360 BOTANICAL GAZETTE [NOVEMBER 



temperatures of 97-100 F. (36-38 C.) are likely to occur every 

 year, and that temperatures approximating these may be prevalent 

 for several days in succession each season. When these extremes 

 coincide with periods of drought, they must act as important checks 

 on the growth of the bog plants, especially the sphagnum. As we 

 pass from northern Indiana along the moraine into Michigan, the 

 gradual increase of bog development, of the variety of bog species, 

 and of the areas covered by sphagnum is very marked. Although 

 other factors are involved, this increase may be correlated with a 

 decrease in summer temperature extremes. 



II. The bogs: their development and ecological conditions. 



PHYSIOGRAPHIC ORIGIN OF THE LAKE AND BOG BASINS. 



In connection with the special consideration of the bog flora, it is 

 of interest to note the origin of the depressions in which this flora has 

 developed and flourished. Indeed, in the morainic belt of the Huron 

 basin it would seem that among the agencies which have produced 

 important topographic changes since glacial times, the bog plants 

 stand near the head of the list. Stream erosion and deposition have 

 been slight, while lake basins have been filled and the level of 

 depressions generally raised by the deposition of plant debris. 



As no attempt has as yet been made at the mapping of peat 

 deposits and muck soils, no reliable estimate of the total amount of 

 aggradation accomplished by plant agencies can be made. Yet the 

 frequency with which in field work one encounters peat soils, in 

 various stages of making or decay, suggests that in the aggregate 

 such deposition has been most effective in this region. The northwest 

 quarter of the Ann Arbor topographic map, which embraces an area 

 of about 215 square miles (55,700 hectares), located in the morainic 

 portion of this basin, indicates approximately 43 square miles (11,500 

 hectares) 20 per cent. as swamp land. It is probable that at an 

 early time this area was very much larger, but with the settlement of 

 the land many extensive areas have been drained and only the dark 

 humous soil remains to suggest its past history. 



The most frequent source of lake and bog basins is here found in 

 connection with the deposits made by glacial drainage. Among the 

 vicissitudes attending the retreat of a glacier are the occasional 



