12 PLANKTON OF WISCONSIN LAKES 



the plankton bucket to a jar with distilled water from a wash bottle. 

 The material was then preserved with a few drops of chloroform and 

 the catch, when completed, was placed in a porcelain dish and evapor- 

 ated to dryness on a sand bath at a temperature of about 60° C. The 

 dry material was carefully removed from the dish, ground up in a 

 mortar, and transferred to a weighing dish. It was then kept in a 

 desiccator for twenty-four to forty-eight hours, depending on the size 

 of the sample, after which the weight of the entire sample was ascer- 

 tained. Thereafter the material was transferred to a bottle and kept 

 for further study. 



At the end of the run made at each depth 10 liters of water were 

 strained through a small plankton net and this catch was used for a 

 numerical study of the various plankton constituents. Once each 

 week these small catches were kept separate for the purpose of deter- 

 mining the vertical distribution of the organisms. At other times all 

 of these small catches were combined into one sample. In order to 

 obtain a check on this method of determining the total number of 

 organisms in the large catch, samples for counting were taken from the 

 large catch itself and the volume of the material was measured when 

 the catch was placed in the porcelain dish for evaporation. With the 

 exception of a few forms which were not well preserved by the chloro- 

 form, the two methods checked as closely as could be expected, the 

 differences usually being well within the limits of error of the methods. 



Centrifuge 



As the work progressed on the net plankton it became increasingly 

 evident that the material which was lost through the meshes of the net 

 should be studied in a similar manner. It was a simple matter to 

 take samples of the overflow water, centrifuge them, and enumerate the 

 organisms therein, but this gave no results that could be compared 

 directly with those obtained for the net plankton. What was desired 

 was some method by which these minuter organisms could be obtained 

 in sufficient quantities to ascertain their weight per unit volume of 

 water, and to permit some studies relative to their chemical composi- 

 tion. Such data would thus permit direct comparisons with the results 

 obtained for the net plankton. 



Various filtration experiments made during the summer of 1914 

 showed that the problem could not be solved by such methods because 

 the various substances used for the filtering process soon became 

 clogged and thus permitted the use of only a small amount of water, 

 not more than two or three liters, usually less. But these experiments 

 served to stimulate a greater interest in the problem since the results 

 showed that the organic matter in the organisms lost by the net was 



