ARTIFICIAL CULTURE OF MvVlilNE PLANKTON ORGANISMS. 405 



to give tlie maximum growth, and the cultures below this 

 temperature were usually superior to those above. 



General Cou elusions. — The general couclusions to be 

 drawn from the experiments described in this section, which 

 were made with a view to determining the conditions that 

 underlie the successful culture of diatoms, may now be dis- 

 cussed. Although the experiments have involved the making 

 of some 750 different cultures, our conclusions on many of the 

 questions raised are still indefinite, and much further work 

 will be necessary before a satisfactory answer can be given 

 to them. 



If we wish to obtain the maximum quantity of healthy 

 growth of a plankton diatom, the diatom must first be 

 obtained as free as possible from all other organisms, if not 

 in a "pure" culture, at least in a "persistent'^ culture. All 

 culture media should be sterilised either by heat or filtration, 

 and the experiments should be conducted under sterile condi- 

 tions. Starting with normal sea-water as the basis for the 

 culture medium, it seems to be first necessary to raise the 

 concentration of the nitrates, and possibly also of the phos- 

 phates, in solution. But this simple addition of nutrient 

 materials will not in itself suffice. Some other action, such 

 as that exerted by Miquel's solution B, by animal charcoal, 

 or by peroxide of hydrogen, seems to be imperative in nearly 

 every case. The exact nature of this action we have not 

 been able conclusively to determine. If the substances con- 

 tained in solution B were purely nutritive in character, we 

 should expect that, when alterations in the amounts of the 

 different ingredients were made, or when any one of the 

 ingredients was omitted altogether, the differences in the 

 quantity of growth would show a direct relation to the kind 

 of modification introduced. But our usual experience has 

 been that solution B can be modified within certain limits, 

 without producing any appreciable effect upon the resulting 

 cultures, whilst, if these limits are exceeded, there is an 

 almost complete inhibition of growth. In supplying a neces- 

 sary increase of phosphates, both Miquel's solution B and 



