245 
and more permanent backwaters. It consists, in the main, of 
Ceratophyllum with some Naias and Elodea. Scattered sprays 
of Potamogeton natans lie in the more open places, and the Lem- 
nacee multiply in the more sheltered nooks, while the yellow- 
flowered Heteranthera abounds at the water's edge and creeps 
out upon the black mud at the margin. This fringe of vegetation 
continues until it is stranded on the shore by the recession of 
the water, washed away by sudden floods which lift it from its 
slight foothold upon the unstable bottom, or pulled out upon 
shore or floated down stream by fishermen’s seines. Thus, of 
the heavy fringe present in June, 1895, only a trace was left by 
September of that year. In the four years following, the river 
levels were such that no vegetation of consequence appeared 
along the shores of the river at any season of the year. 
Even at the time of its maximum development this littoral 
belt of vegetation did not often exceed ten meters in width. It 
is thus a relatively small, as well as an inconstant, factor in the 
environment of the plankton of the river itself. The current 
carries the plankton-laden water through its tangled growth, 
and sessile animals, such as Hydra and the Bryozoa, find in it 
an abundant food supply. These and other organisms which 
find a retreat in the shelter of the vegetation are from time to 
time carried into the channel by the current and serve to diver- 
sify the plankton. On still, warm days Hydra habitually aban- 
dons its sessile mode of life and adopts a limnetic habit, often 
attaching itself to the surface film of water. 
Owing to the changes in levels and to other reasons above 
cited the vegetation of the river, where it occurs, does not con- 
tinue until autumn, and is absent during the ice blockade of 
the winter. In this respect the river environment is in strong 
contrast with that of most of the backwaters, in which the veg- 
etation, though reduced, persists throughout this period. This 
absence of winter vegetation in the river is one of the condi- 
tions favoring the stagnation which sometimes occurs, as in 1898, 
in the Illinois. 
Spoon River (Station M)—Spoon River, throughout the 
