357 
sion of its effect in increasing the plankton of the channel can- 
not be given. Similar marshy regions are found, along the course 
of the river elsewhere, especially above higher bottoms which 
have been built up across the flood-plain by tributaries such as 
Spoon River, and such areas presumably share with Flag Lake 
this contributory function in the maintenance of channel 
plankton. 
In the discussion of production in Quiver and Dogfish lakes 
I have called attention to the similarity in the movement in 
production, these two lakes and in the river. In Flag Lake, 
we are dealing with a very different environment ; bottom, 
shores, vegetation, hydrographic relations, especially in the 
matter of tributary waters, are all diverse. Indeed, the lake it- 
self includes several distinet typesof environment. It is inter- 
esting to note that in so distinct a unit of environment as this 
marsh we find so large a degree of similarity in the movement 
of production as can be traced between its seasonal plankto- 
graph and that of the river and of the lakesthus far examined. 
It should, however, be stated that the similarity is less precise 
here and is more evident in 1896 than in other years, though 
this is probably in part due to the absence of sufficiently fre- 
quent collections. 
The degree of similarity may be seen in the following com- 
parisons. In 17 of the 27 possible comparisons between pro- 
duction in Quiver and Flag lakes (Pl. XXVIL. and XXXIIL.) 
the direction of the change in production coincides. Most of 
the 10 exceptions are due to slight differences in the location of 
apices of pulses, or occur at times of lowest water, that is, of most 
pronounced local differentiation—as, for example, at the drop 
in levelsin May and again inJuly. Thesame number of excep- 
tions similarly located occurs when production in Dogtish Lake 
(Pl. XXXI.) is compared with that in Flag Lake (Pl. XXXIIT.), 
and there are 11 exceptions in the possible 27 in the case of 
the river (cf. Pl. X. and XXXIII.). 
In general terms, the similarity consists in the rise in pro- 
duction, probably obscured in Flag Lake by an overestimation 
