545 



♦Omitting 1895, 1.42. tOmitting 1895, 2.96. |Omitting 1895, i to 4.4. 

 II Omitting 1895, i to 1.3. 



months, from November to March, the backwaters have from 

 8 to 17 times the plankton content of the channel, and in 

 the warmer season, from April to October, from 0.8 to 5 times 

 as much. Omitting the aberrant record of June, 1895, from 

 channel data, it becomes apparent that there is least difference 

 between channel and backwaters in April-May and July, pre- 

 dominantly the months of greatest run-off of impounded back- 

 waters. 



Channel plankton of the Illinois River, therefore, has its 

 source in a large degree in impounded backwaters, and is main- 

 tained to a considerable extent by their run-off. 



INDIGENOUS PLANKTON OF THE CHANNEL. 



It does not seem probable that the channel plankton is 

 only the mingled plankton of the tributaries and backwaters. 

 Growth and reproduction, modified, however, by the cyclic 

 phenomenon, continue in the channel after plankton-laden 

 backwaters unite with it. This increase is facilitated by the 

 fact that channel waters are generally richer (see Tables 

 X.-XIII.) in nitrogenous matters and the other products of 

 decay than the backwaters. During high water, when the cur- 

 rent is more rapid, the time for further breeding is considera- 

 bly reduced, even to five days, from Utica to the mouth of the 



