446 ON THE AETIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



phyll 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 neces- 

 sary, 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 explained, 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 artificially prepared solutions of 

 simple salts. If this end could be satisfactorily attained, the diffi- 

 culties due to the unknown and variable composition of natural sea- 

 water at once disappear. According to van 't Hoff (35) sea-water is a 

 solution containing salts in the following molecular concentrations : — 



NaCl 100-0, KCl 2-2, MgClg 7-8, MgSO, 3-8, CaCl. 1-0 (varies). 



Using these molecular concentrations, a sea-water of any desired 

 salinity can be prepared. The chlorine content of average Atlantic 

 water is about CI =19*4, and samples of artificial sea- water were pre- 

 pared with the same chlorine value, thus : — 



To make this solution comparable to natural sea- water, the " alka- 

 linity " must be raised by the addition of an alkali such as NaoCOg. 

 After the importance of " alkalinity " as a factor had come before our 

 notice, 2*4 cc. M/2 NajCOg was always added to the above solution in 

 order to make the amount of base in equilibrium with COo equivalent 

 to the usual 40 mgrm. 0H7„„ (p. 450). 



* Cf. Zumstein, Zur Mor/iJioIogi*' ». PhysMogie </. Euglena gracilis. Leipzig. 1899. 



