56 PLANKTON OF WISCONSIN LAKES 



tially the same percentages of these two substances ; the differences in 

 each case are no greater than those usually shown by duplicate deter- 

 minations on a single sample. These results indicate, then, that the 

 proteins and fats in these organisms are not affected quantitatively 

 by the centrifuging process. 



Quantity of Material 



The amount of material obtained in the various centrifuge catches 

 in 1917 is shown in figure 27. After the samples were evaporated to 

 dryness and the residues were ground up in a mortar, they were placed 

 in vials until needed for the chemical analyses. When arranged in a 

 series these catches illustrated, in a very interesting manner, the 

 changes in the quantity of nannoplankton during the vernal pulse of 

 this year and a photograph of them was taken. Since these vials were 

 of approximately the same diameter, the height to which they were 

 filled by the different samples indicated roughly these changes in quan- 

 tity. The samples in the first two vials w^ere obtained from 904 and 909 

 liters of water respectively, while all of the others were obtained from 

 a larger quantity of water, namely, from 1,168 liters to 1,183 liters; 

 the former, therefore, ought to be a little more than 25.0 per cent larger 

 than they are for a direct comparison with the other samples. 



Attention may also be called to the fact that this material contains 

 a large proportion of ash, ranging from a minimum of 51.7 per cent in 

 sample No. 718 to a maximum of 64.2 per cent in sample No. 708 ; that 

 is, somewhat less than half of the dry material consists of organic 

 matter. 



For the purpose of indicating more clearly the differences in height 

 two scales marked off in centimeters were included in the photograph. 

 The samples are arranged in chronological order beginning with sample 

 No. 704 (vial No. 1) obtained on February 14, 1917, and ending with 

 sample No. 722 (vial No. 10) secured on June 1, 1917 ; the intervening 

 numbers and dates are shown in the general table for the nannoplank- 

 ton catches. No. 44, p. 207. The series includes all of the centrifuge 

 samples collected in 1917, except No. 702 and this one was omitted be- 

 cause some of it had already been used for a chemical analysis when the 

 photograph was taken. 



Vial No. 2, containing sample No. 706 collected on March 9, 1917, 

 shows the smallest quantity of material and increasing its volume 25.0 

 per cent would not make it as large as that of vial No. 3 (sample No. 

 708), so that there was a distinct increase in material by the latter 

 date, namely, April 18. There followed, then, a substantial increase 

 during the succeeding week, but the most marked increase came between 

 April 25 (vial No. 4) and May 2 (vial No. 5) ; the material in the 



