pempnteees oee 
1903] BOG PLANT SOCIETIES ALT 
in adjacent areas. As we know from the numerous physio- 
graphic studies that have been made of glacial basins, many of 
the lakes were formerly much larger than at present. Some of 
them in early postglacial times had steep banks, which were 
unfavorable to the development of shore vegetation. But by 
the lowering of the water level consequent upon the cutting 
down of the outlet, the shore line at present is a gradually slop- 
ing one, and supports a “drained swamp”? flora. In other cases 
irregular arms, extending away from the main body of the lake 
and protected from wind and wave action, doubtless supported a 
bog vegetation during the tundra dominance. Since then they 
have been separated from the main lake by a lowering of the 
water level. Today we find in many such cases the bog vegeta- 
tion still persisting in the depressions which were formerly arms 
of the lake, while on the shore of the main body, which came to 
be swampy at a much later period, the so-called “drained 
Swamp” flora occurs. One of the best examples of this is Turkey 
Lake, Indiana. Here is an irregular lake several square miles 
in extent, nearly surrounded by high moraines. At its south- 
eastern end, through a less elevated portion of the moraine, it 
formerly connected with several shallow depressions,” all of 
which contain bog plants with varying proportions of swamp 
plants. But on the now shallow margins of the present lake only 
the swamp plants are found. At Eagle Lake® the same obser- 
vations hold for a former extension of this lake toward the north- 
west. But without multiplying examples, the relation of these 
two groups of swamp societies seems to depend largely upon the 
time when the swamps came into existence as swamp habitats. 
If they have existed since the days of tundra conditions they 
may show a bog flora today. If they are of recent origin, the 
plants will correspond to the normal swamp plants of the present 
climatic conditions. If of intermediate age, we may have vari- 
Ous mixtures. of the two. Dr. Cowles informs me that the only 
bog in the sand-dune region near Chicago which contains all 
these typical bog plants is the one that occurs on the Valpa- 
* For map and description of lake see Proc. Ind. Acad. Sci. 1895. 
*3 Map opposite p. 118, Proc. Ind. Acad. Sci. I9OI. 
