44 PLANKTON OF WISCONSIN LAKES 



part of the crude fiber of forage crops is digested by the ruminants, but 

 no data are available to indicate whether any portion of the carbohy- 

 drates in the plankton crude fiber is utilized by the organisms which 

 feed upon this material or not. The chitinous portion of the crustacean 

 shells passes through the alimentary canal of fishes without being 

 affected by the digestive processes, so that this part of the plankton 

 crude fiber may be regarded as having no food value, and it constitutes 

 a considerable portion of the total crude fiber at times. Since the crude 

 fiber constitutes a relatively small part of the organic matter in most 

 instances, and since the chitinous part of it has no food value, it appears 

 that the crude fiber of the net plankton plays a comparatively unimpor- 

 tant role from the food standpoint, even if some of the carbohydrates 

 in it are utilized by the organisms which consume the various plank- 

 tonts. 



In table 13 (p. 189) it will be noted that the crude fiber of the net 

 plankton varied from a minimum of 2.6 per cent of the organic matter 

 in one sample obtained in 1913 to a maximum of 20.2 per cent in a 

 sample collected in 1912. Thus, in one sample out of the 119 on which 

 determinations were made the crude fiber amounted to as much as one- 

 fifth of the organic matter, while in three-quarters of the samples it 

 was 10.0 per cent or less. The mean percentage for the different years 

 varied from 6.3 per cent in 1915 to 10.6 per cent in 1911. 



In terms of milligrams per cubic meter of water the amount of crude 

 fiber ranged from a minimum of 3.7 milligrams in 1911 to a maximum 

 of 67.0 milligrams in 1912, an eighteenfold difference. The greatest 

 difference for a single year was found in 1911 in which the maximum 

 amount was fourteen times as large as the minimum. The mean quan- 

 tity for the different years fell between 17.5 milligrams in 1914 and 30.8 

 milligrams in 1912. 



Nitrogen Free Extract 



The various constituents which have been considered thus far, such 

 as the crude protein, the ether extract, the crude fiber, and the ash, 

 do not comprise the whole of the net plankton; that is, if expressed 

 in percentages of the dry sample they do not constitute 100.0 per cent 

 of the material. In some instances, in fact, these four items account for 

 only about two-thirds of the dry matter. This is due to the fact that 

 there are various carbohydrates present which are not included in these 

 items. More or less carbohydrate material is found in the crude fiber, 

 especially in samples containing large numbers of algae, but, in general, 

 only a relatively small proportion of the total amount of carbohydrates 

 appears in the crude fiber. It is customary to designate all of the car- 

 bohydrates not included in the crude fiber as nitrogen free extract and 

 the quantity of this extract is determined by difference; that is, the 



