74 



SEVENTH REPORT. 



In the accompanying map the ratios, found by dividing the mean an- 

 nual rainfall by the depth of evaporation at the same station, are repre- 

 sented by percentages. If this map be compared with the map pub- 

 lished by Sargent in the Ninth Census report, it will be noted that there 

 is a remarkable coincidence between its divisions and the forests of the 

 eastern United States. The Northeastern Conifer Forest Center is 

 marked by a rainfall-evaporation ratio amounting to between 100 and 

 200 per cent. The Deciduous Forest Center is indicated by the area lying 

 between the 100 and 110 per cent lines. The Southeastern Conifer Cen- 

 ter has a ratio above 110 per cent. The Great Plains are marked by an 



amount of rainfall equal to from 20 to 60 per cent of the evaporation. 

 Where the ratio rises to between 60 and 80 per cent, the prairie region, 

 where dense forests are confined to the river bottoms, is indicated. 

 The region where "open forests," '"oak openings" and "groves" occur on 

 the uplands and dense forests on the low grounds, is indicated by the 80 

 to 100 per cent ratios. The map can only be considered as an approxi- 

 mation, nevertheless it shows why the Northeastern and Southeastern 

 centers are separated from one another in the Mississippi valley and are 

 more or less mixed in the Alleghany region. It suggests that the South- 

 eastern Conifer Center is dependent upon climatic as well as edaphic 

 factors. 



The conclusion seems warranted that a map based on rainfall-evap- 

 oration ratios (since they involve four climatic factors: temperature, rela- 

 tive humidity, wind velocity and rainfall) will show climatic centers which 

 coincide ajjproximately with the centers of plant distribution. In the 



