390 



GRASSES OF IOWA. 



and historical conditions. The ecological factors are discussed under the 

 heads of edaphic, atmospheric, hydrodynamic and biotic. 



Fig. 263. Porcupine grass the Stipa spartea On grassy knolls, drift soil with 



Viola pedata, Echinacea purpurea and Ceanothus Americanus. 



(Charlotte M. King, Photo. ) 



1. The Edaphic. The soil is an important matter with reference 

 to the formation of plant societies. These conditions are often very local. 

 Some soils readily retain moisture, some absorb heat more than others,, 

 and this may determine largely the presence or absence of plants, as in 

 Sporobolus cuspidatus which is so abundant on the loess bluffs and grav- 

 elly knolls of western and northwestern Iowa. The amount of humus 

 m the soil is likewise an important matter. 



2. Under the term atmospheric are included such factors as forms 

 of heat, light and wind. We have only to notice how difficult it is to es- 

 tablish a forest on the open prairie and how much easier it is to establish 

 a forest in the forested area by proper succession of plant growth. How 

 difficult it is tor grasses of the forest to encroach upon the prairie. 



3. Hydrodynamic is used here to designate the action of tides and 

 waves upon strand vegetation, and the action of stream and ocean cur- 

 rents in distributing seeds. This has been an important factor in de- 

 termining the presence of Spartina and Leersia along our streams. 



