474 
imum. In general, this is a period of declining plankton in 
channel waters, where production in these two months falls 71 
per cent. below that in the preceding four months of maximum 
heat, and in Quiver Lake, where it falls 48 percent. below. On 
the other hand, in the rest of the backwaters there is a slight in- 
crease in these two months as compared with the production in 
the period of maximum heat. In Dogfish, Flag, Thompson’s 
and Phelps lakes the October-November increase in percent- 
ages over the average summer production in each of these sev- 
eral localities is 33, 862, 26, and 11 per cent. respectively. In 
view of these divergent tendencies in production under similar 
temperature conditions it is evident that other factors are 
operative, or at least more potent, in controlling autumnal pro- 
duction. The October production is as a rule higher than that 
of November, and suggests a tendency towards an autumnal 
pulse comparable with the vernal pulse but of lesser amplitude. 
The vernal pulse occurs in rising temperatures of 60°-70°, and 
this autumnal one in falling temperatures of 60°—50°. 
The month of December does not on the average quite at- 
tain the minimum winter temperature, though in some years, 
as in 1897 and 1898, it approached closely to it. Neither does 
the plankton production drop to so low a level on the average 
or in individual years in channel waters as during the two 
colder months which follow. In general the same relation ex- 
ists in the backwaters, though exceptions occur—principally in 
Thompson’s Lake. 
Thus, in a large way, temperature plays an important part 
in controlling plankton production. Additional proof of its 
potency is to be found in the correlations between production 
and exceptional divergences from the normal course of tem- 
perature changes, such, for example, as early or late vernal 
rise or autumnal decline. 
The accompanying table (p. 475), kindly furnished by Mr. 
W. G. Burns, Section Director for Illinois of the U. S. Weather 
Bureau, gives the vernal air temperatures for 1896-1898, and 
permits a comparison with the course of plankton production. 
