402 * BOTANICAL GAZETTE [DECEMBER 



Salix discolor, Acer rubrum, Ulmus americana, and Fraxinus 

 americana. 



While it often happens that locally the one occurs only in 

 drained conditions and the other only in undrained situations, 

 field work carried on over any considerable area will show that 

 drainage conditions are not adequate to account for the presence 

 or absence of either of these two distinct types of vegetation. 

 The presence of peat,, with the consequent accumulation of humus 

 acids, has been commonly spoken of as preventing the coming 

 in of the "drained swamp" types. In southern Michigan and 

 northern Indiana, however, there are many swamps with a thick 

 substratum of peat and without an outlet, yet supporting a vege- 

 tation made up wholly or in part of these plants. It is true, 

 however, that the bog societies occur here only in poorly drained 

 situations, underlaid with peat or bogus soil. 



Again, it is not unusual to find these two society groups grow- 

 ing on opposite sides of the same lake, where the underlying soil 

 can be shown to be the same. To account for this, it has been 

 suggested that the depth of water in the two situations is of im- 

 portance. But any ideas of this kind can be disproved by car- 

 rying the criteria into a new locality. Just to the west of Ann 

 Arbor, -Mich., occurs a small glacial lake. This was formerly 

 surrounded by a quite typical group of bog societies. Within 

 recent years the eastern side of the lake has been entirely cleared, 

 and a large part of the original tamaracks on the south and south- 

 west sides have been cut away. There is left a rather pure 

 growth of bog plants on the northwest side. Since the clearing 

 was made on the southwest side there has sprung up a dense 

 growth of herbs, shrubs, and trees, nearly equaling in height the 

 adjoining grove of tamaracks. But if we note the species pre- 

 vailing in this area, we find the plants enumerated above as typi- 

 cal of drained swamps. It is practically impossible to account 

 for this situation on the basis of soil difference, chemical char- 

 acter of the soil solution, drainage conditions, or depth of water. 



In his paper on the "Physiographic Ecology of Chicago 

 and Vicinity" Cowles 2 distinguishes one "drained" and three 



2 Box. GAZ. 31: 145-155. 



