1899] DUNE FLORAS OF LAKE MICHIGAN 97 
along an inland fresh water lake with those along the sea should 
yield instructive results. 
An ecological study of this character has a natural twofold 
division. In the first place the plant formations are to be inves- 
tigated. The species characteristic of each formation must be 
discovered, together with the facts and laws of their distribu- 
tion. The progressive changes that take place and the factors 
in the environment which cause these changes must be discussed. 
This phase of the subject is largely geographic, and will be the 
special feature of the present paper. In another paper it is the 
author’s purpose to discuss the adaptations of the plants to their 
dune environment, paying especial attention to those species 
which show a large degree of plasticity, and which are found 
growing under widely divergent conditions. This second phase 
again has a natural twofold division, one part treating of gross 
adaptations such as are shown in plant organs and plant bodies, 
the other dealing with the anatomical structures of the plant 
tissues, 
The material for the present paper has been gathered chiefly 
wins the study of the dunes in northwestern Indiana in the 
vicinity of Chicago. These studies were carried on in the sea- 
se of 1896, 1897, and 1898, frequent visits being made to 
various points at all seasons of the year. A portion of the sum- 
mer seasons of 1897 and 1898 was spent in a more rapid recon- 
noissance along the entire eastern shore of Lake Michigaa, 
including the group of islands toward the north end of the lake. 
the work resulting in this paper has been carried on in con- 
nection with the Hull Botanical Laboratory of the University of 
see ms the author gratefully acknowledges the kindly 
ec a cooperation shown By his associates among pa 
Har. as Students of the botanical department, — 
author aga John M. Coulter, through whose influence 
caidas irected along lines of ecological research. The 
Rete; Wp = desires to express his great indebtedness to Dr. 
book on ae professor of botany at Copenhagen; his text- 
cology and his treatises on the sand-dune floras of 
