288 



distance from the east and west shores respectively. As will be 

 seen in Plate XL, this was made after nine weeks of uninter- 

 rupted low water, when the river had been standing at 2 ft. 

 for some time. The catches were made between 2 and 4 

 o'clock p. m. There was no vegetation in the river at this 

 point in this season, though both Havana Lake and Quiver 

 Chute, to the north (PI. IL), contained a small amount. The 

 discharge from Quiver Creek and Lake makes its way along 

 the eastern margin of the river, while that of Spoon River un- 

 der these hydrographic conditions hugs the western shore. The 

 effect is seen in the turbidity records, the clearer water be- 

 ing on the eastern side and the more turbid on the western. 



The following table gives the data of collection. There 

 was almost no silt in the catch, and the silt estimates are 

 therefore omitted. 



PLANKTON IN CROSS SECTION OF ILLINOIS RIVER BELOW MOUTH OF SPOON RIVER. 



Num- 

 ber of 

 col- 

 lec- 

 tion. 



Temperature 



(F.) 



Sur- 

 face 



Bot- 

 tom 



Depth 

 in ni. 



Tur- 

 bidity 

 depth 

 (in m.; 



disc 

 visible 



Plankton per 

 m.3, in cm. 3 



Plankton under 

 1 sq. meter 



Vol- 

 ume 



Departure 

 from mean 

 in per cent. 



Vol- 

 ume 



Departure 

 from mean 

 in per cent. 



Departure from mean 



in percent, 

 (omitting- Nos. 1 and 2) 



Under 1 

 sq. ra. 



71.5 



71.5 



70.5 



70.5 



70.3 



70.5 



70 5 



70.5 



71 



71.2 



70 



70 



70 



70 



70 



70 



70 



70.5 



71 



.66 

 1.06 

 1.42 

 1.58 

 1.5S 

 1.68 

 1.83 

 1.88 

 1.83 

 1.^2 



-83 

 -57 

 -20 

 + 4 



- 8 

 +34 

 +56 

 +45 

 -f30 



- 1 



-20 



- 6 

 -17 



+14 

 +22 

 +10 

 + 2 



- 6 



-32 

 -12 

 -22 

 +14 



+33 

 +23 

 +11 

 -15 



Averag-e 



6.00 I ±22.3 



3112.1 



r20.2 



The results of this test are confirmatory of the thesis here 

 maintained, namely, that the distribution of the plankton in a 

 stream does not differ in the main from that thus far observed 

 in lakes in the matter of variations in the plankton content 

 (volumetric) in different localities. The average =t depar- 

 tures from the mean volume, computed per m.' and under 1 

 sq. m., in these ten catches are 22.3 and 33.8 respectively, as 

 compared with 31.8 and 28.8 for Lake St. Clair and 5.52 for 

 the German lakes examined by Apstein. 



