452 BOTANICAL GAZETTE [tune 



ever, when one goes over to the oak and examines the soil in which 



it is growing, the matter is easily explained. These "islands" of 



oak and other upland trees are not growing in peat, but on one of the 



islands formed by projections from the bottom of the lake. Fig. 5 



shows such an island with an oak growing upon it. 



The same relation of present distribution and depth of the original 



post-glacial lake holds true in lakes with several basins, as it did in 



lakes with only one: open water over the deepest and largest basins, 



wide zones of plants where the bottom of the post-glacial lake sloped 



gradually, a definite order of succession of plant zones. Greater 



care must be exercised, however, in the determination of depths by 



the present vegetation, especially in lakes with many small basins 



just at the time when large areas are beginning to become favorable 



for the next group of plants. The occupation will take place most 



rapidly over shallow parts, because here the peat will become firm 



sooner and the conditions will first be favorable for the next group of 

 plants. 



Conclusions.— The chief factor determining the position of the 

 greatest amount of peat deposit and the width of the zones of plants 

 at the local peat bogs is the depth of the water in the different parts 

 of the original post-glacial lakes. The chief factor in determining 



the position of the open water is depth; given time enough, the open 

 water will disappear from all our lakes. 



In places where the water is very shallow the bog flora is unable 

 to get a start because of the wave-action caused by the winds and on 

 account of the shoreward push of the ice. Such places, however, 

 bear no definite relation to the points of the compass. 



Different zones of plants follow in a definite orderly succession; 

 lily, bog sedge, bog shrub, tamarack, maple-poplar. 



University of Michigan 



LITERATURE CITED 



1. Burns, G. P., Formation of peat in Dead Lake. Rept. Mich. Acad. Sci. 

 6:76. 1904. 



2 - > Edaphic conditions in local peat bogs. Science N. S. 29:269. i9°9- 



3. Cowles, H. C, Physiographic ecology of Chicago and vicinity. Bot. Gazette 

 31:147- 1001. 





