406 
of the environment which high water introduces, in unifying the 
course of plankton production in their several areas, and of low 
water in diversifying it, is well demonstrated by these compar- 
isons. 
In the planktographs of Thompson’s Lake and the Illinois 
River there is a striking general agreement in the low vernal 
production and the increased and unusual autumnal production. 
There are also some indications of a pulse-like character of the 
planktograph in the lake, though the collections are too infre- 
quent to demonstrate it. 
1898. 
(Table VIII., XII.; Pl. XXXIX., L.) 
There were 25 collections in this year, at fortnightly inter- 
vals, with an average of 5.71 cm.’ per m.* to 2.13 cm.’ in channel 
waters. The net result of the run-off from Thompson’s lake in 
this year is thus an enrichment of the plankton of channel 
waters. This is true for all of the monthly averages (see table 
between pp. 342 and 343) with the exception of April, and this 
exception is due solely to the distribution of collections on the 
rising vernal pulse, and is more apparent than real (ef. Pl. XII. 
and XXXIX.). The relative plankton content in the two areas, 
as will be seen on a comparison of the planktographs, is not 
subject to great variations in this year aside from January, 
when the ratio of the lake to the river is 1 to 17, and, as above 
noted, in April, when the ratio apparently falls to lto.6. With 
these exceptions, it ranges in the first six months from 1 to 2-3 
and in the last six from 1 to 3-5. These figures express quanti- 
tatively the striking similarity in the planktographs of the two 
areas, which may also be recognized at once in the plates (XII. 
and XX XIX.) in the low winter production, in the meteoric ver- 
nal pulse followed by a minor one in June, and in a low level of 
production during the remainder of the year with fluctuations 
within rather narrow limits. 
The cause of this close resemblance lies in the hydro- 
graphic conditions, which throughout this year favor constant 
