573 
ities but one, but falls much below the summer production in 
Phelps Lake. The ice-sheet is not inimical toa considerable 
plankton production unless stagnation conditions occur. 
19. Light affects plankton production. The half year with 
more illumination and fewer cloudy days produces from 1.6 to 
7 times as much plankton as that with less illumination and 
more cloudy days. Seasons of unusual cloudiness are accom- 
panied by depression in production. 
20. Lakes rich insubmerged vegetation produce less plank- 
ton than those relatively free from it,in an annual ratio of 1 to 
6and a monthly ratio varying from 1.5 to 20 to that of 1 to 20. 
The higher ratios generally prevail in periods of dominance of 
vegetation. Quiver Lake produces more plankton when free 
from vegetation than when it abounds in it. The emergent 
and rooted vegetation of Flag Lake conduces by its autumnal 
and vernal decay to large plankton production, but tends to de- 
press production in summer. + 
21. The normal regimen of the course of plankton pro- 
duction in the Illinois River and its backwaters does not form 
a definite seasonal planktograph, but consists rather of a series 
of recurrent plankton pulses, whose varying amplitudes are 
‘largely determined by the fluctuating environmental factors of 
the unstable fluviatile environment. Hence planktographs of 
the same locality in different years and of the different sta- 
tions in the same year show resemblances only in such funda- 
mental features as the winter minimum and the vernal pulse. 
The relative productive rank of the several localities is gener- 
ally maintained, and more completely in the more stable en- 
vironments. 
22. The plankton of the Illinois River is largely autono- 
mous. Seepage and creek waters are diluents of its plankton 
and add little to its diversity. Even Spoon River is generally 
a diluent, reducing the plankton content 10 per cent. and add- 
ing but few diversifying species to its population. The reser- 
voir backwaters, on the other hand, generally contain a more 
abundant plankton than the channel, the amount, on the bases 
