FACTORS AFFECTING PLANT DISTRIBUTION 27 



The most important chemical fact about humus soils is that 

 they are, under certain conditions, capable of supplying organic 

 substances to growing plants. In addition to this, in humid regions, 

 soils with a high organic content also have a high mineral content, 

 since the growth of plants tends to prevent the normal amount of 

 leaching which otherwise would take place. The prairie soils of 

 North America and the chernoziom (black earth) of Russia are 

 rich in both organic and soluble salts. If the color can be used as a 

 guide to the lime content, (Coffey '09), then the order in which the 

 soils of the United States rank is (1) the prairies, (2) the forests 

 of eastern America. This accumulation of mineral salts in the soil 

 is in agreement with the distribution of total rainfall throughout 

 the year. It is greater in the forests and less in the prairies. In the 

 prairies there has not been enough rainfall, or its distribution has 

 not been such as to leach the lime from the soils at the rate that this 

 is occuring in the light forest soils. Yet the rainfall in the prairies 

 has been sufficient to induce a heavy growth of vegetation, which is 

 only partly decomposed. This is because water standing during a 

 portion of the year in prairie sloughs prevented as complete oxida- 

 tion of the organic material as occurs in forests. The vegetation 

 acted as a series of dams in a country of rolling topography made 

 level by glacial action. These lakes or water-bottomed sloughs 

 kept the forests out until man hastened the gradual drainage proc- 

 ess. Thus we can see how climate and topography acting in unison 

 formed soils in the east north-central states which are entirely 

 different from the soils of the forested regions, though in many 

 instances they have been derived from similar geological for- 

 mation. 



3. Biotic Factors 



The influences of plants and animals upon the developing vege- 

 tation are known as the biotic factors. These are assigned various 

 degrees of importance by different authors. Clements ('05) classi- 

 fied habitat factors into physical and biotic and ('16) he consist- 

 ently eliminated an edaphic group while admitting its convenience. 

 It is true that edaphic factors may be further resolved into topogra- 

 phic, or physiographic, yet this does not detract from the import- 

 ance of having all the subterranean plant environment in a single 

 group. If the resolution be carried out far enough, the last result 

 will be something like Shreve's ('16), in which all the factors are 

 physical. Cowles ('01, '11) has carefully distinguished the biotic 



