285 



Illinois as compared with other streams, as shown by th 

 slight ratio of development of the stream (see p. 102), tends to 

 prevent the rapid mingling of channel and marginal waters, 

 and thus gives cumulative effect to their differential charac- 

 ters. 



In order to trace the quantitative effect of the shore and 

 determine the variation in transverse distribution, I made two 

 series of ten collections each along a transverse line, the first 

 at our usual plankton station and the second below the mouth 

 of Spoon Kiver (see PI. II.). The results of the first-named 

 test, made August 26, 1896, are given in the following table, to- 

 gether with conditions of distance from shore, depth, tempera- 

 tures, and turbidity. The river at this time stood at 6.5 feet 

 above low water, and had a width at the station of 150 meters. 



PLANKTON IN CROSS-SECTION OF RIVER AT STATION E. 



The collections were made with the pump, one fourth of a 

 cubic meter of water taken from bottom to surface being 

 strained in each catch. 



The variation in the catches is much greater in the cross- 

 section than in limited longitudinal tests, in accord with the 

 greater contrast in environmental conditions. The marked 

 decline near the western shore may be due to the marginal 

 belt of vegetation then present along that side of the river, and 



