ON THE ARTIFICIAL CULTURE 01" MARINE PLANKTON ORGANISMS. 455 



different uniform temperatures from 10^ to 25° C, by means of hot 

 air, was used, but no really satisfactory result could be obtained. 

 About 17" C. seemed to give the maximum growth, and the cultures 

 below this temperature were usually superior to those above. 



General Conclusions. The general conclusions 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 discussed. Although the experiments have in- 

 volved 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 sterilized either by 

 heat or filtration, and the experiments should be conducted under 

 sterile conditions. 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 phosphates, in solution. But 

 this simple addition of nutrient materials will not in itself suffice. 

 Some other action, such as that exerted by Miquel's sol. 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 substance contained in sol. B 

 were purely nutritive in character, we should expect that, when altera- 

 tions 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 

 sol. 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 necessary increase of phosphates, both Miquel's sol. B and 

 animal charcoal may and probably do act as " nutritive " substances, 

 but, since the addition of phosphates alone does not yield cultures 

 comparable with those produced by either of these, and since, excepting 

 phosphates, there is no possible common nutritive substance in their 

 composition, we are led to conclude that, in addition to any nutritive 

 effect, they must exert some other action. This view is supported by 

 the results obtained by using HoO.,. This substance cannot be directly 

 " nutritive," although it may be so indirectly, by oxidizing into useful 



NEW .SEKIE.S.— VOL. VIII. XO. 5. MAl'.ril, 1910. 2 I 



