ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANIS^IS. 381 



are dealing; with a solutiou of a very complex and very 

 variable character, the exact nature of which it is extremely 

 difficult to determine. The most direct method of research, 

 namely, chemical analysis, has not proved of much service, 

 owing to the vmcertaiuty, and in many cases impossibility, of 

 accurate determinations, in sea-water, of such minute quan- 

 tities of substances as those upon which the growth of 

 plankton diatoms has been found to depend. 



We have bad, therefore, to rely, for the most part, on the 

 lengthy and tedious process of analysis by "trial and error," 

 the experiments being largely conducted on lines suggested 

 by Liebig^s well-known "law of minimums" (Pfeifer, vol. i, 

 p. 413). The ideal at which we aim is to find a culture 

 medium with artificially prepared sea-water as its basis, such 

 that the absence, or diminution in quantity, of any one of its 

 constituents would have a profound effect upon the growth of 

 diatoms in it. Whether the conditions regulating growth in 

 such a medium would be at all comparable to the natural 

 conditions of life in the sea is a question that would have to 

 be decided by experiment, but in any case this could be made 

 a starting point for much more definite research than has yet 

 been attempted. Up to the present time we have not, unfor- 

 tunately, succeeded in finding such a, culture medium. 

 Throughout the work we have had very great difficulty, in 

 spite of much care and many precautions, in obtaining 

 consistent results. It may even happen that in two flasks 

 containing the same culture medium, inoculated with the same 

 culture of diatom and standing side by side, under exactly 

 identical conditions, as far as can be recognised, quite 

 different degrees of growth will be observed. All experiments 

 must therefore be frequently repeated before entire confidence 

 can be felt in any conclusions which they seem to indicate. 



It must be remembered, also, that in all the persistent 

 cultures of diatoms that we have used, bacteria have pro- 

 bably been present, and this fact has probably had some 

 influence on the result. Unfortunately our attempts to 

 obtain absolutely pure cultures have not met witli success. 



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