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teristics these situations are quite similar to lake beaches. The im- 

 portant difference is the slow movement of the water in the rivers, 

 which are not sufficiently extensive to permit the wind to raise waves 

 which could destroy the vegetation along the shores. The bottom of 

 these rivers is seldom more than one or two meters below the level 

 of Lake Michigan. 



The associations which occur in these localities are characterized 

 by the great abundance of a very few species. The associations are 

 restricted to narrow bands which spread out horizontally for many 

 meters. This gives rise to zones of associations around the ponds 

 and along the streams. The associations may alternate to a limited 

 extent. They are, however, sharply separated from one another by 

 definite tension lines, which are sharpest between the associations 

 farthest out in the water. Landward the tension lines are occu- 

 pied by species of both of the bordering associations and in many 

 cases by small plants which occur there only. 



THE PLANKTON ASSOCIATION 



The free-swimming protozoans and algae which enter into the 

 plankton were not investigated, owing to lack of proper facilities 

 for such work. 



THE CHARA ASSOCIATION 



The bottom of the deeper parts of lakes and ponds in northern 

 Illinois and Indiana is usually covered with an alga, Chara, con- 

 stituting the Chara association. There are no secondary species with 

 the Chara, as it normally occurs in this area. In streams with vis- 

 ibly running water there is no Chara. The accumulation of Chara 

 furnishes a lodging place for the seeds of Potamogeton, giving rise 

 to the following association. 



THE POTAMOGETON ASSOCIATION 



This association occurs in both quiet and running water, al- 

 though usually with different dominant species in the two cases. The 

 association consists mainly of plants that are entirely submerged, 

 although some of them may mature their flowers and fruit at the 

 surface of the water. This association frequently starts near the 

 edge of the Chara, or it just as frequently has its beginning in ponds 

 in which there is no Clmra. In the main part of Dead Lake the as- 

 sociation is characterized by a single species, Potamogeton natans. 

 In some of the ponds, where the water is not so deep, it may have 



