30 BOTANICAL GAZETTE [SEPTEMBER 
formation, and swamp and bog formations. It occupies a region 
which consists of sand ridges with depressions containing bodies 
of water of all sizes, from a few yards to a mile or more in diameter. 
Geologically the region was a shallow bay of Lake Algonquin 
drained by the recession of the waters of the lake with the melting 
of the ice barrier in the Straits of Mackinaw. 
4. The first stages of the sand succession (psammosere) are 
found in the moving dune belt along the shore, but they do not 
lead to a complete stabilization of the sand. The later stages 
appear when the sand stops moving as a result of a checking of the 
force of the wind, due to distance from the shore or the formation 
of high dunes which act as windbreaks. The pine-oak stage shows 
a progressive change from the less mature areas near the lake to 
the more mesophytic areas in the southern portion of the region. 
Soil moisture content and the amount of humus in the sand seem 
to be important factors in this change. 
5. The clay-gravel succession (geosere) has reached the climax 
stage as a beech-maple-hemlock forest on the surrounding morainic 
upland, and is found invading the pine-oak formation along the 
borders of the region, and especially along a narrow morainic tongue 
which extends completely through the sand ridge substratum to 
Lake Michigan. The controlling factor in this invasion seems to be 
primarily the chemical character of the soil, glacial material ranking 
close to humus in importance, and secondarily the soil moisture 
content. 
6. The various ponds and lakes all show the normal early stages 
of the aquatic succession (hydrosere), which lead either to swamp 
meadows or to bog forests, the line of development followed being 
determined chiefly by depth of water and exposure to the heat of 
the sun. 
NORTHWESTERN UNIVERSITY 
Evanston, Ibi. 
LITERATURE CITED 
. CLemeEntTs, F. E., Plant succession. Carnegie Publ. no. 242. 1916. 
. Cooper, W. S., The climax forest of Isle Royale, Lake Superior, and its 
development. Bort. Gaz. 55:1-44. 1913. 
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