40 BOTANICAL GAZETTE [JANUARY | 
A study of the transition zone between these two areas will be i 
necessary before the working out of the exact relation of the various 
societies can be attempted. 
RELATION OF THE VEGETATION TO THAT FARTHER NORTH. 
The student of plant distribution who has formed any definite — 
theories as to the relation of plant societies to topography and soils — 
from the southern part of the state and from Indiana and Mlinois 
will be much surprised if not actually shocked by a visit to the region | 
of the straits of Mackinaw. On the island of Mackinac, for example, |. 
occur most of the tree forms found in Kent county, but growing : 
apparently without relation to the soil condition. Norway and ]{; 
white pines, beech, sugar maple, red maple, tamarack, arbor vitae, | 
balsam fir, basswood, etc., will be found growing side by side on the |/ 
well-drained uplands in clay or sand or loam, or even partially bare } 
rock. As to the reasons for this nothing can be said before more of 
the area has been studied in detail. 
CONCLUSION. 
It appears from these investigations that the main factor in deter- 
mining the distribution of the forests on the uplands of this region 
is that of the size of soil particles, the sorting of which dates back 
- almost entirely to the close of the last glacial epoch. The size of 
particles determines the amount of air and moisture in the soil, 
and these in turn determine the amount of humus formation and 
the growth of nitrifying organisms, and perhaps also to a certain 
extent the amount of soluble salts in the surface layers. 
3 ae rake 
se ee 
A factor of less importance, because applicable only over small j- 
areas, is the nearness of the underground water level to the surface. ) 
This affects the uplands only along swamp borders. 
In a broad way, physiography may be said to determine the 
vegetational distribution here.77 The physiographic features are 
largely ones of glacial topography or are traceable directly to these. 
logical factors, in one way or another, have of course dete — ¢ 
aay ae 5 PaaS ae ly Har f this idea see CowLeEs, H. C., The 
oS ecology of — and vicinity. Gaz. po hig Ans, The ast 
mc. 
