250 
emergent vegetation, which reaches a great development here, 
adds its contributions to the enrichment of the marsh waters 
with the return of flood conditions and temperatures facilitat- 
ing decay—conditions prevailing in the spring at the time of 
plankton maximum. The growth of vegetation in the spring, 
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 fluctuating 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 gaina 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 itis 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 (’98) 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- 
