230 NATUEAL HISTOEY BULLETIN 



ing was reduced to a minimum. Moreover the species are of 

 such character that we can easily account for their later intro- 

 duction by birds, and other animals, and by wind. The nearest 

 native groves containing these species are several miles away, 

 along the Missouri and Little Sioux rivers. This case, with many 

 others like it, illustrates the effect of shelter on the light-loving, 

 drouth-resisting prairie flora. 



SUMMAEY OF CONCLUSIONS 



The conclusions for our region may be briefly summarized as 

 follows : 



1. Exposure to evaporation as determined by temperature, 

 wind, and topography is the primary cause of the treelessness 

 of the prairies. 



2. The prairie flora persists on the exposed areas because it 

 is xerophytic. 



3. Kainfall and drainage, while of importance because deter- 

 mining the available supply of water in both soil and air, are not 

 a general, determining cause, both frequently being equal on 

 contiguous forested and prairie areas. 



4. Soils and geological formations are of value only in so far 

 as they affect conservation of water; the porosity of the former 

 determining its power of holding moisture, and the latter often 

 determining topography. 



5. Prairie fires were an effect rather than a cause, and where 

 acting as a cause were local. 



6. Seed-dispersal probably largely accounts for the grouping 

 of plant societies on the prairies, but does not account for the 

 presence of the prairie flora as a whole. 



7. Other assumed causes, such as the bison, the sea, etc., are 

 of remote interest and not to be taken into account in any 

 attempt at the explanation of the prairie as a whole. 



Acknowledgements. 



The work on evaporation at Missouri Valley was done in con- 

 nection with field-work for the Iowa Geological Survey, and is 

 here reported with the consent of the Director, Professor Samuel 

 Calvin. 



