243 



and more permanent backwaters. It consists, in the main, of 

 Ceratophyllum with some Naias and Elodea. Scattered sprays 

 of Potamogeton nutans lie in the more open places, and the Lem- 

 nacecB 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 1895, 

 in the Illinois. 



Spoon River (Station M). — Spoon River, throughout the 



