XI 



PAGE 



Preservation and Measurement. 254 



Fixing and preserving fluids 254 



Measurement by centrifuge 254 



Method and extent of its use 255 



Comparison with gravity method 255 



Statistics of decrease in volume 255 



Factors determining decrease 256 



Composition of plankton 256 



Silt 257 



Comparisons with results of other investigators 257 



Formula for and computation of specific pressure 257 



Effect upon plankton organisms 258 



Priority in use of centrifuge for plankton measurement 259 



Silt estimation 260 



The clogging of the net 261 



Correction by computed coefficient 261 



Computed coefficients of our nets 262 



Inadequacy of such means of correction. 262 



Determination of empirical coefficients. 263 



Progressive clogging of the drawn net 263 



Reasons for adoption of empirical coefficients 264 



Volumetric Examination of Plankton of Channel and Backwaters 265 



Plankton of Illinois River channel 266 



Description of locality of collection 266 



Modifications of methods of collection at this station 267 



Chronology of collections 268 



Local distribution of plankton and its relation to limit of error in the 



method 269 



Results of other investigators 270 



Discussion of Reighard's method of computation 271 



Problem stated for Illinois River 272 



Longitudinal distribution of plankton in Illinois River 273 



Test from anchored boat 273 



Comparison with Apstein's results in German lakes 274 



Test from floating boat 276 



Comparisons and conclusions 276 



Tests from chronological series 277 



Test in traverse of river from Hennepin to mouth 279 



Method of collection 279 



Conclusions as to distribution in river as a whole 280 



Sources of error and disturbing factors 280 



Effect of hydrographic conditions 281 



Distribution in minor units of environment 281 



Transverse distribution of plankton in river 283 



Relation of shore to plankton 283 



Plankton in cross-section of river at Station E 285 



Variations in plankton per m. 3 286 



Variations in plankton under one square meter 286 



