THE LIFE OF DEVILS LAKE 35 



apparatus are found in the works of many authors,* and need 

 not be repeated here, where it may suffice to summarize the diffi- 

 culties involved in their use. 



1. Nets, even of the finest bolting cloth available, allow the 

 escape of a large part of the nanno-plancton. 



2. The amount of water filtered by the net is a function of its 

 form, the rate at which it is handled, the age of the silk and the 

 consequent extent of clogging, and probably also of the temperature 

 and the amount of plancton present. 



3. Plancton traps and pumps which depend upon the use ol 

 nets for the concentration of the catch are subject to the first diffi- 

 culty above mentioned. 



4. Pumps are open to the further objection that their current 

 may serve to drive away some of the more active negatively rheo- 

 tropic organisms from the intake. 



5. Water bottles collect only small amounts of water, and fur- 

 ther cannot be filled without the creation of some current, sub- 

 jecting them to the same criticism as applies to pumps. 



6. The centrifuge in general handles only small amounts of wa- 

 ter, and if constructed of larger capacity is bulky, and consequently 

 impractical for general field work. Further, unless run at a high rate 

 of speed, some of the organisms, such as filamentous algae, especially 

 diatoms with flotation spines, may not be fully thrown down. The 

 large centrifuges require electricity, while the small hand instru- 

 ments require considerable strength and endurance on the part ot 

 the operator to run at a speed of 3,000 revolutions per minute, 

 for periods of one minute, which is hardly adequate for the pre- 

 cipitation of the lighter forms. 



7. The Sedgwick-Rafter method, involving the use of a funnel 

 tube closed by a piece of bolting cloth and a column of sand, is 

 open to objection 1 above, since the sand in combination with the 

 bolting cloth does not retain all of the finer organisms. When 

 there is sufficient amorphous material and zooglea in the water to 

 retain the nanno-plancton it retards filtration, and the apparatu& 

 works very slowly, if at all, six hours or more being required with 

 some collections, even tho the scum on the surface of the sand b(» 

 broken occasionally with a glass rod. Many of the organisms are 

 retained by the sand when filtration is ended and some may be 

 retained by the walls of the tube itself. A careful discussion of its 

 accuracy has been given by Whipple (1914) who considers it accui- 

 rate to 10%, In obtaining samples for filtration, moreover, the 

 difficulties mentioned above under 4 and 5 apply. 



*See especially Steuer (1910), Whipple (1914), Reighard in Ward and Whipple 

 (1918, Chapter III), and Juday (1916). 



