546 
Illinois. Under these circumstances the plankton indigenous 
to the channel itself is of small volume as compared with that 
contributed from backwaters. At low water, however, it may 
take 23 days, or more, for the water to traverse this distance. 
This frequently results in such a development of channel 
plankton that it not only rises above that in many of the 
backwaters, but also takes on a characteristic facies which 
at once stamps it as largely indigenous in the channel itself in 
its origin. Instances of this phenomenon appear with pro- 
longed low water, as in June-July, 1895, when channel plankton 
exceeded that in ail the backwaters and was characterized by 
the great abundance of Moina. In hke manner in November- 
December of every low-water year there has been an unusual 
development of Ciliata, principally Carchesium lachmanni and 
predatory forms feeding upon it. This, at least as shown by, 
the catches of the silk net, does not often exceed the plankton 
content elsewhere, but its dominant organisms form relatively 
a small proportion of the backwater plankton at such times. 
The channel plankton is, then, largely indigenous. The autumn 
of 1897 saw a similar indigenous devevelopment of Chlamy- 
domonas in the river which was not equaled in the backwaters. 
The plankton of the Illinois River is the result of the 
mingling of small contributions by tributary streams, largely 
of littoral organisms and the quickly growing alge and flagel- 
lates, and of the rich and varied plankton of tributary back- 
waters, present to an unusual degree in the I|linois because of 
its shghtly developed flood-plain, and from which it is never 
entirely cut off even at lowest water. Data are lacking as to 
the effect of the contributions of the [linois and Michigan 
Canal upon the plankton of the river. To these elements is 
added such further development of the contributed or indigenous 
organisms as time permits or the special conditions of nutri- 
tion and sewage contamination facilitate. Though continually 
discharging, the stream maintains the continuous supply of 
plankton, largely by virtue of the reservoir backwaters—the 
great seed-beds from which the plankton-poor but well-fertilized 
