NET PLANKTON OF LAKE MENDOTA 43 



4. The aqueous extract of the sample consisting of diatoms and Crus- 

 tacea gave an amorphous brown precipitate when treated with phenyl- 

 hydrazine hydrochloride and sodium acetate. This precipitate did not 

 contain an osazone that was soluble in hot 60.0 per cent alcohol or in 

 pyridine. Neither did it contain disaccharides or glucosides. 



5. No substance comparable to the algin of marine algae was found. 

 The samples of net plankton yielded measurable amounts of furfurol 



when distilled with hydrochloric acid; the furfurol was collected as 

 the phloroglucide, weighed, and the result calculated to pentosans. 

 Such quantitative determinations were made on 95 of the 134 net 

 samples that were collected between 1911 and 1915. Table 12 gives a 

 summary of the results obtained on the material from 1912 to 1915; 

 only three determinations were made on the material collected in 1911 

 and none at all on that of 1916 and 1917, so that these three years do 

 not appear in this table. All of the analyses are shown in the general 

 table, however (No. 43, p. 202). The pentosans constitute a relatively 

 small proportion of the organic matter in the net plankton. In only 

 two samples out of the 95 that were analyzed did the pentosans amount 

 to as much as 5.0 per cent of the organic matter, while the annual 

 means range from a minimum of 1.7 per cent in 1914 to a maximum 

 of 3.4 per cent in 1915. 



The total quantity of the pentosans was relatively small ; it exceeded 

 30 milligrams per cubic meter of water in only two of the samples on 

 which determinations were made. One sample collected in 1912 yielded 

 36.1 milligrams and another obtained in 1915 gave 33.6 milligrams per 

 cubic meter of water ; both of these amounts were found during the au- 

 tumnal maxima of organic matter. The annual means for the pentosans 

 range from a minimum of 5.4 milligrams in 1914 to a maximum of 15.3 

 milligrams per cubic meter of water in 1912, almost a threefold varia- 

 tion. Since the observations were discontinued on July 2, 1914, the 

 results cover only the first half of this year; therefore, it can not be 

 compared fairly with the full years. The minimum of the complete 

 years is 10.9 milligrams which is two-thirds as large as the maximum of 

 1912, thus showing a very much smaller range of variation when the 

 three complete years only are considered. 



Crude Fiber 



The term ** crude fiber'' is applied to the organic material which 

 remains undissolved after the plankton is digested for half -hour periods 

 with sulphuric acid and sodium hydroxide solutions having a specific 

 gravity of 1.25. In the mixed plant and animal material of the net 

 plankton, the crude fiber consists of carbohydrates derived chiefly from 

 the former and of chitin from the shells of the Crustacea. A certain 



