tort] BURNS—HURON RIVER VALLEY II5 
Water table 
The variations in the position of the water table from year to 
year and from month to month play a very important part in the 
succession of plant societies in these bogs. In 1908 the level of 
the water in all bogs of southern Michigan was high. In 1901- 
1904, When the maps of Dead Lake were made (3), there was an 
island near the center, and it was customary to row out there and 
leave the supplies when working at the lake. That island in 1908 
was submerged, and one could row over it in 18 inches (45.8 cm.) 
of water. The flora of upland forms had disappeared with the 
exception of some stunted willows, and in its place were a few 
potamogetons, chiefly. P. helerophyllus. At Whitmore Lake, north 
of Ann Arbor, the same rise in water has occurred, and what was 
a peninsula in 1904 is now an island. The same fact is recorded 
by Davis (9, p. 162). 
During a series of wet years the change in water level affects 
the vegetation in all zones except perhaps on the lakeward side of 
the floating sedge, which rises and falls with every change of level. 
Along shallow shores the factor is sufficient to control the char- 
acter of the vegetation, as has been pointed out in the case of the 
island at Dead Lake. Its influence is also profound in those zones 
where the peat is solid. It may rise above the surface to a depth of 
several inches. With this rise there is also found an increase in 
the humidity of the air. During such periods Sphagnum sp. 
spreads rapidly toward the shore and may assume quite an impor- 
tant position in all zones except perhaps the marginal. An inter- 
esting example of the behavior of this moss during wet and dry 
periods is found at Mud Lake. Here a section was made which 
showed alternating layers of Sphagnum and Polytrichum corre- 
sponding to the wet and dry periods of previous years. As many 
as four layers were easily distinguished at Mud Lake. 
In one case measured the shrubs stood in water which was 
18 inches (45.8 cm.) deep one summer, and in water of various 
depths less than that for a period of at least three years. These 
plants then must be able to endure submergence for a long period, 
as has been pointed out by Davis. 
