ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 365 



weeds, such as Zostera. Macerations of these should be made 

 up separately some time before they are required for use, 

 and should be carefully filtered and sterilised. Organic 

 matter must, however, be used very sparingly, or else putre- 

 faction will set in and the cultures will be irrevocably lost." 

 As a matter of fact we have found that such organic infusions 

 are unnecessary when dealing with plankton diatoms, and it 

 has not been our practice to employ them (cf., however, p. 392). 



Miquel obtained cultures of single species of diatoms either 

 by picking out individual diatoms under the microscope and 

 introducing them into the prepared water, or by adding a, 

 small quantity of water containing a mixture of diatoms and 

 other organisms to some prepared Avater, and subdividing 

 this into a number of tubes. If the subdivision has been 

 carried out sufficiently some of the tubes may contain one 

 kind of diatom only, from which fresh cultures can be made. 

 In this way, by repeated subdivision, cultures can be obtained 

 which, by inoculating fresh quantities of prepared water from 

 time to time, may, with care, be maintained indefinitely. Such 

 cultures, however, must practically always contain bacteria, 

 and Miquel distinguishes them frotn bacteria-free cultures, 

 which he terms " cultures des diatomees a I'etat de purete 

 absolue." The latter he found very difficult to obtain, but 

 through repeated washing in sterile water, followed b}^ frac- 

 tional subdivision, he succeeded in getting- some in which he 

 could find no trace of bacteria by ordinary bacteriological 

 methods (Miquel [11], p. 155; cf. also Richter [16-18]). 



We propose to call any diatom culture which can be' 

 carried on practically indefinitely by inoculating fresh 

 supplies of prepared water a "persistent" culture, the 

 term "pure" culture being reserved for cultures which 

 can be proved to contain not more than one organism. We 

 are not satisfied that we have yet succeeded in obtaining 

 cultures of the latter kind. For the most part our persistent 

 cultures contain one species of diatom only, and are free fmm 

 all organisms larger than small flagellates. ' '' "^ 



In our earlier experiments with plankton diatoms Wei 



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