Sls) 
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. Itas 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 nnaghee 
with the more typical planktonts. 
Zoological Laboratory, 
University of California, 
May 10, 1904, 
