64 PLANKTON OF WISCONSIN LAKES 



continued until June 1, 1917. With the exception of January, 1916, 

 observations were made every month during this period, and runs or 

 catches were made twice a week in most instances during the time that 

 the lake was free of ice, i. e., from April to December. In 1915 the 

 centrifuge runs totaled 65 in number, in 1916 there were 69, and in 

 1917 there were 11 up to June 1 when the observations were discon- 

 tinued. The number of runs and the amount of water centrif uged each 

 year are shown in table 3 (p. 182). The total for Lake Mendota for 

 the three years was 145 runs in which 179,506 liters of water were cen- 

 trif uged, an average of about 1,240 liters for each run. 



Effect of Centrifuging 



The centrifuge bowl has an inside diameter of 24 centimeters and the 

 maximum speed at which it can be run with safety is 6,000 revolutions 

 per minute. For all of the runs made in these investigations the speed 

 was kept a little below the maximum so that it ranged from 5,600 to 

 5,800. Experiments showed that this high speed was necessary to 

 remove the organisms effectively; only about 70.0 per cent as much 

 material was obtained at 4,000 revolutions as at 5,800. Taking 5,700 

 revolutions per minute as the average speed of the centrifuge, the 

 inner surface of the bowl, on which most of the material was deposited, 

 moved at a rate of about 4,300 meters per minute, or 71.6 meters per 

 second. 



The material was frequently examined with a microscope in order to 

 ascertain whether the various organisms showed any signs of serious 

 injury as the result of being centrif uged. In general those forms which 

 possess a fairly firm cell wall did not show any evidences of serious 

 injury. Colonies of Pandorina were found intact, for example, and 

 they promptly renewed their normal activities after being centrif uged ; 

 while Euglena showed no signs of physical injury, it remained in a 

 contracted state for about an hour before it became active. None of the 

 algae showed any evidences of injury to the cells. 



The more delicate forms, such as Amoeba and the monads, appeared 

 in the material in good condition but their numbers were not as large as 

 expected from enumerations made previous to centrifuging. It seems 

 probable, therefore, that some of these organisms suffered more or less 

 damage, but it is not believed that this was sufficient to affect appre- 

 ciably the total quantity of the centrifuge material nor its chemical 

 composition. Such an injury might result in the loss of some of the cell 

 fluids which have about the same density as water, but the protoplasmic 

 portion would be retained in the centrifuge. A slight loss as the result 

 of more or less injury to these forms, however, would have little signifi- 



