525 
ment-basin of the Illinois. It also receives a moderate amount 
of sewage from the cities of Lewistown and Canton, and the 
drainage from a considerable number of towns. Its diluent 
effect upon the plankton of the Illinois is thus not due to the 
poverty of its own waters but rather to the excessive fertility 
of the main stream, a fertility resulting from the sewage and 
industrial wastes received by that stream from che cities of 
Chicago and Peoria. 
The contrast in fertility as indicated by the analyses tabu- 
lated above is not as great as the differences in the plankton 
production of the two streams. The ratio of nitrates which 
perhaps most fully expresses their relative fertility is 1.01 to 
1.58, while the ratio of the plankton production as expressed 
in the average of the monthly means of the catches by the 
silk-net method is 0.256 to 2.71. The failure of Spoon River to 
develop a more abundant plankton is thus apparently due to 
some other cause than the lack of nutritive elements in the 
water for the support of the plankton. The development of a 
considerable volume of plankton in it at times of low water 
and slack current makes patent the probability that the lack of 
time for breeding is at least one of the important factors in the 
relative paucity of the plankton of this tributary stream. 
QUANTITATIVE COMPARISON. 
A comparison of the quantities of plankton taken by means 
of the silk net in the two streams affords a fair contrast of 
their relative productivity. Certain sources of error are, how- 
ever, present in the data of comparison, and as they are not 
equally distributed in the case of both streams they invalidate 
to some undetermined extent precise comparisons. These 
sources are the leakage of the plankton through the silk and 
the presence of silt. The plankton escaping through the silk 
is largely made up of the Mastigophora and small diatoms and 
algw, and they are found alike in both streams. The presence 
of a more abundant plankton in the Llinois and the somewhat 
flocculent nature of much of its silt tend to induce more per- 
