THE PEAIEIES 221 



the margin in this direction not a single prairie species could be 

 found. 



The absence of trees and the presence of a prairie flora on these 

 areas which are in every way typical prairie areas, cannot be 

 explained on the ground of any other factors thus far considered 

 excepting that of exposure to evaporation. Rainfall is the same 

 upon them and the adjoining slopes; frequently the slopes are 

 the same and affect run-off equally ; the underlying formations 

 are the same ; excepting for a rather thin veneer of leaf mould 

 formed in the forest the soils are the same, and moreover are as 

 variable in different openings as they are on the general prairies ; 

 and general climatic conditions cannot be materially different in 

 such contiguous forest and prairie areas. Furthermore it is in- 

 conceivable that fires could have cleared these small areas and 

 kept them cleared year after year without destroying the nearby 

 forest, nor can the tramping of bisons be here considered as a 

 cause. 



As stated, these prairie openings are alike in exposure and in 

 flora, and in these particulars they agree also with the general 

 prairie areas. Exposure here periodically produces xerophytic 

 conditions, and the prairie flora, which is more or less xerophytic 

 in its structural adaptions, is alone able to persist. 



Sand-Dunes 



Further confirmation of the conclusion that the floral con- 

 ditions of the prairie are not due to fires, soil, etc., is furnished 

 by the sand-dune areas of Iowa and surrounding territory. 



While the flora of the sandy areas presents certain special 

 features, by far the greater part of it consists of typical prairie 

 plants, as shown in column VI of the prairie plant list. In this 

 list are included several species which appear as occurring only 

 on sand-dunes, but they are also found on other dry surfaces in 

 other areas than those considered specifically in connection with 

 the list. They are the following, and all but the last show a 

 decided preference for sandy areas : 



Apocynum cannabinum var. pubescens (E. Br.) DC. 

 Astragalus distortus T. & G. 

 Draba caroliniana "Walt. 

 Euphorbia serpens HBK 



