246 ILLINOIS ACADEMY OF SCIENCE 



frequent observation that in sand areas where dune build- 

 ing has been extensive and dunes of considerable height 

 have developed there occur sheltered depressions where 

 mesophytic vegetation soon becomes established on ac- 

 count of the lower evaporating power of the air and 

 the consequent increased accumulation of humus. Such 

 islands or centers of mesophytism are too well known 

 and their occurrence will be too readily admitted to re- 

 quire further demonstration, but their importance as 

 centers from which mesophytism spreads to surround- 

 ing territory has not been sufficiently recognized. From 

 such centers go the seeds or other organs of dissemina- 

 tion to the fringe within shelter of the central forest as- 

 sociation, and so completely is the spreading of the mem- 

 bers of the climax forest accomplished that soon the 

 very tops of the higher dunes are covered with beech 

 and maple while adjacent plains remain in xerophytic 

 scrub pine. 



The successional stages may include as in southern 

 Michigan, a pioneer pine forest with an evergreen under- 

 growth of Jiiniperus, Arctostaphylos, Pyrola, Linnaea 

 and associated forms, a succeeding xero-mesophytic oak 

 forest dominated by Quercus velutina and becoming grad- 

 ually more mesophytic as indicated by the invasion of Q. 

 alba and Q. rubra, and this in turn gradually giving way 

 before the encroachment of Tsuga canadensis, Acer sac- 

 cliarum, and Fagus grandifolia. Farther to the north the 

 oaks disappear but the maple and hemlock remain with 

 equally mesophytic undergrowth, and such tree associ- 

 ates as the white spruce, Picea canadensis, and the yellow 

 birch, Behda lutea. Whatever may be the modifications 

 in the rate and phases of the succession the ultimate and 

 comparatively speedy arrival at a rich mesophytic climax 

 forest association leads to the conclusion that the high 

 relief of sand dunes affords an excellent demonstration 

 of the importance of mesophytic centers, developed in 

 protected local areas, in inducing and hastening the de- 

 velopment of the more advanced stages in plant suc- 

 cession. 



