526 



of plankton per m.^ In November and December, however, the 

 mean production in the tributary, for the only time in our rec- 

 ords, exceeds by 67 and 7 per cent, the production in the chan- 

 nel. On the average, however, production in the tributary, 

 even in most favorable conditions, is less than in the main 

 stream, and the contributions of the tributary continue to be 

 generally a diluent of the channel plankton. 



If the means of the monthly averages are compared, the 

 ratio, when all collections are included, between production in 

 Spoon and Illinois rivers rises from 1 to 4.7 to 1 to 10.6. This 

 ratio, which eliminates somewhat of the error resulting from 

 differences in the number of collections in the two streams, 

 probably represents more nearly the actual ratio of production 

 in the two streams derivable from our data. 



In the table which follows, the catches in the two streams 

 upon coincident — approximately so in hve instances — dates 

 only are averaged. This reduces the number of catches in the 

 Illinois River from 235 to 33, modifies some of the monthly 

 averages, and changes the ratios of production in the Illinois 

 and Spoon rivers, based on means of the monthly averages, 

 from 1 to 10.6 to 1 to 9.9, but does not otherwise materially 

 alter the relationship of production in tributary and channel 

 waters as determined on the basis of all collections. 



If the silt is not eliminated by estimation the average 

 amount of the silk-net catch in the two streams is respectively 

 1.41 and 3.98 cubic centimeters per cubic meter. The change 

 in the ratio is due to the greater proportion of silt carried 

 by Spoon River. 



In the table of plankton comparisons following page 342, 

 the averages of the amount of plankton per cubic meter of 

 water in the two streams is given by months for the years of 

 our operations. Decimals to three places are not here indica- 

 tive of great accuracy of measurement, They result from ef- 

 forts to represent the small proportion which the plankton 

 forms of the silt-laden catches from Spoon River. Considera- 

 ble differences in these small quantities in this table and else- 



