248 
Late summer and early autumn see the decay of much of 
the more succulent vegetation, such as the water-lilies and 
lotus, the arrowleaf, the Potamogetons, and some of the Elodea 
and Ceratophyllum, while the emergent and more resistant 
rushes, sedges, and grasses yield more slowly and later to the 
accidents of flood and ice. and do not reach the late stages of 
decay until the following spring. 
The vegetation of Thompson’s Lake is subject to consid- 
erable fluctuations, due to other than seasonal changes. 
‘These are variations in river level, the seining of fishermen, 
and the movements caused by flood, wind, and ice. The 
changes in level, owing to the very gentle slope of most of 
the shore of this lake, greatly contract the littoral zone as the 
spring flood recedes, and restore more or less of it with each 
recurrent rise—changes which facilitate the decay of whatever 
vegetation of the submerged type develops in this zone. The 
location of the lake with its long axis in the direction (S. W. to 
N. E.) of the prevailing winds, gives a force to the waves suffi- 
cient at times to tear isolated patches of “moss” from their 
sight hold on the unstable bottom and drive them toward the 
northern end of the lake. This is an important factor in keep- 
ing the greater part of the lake free from vegetation. 
The vegetation of Thompson’s Lake is thus a considerable 
factor in the environment of the plankton. It furnishes a con- 
siderable quantity of decaying organic matter in fall and spring, 
both being periods of marked plankton development. At high 
and moderate stages of water, when a gentle current passes 
through the lake, its influence must be generally diffused. At 
low-water stages, when the current is cut off, its effect is much 
more local. At such times no open channel is maintained 
through the vegetation at the northern end of the lake (PI. II.) 
to the outlet, as in the case of Quiver Lake. The movements 
in the lake attending change in level tend to mingle the plank- 
ton of regions full of vegetation with that of the open lake or 
vice versa, thus tending to diversification. There still remains 
at all times a large tract of open water in which for considera- 
