ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 393 



tnobiliensis in another, was obtained, the growth lasting 

 for some considerable time. 



Infusions, made in the same way from a small piece of fresh 

 fish, gave the same results, and although growth was rather 

 slower at first, the final result was, if anything, slightly 

 better. As Miquel points out, these infusions must be made 

 very dilute, otherwise growths of bacteria, moulds, etc., will 

 completely swamp the diatoms. Karsten (7), in some interest- 

 ing' experiments, showed that Nitzschia palea (Kutz) 

 W. Sm. could be made to alter completely its mode of nutri- 

 tion. On placing this diatom in organic nutrient solutions, 

 it lost all chlorophyll and became colourless, but in saline 

 media the chlorophyll would not regenerate, and the nutrition 

 change back from heterotrophic to autotrophic.^ 



Of course, with our infusions, it cannot be said that the 

 diatoms were necessarily feeding on dissolved organic 

 material, as some necessary, saline, nutritive materials could 

 have dissolved out from the weed or fish. If the former is 

 the case, it might explain the superiority of tank-water over 

 outside water, since the tank-water must contain a much 

 higher percentage of organic substances in solution. If an 

 alternative mode of nutrition, autotrophic or mixotrophic, 

 could be proved, especially in the case of the '^ bottom" 

 forms of diatoms, a great many phenomena could be ex- 

 plained, but the evidence is as yet far too slight to warrant 

 any such assumption. 



Artificial Sea-water. — As we have explained in a 

 previous section, the ideal aimed at in this part of our work 

 has been to obtain strong growths of Diatomaceae in purely 

 ai'tificially prepared solutions of simple salts. If this end could 

 be satisfactorily attained the difficulties due to the unknown 

 and variable composition of natural sea-water at once dis- 

 appear. According to van 't Hoff (35) sea-water is a solution 

 containing salts in the following molecular concentrations: 

 NaCl 100-0, KCl 2-2, MgCL 7-8, MgS0^3-8, CaCL I'O (varies). 



^ Cf. Zumstein, ' Zur Morpliologie n. Pliysiologie d. Euglena 

 gracilis,' Leipzig, 1899. 



