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.; PI. 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 ran-off from Thompson's lake in 

 this year is thus an enrichuient 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 (cf. PI. Xll. 

 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 1 to .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 (XEI. 

 and XXXIX.) 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 



