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forms (ci. PlatesglIT. and IV. with I. and II.) coincide with or 
follow shortly after those of the synthetic planktonts on which they 
feed, we may conclude that the cause of the periodic movement of 
these animal groups lies in the periodic fluctuations of their food 
supply. In the causes which control this periodic growth of the 
chlorophyll-bearing organisms will be found the solution of the 
general periodic phenomenon in plankton. 
This rhythm is primarily one of growth and reproduction, and 
its solution must be sought in the forms of matter and energy which 
affect these processes. The nutrition of the chlorophyll-bearing 
organisms is drawn from matter in the river water. The analyses 
contained in Part I., Table X., and graphically presented on Plates 
XLIII. to XLV. trace the seasonal fluctuations in the nitrates—one 
of the important constituents of plant food. Neither in the seasonal 
curves of this or other forms of nitrogen delineated in the plates is 
there any such rhythm of occurrence, though, as has been pointed 
out in the discussion of the chemical conditions, there are instances 
of apparent correlation of plankton and nitrate pulses. They occur 
at irregular intervals, and do not form a continuous series. That 
there might be a rhythm in the utilized nitrates (the analysis repre- 
sents only the unused residuum) is of course possible, or that it 
might occur in some other constituent of the food not determined 
in the analysis is not impossible, but we have no evidence of its 
existence. 
The chlorine in our river waters is a fair index of the amount of 
sewage or pollution by animal wastes. It is subject to considerable 
fluctuations, resulting in part from dilution by floods or concentra- 
tion in low waters, and there are other pulses not traceable to 
hydrographic conditions, which perhaps result from industrial 
wastes. These fluctuations in some instances coincide with those 
of the phytoplankton in question, but the instances are few and 
the correlation is incomplete. Upon investigation I find that 
sewage pumpage at Bridgeport, which discharged the sewage of 
Chicago River into the Illinois and Michigan Canal and thence into 
the Illinois River, was practically continuous, and could not produce 
the rhythm in question. The sewage of Peoria has a much more 
immediate effect upon the chemical conditions in the river at 
Havana than has that of Chicago. The sewers of this city, I am 
informed by Mr. H. E. Beasley, City Engineer, are flushed as 
