NET PLANKTON OF LAKE MENDOTA 41 



wide range in the percentage of fat in Daphnia ; this variation appears 

 to be more or less closely correlated with their stage of development. 

 Those which are in an active stage of reproduction and are carrying 

 many embryos in their brood chambers, contain the largest amount of 

 fat ; immature individuals or adults with few embryos yield a smaller 

 percentage of extract. In the former individuals the amount of fat 

 may exceed 21.0 per cent of the dry weight while in the latter it may 

 fall as low as 3.9 per cent. 



Thus the percentage of ether extract in the net plankton depends 

 upon three factors, namely, (a) the relative proportion of plant and 

 animal material present, (b) the predominant form of crustacean, and 

 (c) the developmental stage of the Daphnias present. 



The range of the variation in the percentage of the ether extract is 

 shown in the first part of table 10 (p. 188) where the results are stated 

 on an ash free basis ; the second part of this table gives the variations 

 in terms of milligrams per cubic meter of water. The maximum per- 

 centage in the net plankton was found in 1911 ; in one sample obtained 

 this year, the ether extract constituted 26.58 per cent of the total or- 

 ganic matter. The minimum for this year was also higher than in any 

 of the other years; the maximum percentage for 1911 was only about 

 three and a half times as much as the minimum. The lowest maximum 

 percentage was noted in 1914 and the lowest minimum in 1913. The 

 maximum percentage in the latter year was nearly seven times as large 

 as the minimum ; in the other years the maxima were from three to four 

 times as large as the minima. The highest mean percentage was found 

 in 1914 and the lowest in 1913. 



When stated in terms of milligrams per cubic meter of water, as in 

 the second part of the table, the largest amount of ether extract was 

 found in 1912 and the next largest in 1915. A sample obtained in 1913 

 showed the smallest amount per cubic meter and the maximum quantity 

 for this year was a little more than twenty-five times as large as the 

 minimum; this was the largest annual difference noted during the pe- 

 riod of these observations. In the other instances, the maxima were 

 from eight to twelve times as large as the minima of the corresponding 

 years. The mean amount for 1911 was far below those of the other 

 years, being only about half as large as three of the others. The 

 largest average amount for a whole year was found in 1915, namely, 

 46.6 milligrams per cubic meter of water. 



The curves marked C in figures 11 to 14, inclusive, show graphically 

 the quantities of ether extract in milligrams per cubic meter of water 

 for the different years ; they also indicate the relative proportion of this 

 material in the organic matter as well as the relation of the amount of 

 ether extract to that of crude protein. 



