METHODS AND APPAEATUS 17 



24 hours the precipitate was filtered off, dried, and weighed ; the results 

 were then calculated to pentosans. 



The amount of crude fiber was determined by digesting the material 

 for half -hour periods with sulphuric acid and sodium hydroxide solu- 

 tion having a specific gravity of 1.25. The undigested material was 

 filtered in a tared Gooch crucible, washed, thoroughly dried at 100°, 

 weighed, and then ignited. In the pure crustacean material the crude 

 fiber has been regarded as chitin. 



The plankton which was deposited on the filter papers of the centri- 

 fuge could not be removed for direct estimation since it adhered too 

 closely to the papers. So it was necessary to determine the quantity of 

 this plankton by an indirect method; that is, the filter papers were 

 dried and the total nitrogen in them was determined. One-eighth of 

 each filter paper in the set of nineteen was used for this analysis and 

 the nitrogen in excess of that obtained from a corresponding set of 

 blanks was regarded as belonging to the plankton. The quantity of 

 organic matter in this material was then estimated by multiplying the 

 excess nitrogen by the factor representing the ratio of the nitrogen to 

 the organic matter in the regular centrifuge catch. This estimated 

 organic matter was then added to that obtained from the bowl of the 

 centrifuge and the sum of these two constitutes the total organic matter 

 in the catch. The quantitative results are based on this total. In most 

 instances the nitrogen in the filter papers constituted between five per 

 cent and ten per cent of that in the material from the bowl of the 

 centrifuge. 



Purposes of the Investigation 



These studies were undertaken with a twofold purpose in view ; first 

 to obtain data on the annual plankton production of a lake with special 

 reference to the quantity of organic matter involved as well as the 

 chemical composition of this material; second, to obtain similar data 

 with respect to the various kinds of organisms making up the plankton. 



Concerning the first problem it may be said that it has been the aim 

 to secure results which are comparable in a general way to those that 

 have been obtained for the land. The agriculturist knows either exactly 

 or approximately the amount of his crops each year in terms of some 

 standard unit and he also knows approximately the area of land from 

 which these crops have been harvested; thus he can state his season's 

 results quantitatively per unit area of land. Furthermore numerous 

 chemical analyses enable one to obtain some idea of the food value of 

 the various agricultural products. But we have substantially no infor- 

 mation of this character with respect to the productivity of our fresh 

 waters; only estimates based on wholly insufficient data are available. 

 Also, with the exception of the fishes, our knowledge of the chemical 

 composition of the various fresh- water organisms is very scant indeed. 



