128 



The formation of the dunes along the present shore of the head 

 of Lake Michigan was considered, and also the changes in the flora 

 that may be noted as one passes from the naked shifting dunes and 

 extremely xerophytic conditions of those recently fixed, to the 

 dunes farthest inland, where mesophytic conditions prevail. Cer- 

 tain grasses, species of Calaviagrostis, Andvopogon, Auniwphila^ 

 and Elymus, do much to bind the dunes. The first trees to ap- 

 pear are the cottonwood and certain willows, which are also of 

 value in fixing the dunes. The scrub-oak and black oak soon 

 appear and are followed by the bur-oak, the white oak, and the 

 red oak. Pitms Banksiana is followed by the white pine ; the 

 pig-nut hickory is succeeded by the shag-bark ; other trees, such 

 as the basswood, ash, cherry, and black walnut, come in, and on 

 the most mesophytic slopes of the oldest dunes and beaches one 

 finds the sugar-maple and, more rarely, the beech, hemlock, and 

 southern tulip-tree. Corresponding changes in the shrubby and 

 herbaceous vegetation occur, and at Stevensville and Porter, one 

 may pass in a short time, from extreme desert conditions through 

 successive stages of the open forest of low trees and shrubs to 

 the oak-hickory type and finally to the beech-maple-hemlock 

 combination, which indicates the culmination of the forest in this 

 region. 



The usual ecological factors, heat, light, water, soil, wind, and 

 direction of slope, all have their influence on the floral distribu- 

 tion. Conditions in the dunes are extreme. Thus, for example,, 

 the trailing-arbutus and the bearberry, both northern types, may 

 appear on the north-facing slope of a dune, while just over the 

 crest, on the south-facing slope, the cactus may flourish. 



Emphasis was laid on the fact that species v^ary with environ- 

 ment, often losing more or less of their xerophytic adaptations 

 under mesophytic conditions ; that a plant-society is only a 

 stage in the development of a region ; that the apparent tendency 

 is for all to approach the mesophytic condition. 



The paper was discussed by Dr. Grout and Dr. Rydberg. 



"Some Relations between Habitat and Structure in Mosses," 

 by Dr. A. J. Grout. 



Xerophytic mosses apparently tend to develop short, thick- 



