150 



those forces or conditions which have or have had direct influ- 

 ence, and among them may be mentioned the historical factors 

 of migration, succession, and the like; the phj^sical factors of 

 water, temperature, and soil; and the biotic factors, including 

 competition, layering, and others. The various factors are by 

 no means independent, but each influences the others and is to 

 some degree influenced by them. The biotic and historical 

 factors are here mentioned either incidentally or under the head- 

 ing of phytogeography, but the physical factors are in many 

 cases so different from those normal to other parts of Illinois 

 that they are discussed more in detail. 



The physical factors which aid in the control of the vege- 

 tation of any area have been divided by Schimper ('98, p. 174) 

 into two groups, climatic and edaphic. The climatic factors are 

 temperature and rainfall, and they determine respectively the 

 specific composition of tlie flora and the general character of 

 the vegetation, whether forest, prairie, or desert. Similar 

 climatic factors are operative over broad areas, and the changes 

 from one type of climate to another are usually very gradual. 

 Local variations in the vegetation are due to the physical or 

 chemical composition of the soil, to its exposure to the sun, to 

 the available supply of water, and to other such factors, desig- 

 nated collectively by the term edaphic. Edaphic factors are 

 always influenced by, and are sometimes the direct result of, 

 the climate. They are also modified to a greater or less extent 

 by the plant-covering, as will be shown later in the discussion 

 of the plant associations. 



Although the climate of the lllinoisRiver valley sand region 

 is in every essential respect like that of other parts of central 

 and western Illinois, a strikingly different vegetation has been 

 developed. Receiving the same amounts of heat, light, and 

 rainfall, and exposed to the same winds, the differences in veg- 

 etation are due in every case to the sand in its? relation to wind, 

 moisture, and available food supply. 



The designation Miami fine sand has been applied by the 

 United States Bureau of Soils to the sand composing these 

 extensive deposits along the Illinois River, and the following 



