28 BOTANICAL GAZETTE [JULY 
It is the lack of moisture, and not low temperature that will arrest 
the growth and reproduction of the plants concerned, and the 
disintegrating action of fungi and bacteria. This factor in plant 
growth, not previously important to the plants of the sedge, 
shrub, and thicket growth, then becomes operative selectively, 
leading to the establishment of a xerophytic plant association. 
At present, however, there is little indication of the appearance 
of an association of that kind; the climatic trend favors broad- 
leaved forests, and the supposed physiographic characteristics 
leading to a xerophilous climax association assume nowhere on 
the island any considerable importance. 
There are conditions, however, which would indicate a rever- 
sion to a hydrophytic association. Adjoining the maple-alder 
zone on the southeast side are several extensive areas which do 
not respond quickly to changes in the water level; fig. 7 illus- 
trates a part of such an area. Through the accumulation of 
vegetable débris, the replacement of air and other gases held in 
the mat by water, but especially through the increased load upon 
the surface of the mat after the heavier tree association became 
established, a settling and shrinkage of the peat occurred, which 
ultimately resulted in the sinking of the mat several feet below 
the water level. The cutting of the timber reestablished equilib- 
rium and rejuvenation. The species now tenanting the mat in- 
dicate a tendency toward the development of a hydrophytic vege- 
tation approaching the type of the border zone described. The 
marked difference between the vegetation of the central zone 
and the one establishing itself is worthy of special note. Except 
such portions of the fibrous mat as were long ago broken off from 
time to time by the action of wind and waves and drifted about 
as floating islands, the rejuvenated “sunken” mats, and such 
areas as annually rise in the early summer and disappear again 
beneath the water in late autumn concomitant with the “‘over- 
turn”’ of the lake, show nowhere members of the cranberry-sphag- 
num zone. They illustrate most forcibly the fact that under these 
conditions a very different set of plants spring up and become 
dominant, although the true bog plants are near at hand. 
