36 THE LIFE OF DEVILS LAKE 



8. Concentration of the planctou by filtration thru paper, in 

 addition to the errors involved in collection of the samples ana 

 enumerated above, involves also one due to the adherence of a 

 considerable amount to the paper and its consequent loss in the 

 counting cell. 



Were the errors involved in any method constant, they would 

 not cause any serious difficulty in plancton investigations, but i.a 

 every case they are the function of several variables and hence are 

 themselves variable. Thus in the use of a pump the current at 

 the intake depends on the rate of pumping, which is necessarily 

 inconstant for hand driven pumps, while machinery for the purpose 

 is expensive, bulky and generally impractical for field work. 



This variation in current rate at the intake may influence the 

 number of organisms which escape due to their rheotropism. Further- 

 more the movement of the end of the hose due to the rocking 

 of the boat, which is obviously a very variable quantity, may influ- 

 ence their reaction, and the same objections apply in the case of 

 the water bottle, where the rate of flow is a function of the pressure 

 and this in turn of the depth. 



Furthermore the rheotropic reaction on the part of plancton 

 organisms is very probably a function of temperature and other 

 less obvious factors in the life of the organism. 



Satisfactory preservation of material is one of the most serious 

 difficulties in any plancton investigation. If the material is to be 

 counted fresh, it must be done promptly, especially in warm weather, 

 or it will deteriorate, and this is impractical in the case of collec- 

 tions made in the field at some distance from a laboratory. If the 

 material be living when counted the movement of larger forms such 

 as crustaceans and rotifers disturb the contents of the counting 

 cell so that accurate counting is impossible. Furthermore at the 

 time collections are made, press of more immediate duties may 

 render counting impractical. On the other hand preserved material, 

 especially those forms such as the alga Dictyosphaerium and bac- 

 terial zooglea, which are held together by delicate gelatinous sheaths, 

 is almost certain to deteriorate in course of time, so that if counting 

 be delayed for months or years a very serious error is introduced. 

 The enumeration of the plancton has to be made with low 

 powers (about 100 diameters) which renders the specific determina- 

 tion of some of the smaller forms difficult or impossible. 



Still another difficulty in obtaining accurate plancton records, 

 and one to which sufficient consideration has not been generally 

 given, is the irregularity in the distribution of the plancton organ- 

 isms themselves. This irregularity has been noted by occasional 

 investigators in the past, but it has nevertheless been generally 

 assumed that one sample taken at any point in a given body of 



