260 



emergent vegetation, which reaches a great development here, 

 adds its contributions to the enrichment of the marsh waters 

 with the return of Hood conditions and temperatures facilitat- 

 ing decay — conditions prevailing in the spring at the time of 

 plankton maximum. The growth of vegetation in the springy 

 and the choking up of the lake which attends its transforma- 

 tion into a marsh seem, on the other hand, to introduce condi- 

 tions of nutrition and light inimical to the development of the 

 plankton. 



PLANT ZONES. 



The classification of the aquatic vegetation of our locality 

 and its divisions into definite zones or belts is made difficult by 

 the fiuctuating character of the environment. The average 

 range in river level is almost fourteen feet, and with the changes 

 in level go advance, recession, or even obliteration of the 

 shore-line in a large part of the territory included in our field 

 of operations. Drouth and untimely flood also bring wide- 

 spread catastrophe to the aquatic vegetation. The depth, even 

 if it was constant, is insufficient to provide for any extreme 

 differentiation of plant zones, while its inconstancy obliterates 

 any which may gain a partial development. The greatest depths, 

 7-8 meters, occur temporarily in maximum floods in Spoon and 

 Illinois rivers. The depth at such times over areas of vegeta- 

 tion in the backwaters rarely exceeds 5-6 meters, while during 

 the period of spring growth it is usually less than 3-4 meters and 

 in large areas less than one. The whole area of vegetation 

 thus lies within the depth assigned by Magnin ('93) to the lit- 

 toral zone. Under these conditions the origin and mainte- 

 nance of permanent zones of vegetation is far less possible in 

 this fluviatile environment than it is in the more stable condi- 

 tions of the typical lake. 



An attempt to classify our water meadows according to the 

 scheme adopted by Magnin ('93) for lakes in the Jura, and by 

 Pieters ('94) for Lake St. Clair cannot succeed in our fluviatile 

 environment. The zone of deep and colder water character- 



