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 débris. 
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, 1n 
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 40 are 
of about 215 square miles (55,700 hectares), located in the moralni¢ 
portion of this basin, indicates approximately 43 square miles (11,500 
hectares)—20 per cent.—as swamp land. It is probable that at . 
early time this area was very much larger, but with the settlement 
the land many extensive areas have been drained and only the 
humous soil remains to suggest its past history. oe 
The most frequent source of lake and bog basins is here i | 
connection with the deposits made by glacial drainage- Apes ot 
vicissitudes attending the retreat of a glacier are the occash 
