Part II. A Botanical Survey of the Illinois Hirer Valley 

 Sand Bee/ion.* — By Henry Allan Gleason. 



Introductory. 



The ecological study of sand-dune vegetation has in recent 

 years attracted the attention of numerous American botanists, 

 and many noteworthy contributions to it have been made. 

 Dune vegetation is especially well adapted to ecological inves- 

 tigation, since the changes in the physical factors of the envi- 

 ronment are usually considerable, the component associations 

 are sharply distinguished, physiographic processes go on with 

 comparatively great rapidity, and the plant inhabitants show 

 characteristic features in habit and structure. The vegetation 

 of the dunes bordering Lake Michigan has been studied in de- 

 tail by Cowles, while Rydberg, and Pound and Clements have 

 described the sand-hills of Nebraska. Intermediate in position 

 between these two regions lie the sand deposits now to be con- 

 sidered, and it is hoped that the matter presented may be of 

 some value, not only in the study of sand vegetation in general, 

 but in extending a knowledge of the origin of the prairie flora 

 and of the relation of the forest to the prairie. 



The field work upon which this study is based, was carried 

 on during the summers of 1903 and 1904, mainly in the vicinity 

 of Havana, Mason county. 



Ecological Factors. 



The varying structure and distribution of vegetation are 

 an expression of the various conditions to which it is subjected. 

 Any force or condition, present or past, which has in any way 

 an effect upon vegetation may be termed an ecological factor. 

 The use of this term is, however, more generally restricted to 



* Mr. Gleason's manuscript for this paper was filed for publication early in 1905. 

 Its appearance has been retarded by inevitable delays in the completion of the con- 

 tributions of Mr. Hart, it being the intention and desire of both the authors that their 

 work should be published jointly in a single paper.— S. A. F. 



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