the water and rnarsli surfaces, and the extent of still undisturbed 

 prairie, and of prairie groves and fringes, make this in many re- 

 spects the most interesting region in the state for ecological 

 studies. 



Each of these regions possesses the characteristic floral features 

 of its type, and each presents points of special interest. To some 

 of these brief attention is here given : 



THE PRAIRIE 



The prairie of this region is of special interest because it pos- 

 sesses certain peculiarities which throw Hght upon the cause of 

 the treelessness of such areas. In its topography and flora it 

 does not differ materially from much of the prairies of the state, 

 but the presence of large bodies of water, with the prairie ex- 

 tending in many cases almost to their shore lines, constitutes an 

 unusual features well worthy of attention. 



In the paper entitled "The Prairies" * the writer offered evi- 

 dence to show that factors operating in the atmosphere rather 

 than in the soil were the chief cause of treelessness, and the fol- 

 lowing conclusions were reached : 



"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. — Rainfall and drainage, while of importance, because de- 

 termining the available supply of water in both soil and air, are 

 not a general determining cause, both being frequently 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 lat- 

 ter 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 accounts largely for the gi^oup- 

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

 the presence of the prairie flora as a whole. 



4 Bulletin from the Laboratories of jSTatural History, State University of Iowa, 

 vol. VI, pp. 169-240, 1911. 



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