesses 

 the same ecological characters as that in the vicinity of Orono 

 and Burlington, the higher levels of the Santa Catalinas 

 being occupied by pine, spruce, maple, alder, elder, cornel, 

 brake, columbine, violet, etc. Furthermore, plants in Maine 

 and Vermont were in about the same condition as those at our 

 upper station at this time of year. The season at Raleigh 

 was of course much farther advanced at this time, so that 



