405 
that in other years, and this also tends to lower the relative 
productiveness in the lake. It is evident that the seasonal dis- 
tribution of the period of flood waters and the resulting im- 
pounding function of the lake affect greatly its contributions 
to channel plankton. In this year flood waters are largely 
confined to the colder and less productive season, when the 
run-off contains little plankton and its contributions are small, 
while in 1897 recurrent floods throughout the year afforded a 
run-off in seasons of Jarger production, and this tended to 
greatly increase the enrichment of channel waters in that year 
as compared with 1897. 
Plankton content in 1897 in Thompson’s Lake was in ex- 
cess of that in the river in the case of coincident or approxi- 
mate collections in 16 of the 15 instances, the two exceptions 
appearing in July and September, when pulses in channel plank- 
ton rise above the recorded production in the lake as a result 
of some undetermined factor. 
The similarity in the course of plankton production here 
and elsewhere is most marked in the first part of the year, 
and decreases in the time of low water. Thus, on comparison 
of the planktographs of Thompson’s Lake (Pl. XX XVIII.) and 
the Illinois River (Pl. XI.) we find 14 out of 18 changes in the 
course of production coincident in the two regions, the four ex- 
ceptions occurring in May (1), July (2), and September (1). 
The environmental differences between Thompson’s Lake and 
the river are much less than between this lake and Quiver, and 
we find a corresponding disagreement in their planktographs, 
only 10 out of 18 changes being in the same direction, and six 
of the ten are in the period of high water, when local differen- 
ces are submerged. In the cases of Flagand Dogtish lakes col- 
lections extend only to July, with agreement in 5 cases in each 
out of a possible 7 and 6 respectively. In the year as a whole 
and including all the above localities we find 54 agreements to 
15 exceptions, in January—June the ratio being 3 to 21 for 4 lo- 
calities, and in July-December, in low water conditions, 12 to 13 
for from 2 to 3 localities. The effect of the common elements 
