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tion of the basin and in the matter of movement in the water, but 

 also in the constitution of the plankton itself. The examination 

 of the plankton of the Illinois River, and of its backwaters and 

 tributaries, has shown that the plankton of the channel is not im- 

 mediately derived from the tributaries, but comes in large part 

 from the impounding backwaters, and at low-water stages is almost 

 exclusively indigenous in the channel itself. Upon the basis of the 

 data from the Illinois River the potamoplankton is distinguished 

 from the other types named by the following characters : 



1. It is a polymixic plankton. This is due to the mingling of 

 planktons from all sources in the drainage basin, especially from 

 tributary backwaters, and the consequent seeding of the channel 

 waters with a great range and variety of organisms. In all of our col- 

 lections in channel waters monotonic planktons can scarely be said 

 to be present. The nearest approach to such conditions occurred 

 at low- water stages, when channel waters are most fully isolated. 



2. It is subject to extreme fluctuations in quantity and con- 

 stitution. This naturally follows from the manifold factors of the 

 fluviatile environment and the directness with which they impinge 

 upon the plankton. Changes in volume, contact of shore and 

 bottom, access of heat and light, and changes in chemical con- 

 stituents are frequently both more extensive and more widely 

 effective in the stream than they are in the other types of aquatic 

 environment. In consequence, the plankton of the stream is sub- 

 ject to more catastrophic changes than that of the lake. 



3. The potamoplankton is not characterized by any species 

 peculiar to it, nor by any precise assemblages of eulimnetic organ- 

 isms. It may be distinguished, in a general way only, by the 

 greater proportion of littoral or benthal forms which are mingled 

 with the more typical planktonts. 



Zoological Laboratory, 



University of California, 

 May 10, 1904. 



