34 



DJFMAMJ JASON 



D 



Fig. 3. Sunshine curves for Dubuque and Peoria, from December to Novem- 

 ber, expressed in per cent, of total possible sunshine. 



The Ecological Environment 



In each area the sand has essentially the same structure. It is 

 fine grained, yellowish brown in color, and virtually free from or- 

 ganic matter except in the upper layers. In those portions occupied 

 by prairie, and in a part of the forests, the surface is exposed and 

 considerable loose sand is shifted by every wind. 



The ecological nature of sand as an environment for plants has 

 been so frequently described that further discussion here is unneces- 

 sary, especially since it is probable that no important additions can be 

 made to our knowledge of the subject without careful field experimen- 

 tation. Some of the best treatments of the matter in English may 

 be found in the works by Cowles (iSpp), Warming ( ipop), Olsson- 

 Seffer (ipop), and Schimper (ipoj), and in them fuller reference 

 is made to the literature. Briefly summarized, it may be stated that 

 in sand deposits ( i ) the temperature shows a great variation from 

 day to night and from surface to subsoil; (2) in open associations 

 the insolation is increased by reflection; (3) the water capacity is 

 low and the available supply is small in amount, but constant, because 

 of atmospheric condensations; (4) the amount of soluble inorganic 

 salts and of organic matter is small; (5) in open associations the 

 surface sand is constantly shifting, resulting- always in an unstable 

 environment and sometimes in large excavations or accumulations. 



Of these conditions, the last exerts the most apparent and the most 

 important influence on the associational distribution of the vegetation. 



