BOTANICAL GAZETTE [DECEMBER 



raiso moraine. When this basin was formed the area occupied 

 by the other bogs was still covered by the waters of Lake Chi- 

 cago (now Michigan). At the present time new bog areas are 

 being continually added by the interference of the moving dunes 

 with drainage lines. And these new areas frequently contain a 

 number of the bog plants. This, however, does not invalidate 

 the explanation here suggested. The bog habitat has been con- 

 tinuous since early postglacial times; only its position and 

 extent have been variable. 



This same observation holds in the case of certain lakes 

 which have long supported a growth of the bog plants at some 

 part of their shore line. By recent gradual changes of level, or 

 by the development of a floating sedge and cassandra zone, these 

 areas have been greatly enlarged in recent times. Usually, how- 

 ever, such formations are partially made up of swamp species. 



It is a well-known fact that in many localities where the bog 

 societies formerly existed, they have partially or entirely disap- 

 peared. Since the settlement of this region, extensive bog areas 

 have been cleared and drained. Fires have aided in the destruc- 

 tion of the tamaracks, and in many places the sudden lowering 

 of the water level due to ditching has resulted in the killing of a 

 large part of the original bog flora. 



In this connection it is to be noted that the partial clearing 

 or burning of a swamp area opens up a new territory for occupa- 

 tion, either by the bog plants or the swamp plants. The pres- 

 ervation of the underground stems of many of the bog species 

 makes their chances more favorable for capturing the area in 

 question. But there are many areas to the west of Ann Arbor 

 which show that these bog plants cannot compete with the 

 swamp plants in the occupancy of new territory, even though the 

 bog plants be in complete possession previous to the clearing. 

 We may say that the chances of capturing newly exposed land 

 areas at the present time are all in favor of the swamp plants, 

 largely because of their greater production of seeds, more ade- 

 quate means for seed dispersal and better adaptations to present 

 climatic conditions. In early postglacial times, however, the 

 conditions were far different. The swamp plants had been 



