6o 



re;ports of invi^stigations and projects. 



(7) The species considered (less than 500 in number) are of diverse geo- 

 graphical origin, including forms whose centers of dispersal range from sub- 

 tropical America to regions near the Arctic Circle. The major movements by 

 which these forms have come to their present occurrence have been gradual 

 and continuous since the Tertiary Period. Some portions of the area studied 

 are of more recent origin, but these have received a floral population from 

 the immediate neighborhood. The total amplitude of climatic change since 

 the Tertiary Period may be taken to be comparatively small, no greater per- 

 haps than the range now presented by the location of the Desert Laboratory 

 (2,663 feet), and the summit of the Santa Catalina Mountains (9,225 feet). 

 The present floras of these two places are probably more diverse in character 

 than the Tertiary and present floras of the Laboratory domain. During all of 

 the period in question the flora has been composed of two distinct elements — 

 the mesophytic species of the moister elevated regions and the xerophytic 

 forms of the arid slopes and plains. 



CORRELATED PROBLEMS. 



Bvaporation and Plant Distribution.— The investigation of the evaporating 

 power of the air as a total expression of the factors that determine the general 

 distribution of plants has been carried forward by Dr. Livingston and Dr. 

 Shreve, with peculiarly satisfactory results. 



Atmometers have been kept in operation during the season of 1908 at 50 

 stations (see p. 68), and the data at hand establish with fair conclusiveness 

 that the amount of water taken up by the air at any given locality during the 

 vegetative season is an infallible index of the character of the vegetation of 

 the encompassing region. As an illustration the following table may be cited : 



Relative Evaporating Pozvcr of the Air at 16 Stations in the United States, 

 June 3 to September 30, 1907. 



Mecca, California 



I.aramie, Wyoming 



Tucson. Arizona 



ShU Lake City, Utah 



Austin, Texas 



Raleigh, North Carolina. . 



Eugene, Oregon 



Gainesville, Florida 



St. Louis, Missouri 



Burlington, Vermont 



Lincoln, Nebraska 



Chicago, Illinois 



Orono, Maine 



Newark, Delaware 



Grand Rapids, Michigan. 

 New York City 



June. 



349 

 -522 



338 



92 



225 



120 



90 



112 



120 



96 



93 

 81 

 98 

 70 



July. 



368 



3n 

 228 



2S5 



166 



188 



196 



133 



157 



130 



90 



95 



91 



97 



82 



303 

 286 

 212 

 272 

 168 

 269 

 208 

 169 

 140 

 132 



113 



109 



96 



77 



105 



63 



September. 



278 

 274 

 282 

 227 

 215 

 151 



98 

 122 

 137 



63 

 144 



95 

 67 

 63 

 54 

 35 



Average. 



323 

 298 

 262 

 222 

 192 

 158 

 152 



*I43 



137 



112 



III 



98 



84 



83 



79 



*50 



♦Average for 13 weeks iustepd of 17. 



