INTRODUCTION. 



In recent years a great impulse has been given to the study of 

 plants in their environment by the works of Warming, Schimper, and 

 other European botanists. The subject that deals with this part of the 

 botanical field is now called plant ecology — the science of plant house- 

 keeping, or, as some would say, plant sociology. One phase of ecology 

 deals with the meaning of plant structures, such as leaves, roots, flow- 

 ers ; the variation of these organs is investigated in relation to the 

 influence of external agents, and attempts are made to work out the 

 causes which determine plant forms. This phase of the subject is 

 presented in Kerner and Oliver's Natural History of Plants and in Dr. 

 Coulter's Plant Relations. 



A second phase of ecology, and the one that concerns us here, has 

 to do with plants not as individuals, but as grouped in societies. Very 

 superficial observation shows that certain plants grow in swamps, others 

 in forests, and still others on sand dunes. Warming, in his Ecological 

 Plant Geography, published at Copenhagen in 1895, gave the results 

 of a long serifis of investigations as to the causes determining these 

 diversities in the distribution of plants. He divided the plants of the 

 world into four great groups : hydrophytes, or plants which grow in 

 water or wet places ; xerophytes, or plants which grow in dry habitats ; 

 mesophytes, or plants which grow in places of medium moisture, such 

 as ordinary forests and meadows ; and halophytes, or plants which 

 grow in salt-water or alkaline soil. It will be seen that all of these 

 groups except the last are related to water, which is commonly re- 

 garded as the most important factor in determining local differences in 

 plant societies. Most botanists have accepted Warming's classification 

 of plant societies as a more or less complete organization of this part 

 of the ecological field. 



The present paper attempts to relate the plant societies not 

 only to water, but also to soil, and more especially to the physi- 

 ography. The geographic and ' physiographic features of the Chi- 

 cago region have been admirably presented in papers by Leverett,' 



'Leverett, F. : The Pleistocene features and deposits of the Chicago area. Chicago. 

 1897. 



