ON THE ARTIFICIAL CULTUEE OF MARINE PLANKTON ORGANISMS. 439 



Since cultures can be obtained with no appreciable difference by 

 using media prepared by adding either of these solutions, together 

 with Miquel's solution A, to sea-water, a considerable latitude in the 

 proportions of the salts present is tolerated. 



We must now consider what is the role of the various constituents 

 in Miquel sea-water. The part played by any salt of a culture medium 

 may be considered as being either, firstly, "nutritive," or secondly, 

 "protective."* Under the first heading, any direct addition of food 

 material must be included ; under the second, any removal or neutral- 

 ization of harmful substances, such as toxins and possibly bacteria, 

 and any more remote effects, which, although influencing growth, do 

 not directly enter into the metabolism of the plant. 



Our experiments have proved that solution A can be reduced to a 

 simple solution of potassium nitrate f without detriment (cf. p. 427), 

 and that the amount of growth is, within limits, roughly proportional 

 to the amount of KNO3 added, as the following experiment shows : — 



Inoculated from persistent culture of TJialassiosira decipieiis. 



A. Normal Miquel sea-water. 



Growth as usual. 



B. Ditto, but only one-half amount of sol. A. 



Good growth at first, but exhausted sooner than A. 



C. Ditto, but 2i times amount of sol. A. 



Was slower than either A or B at start, but afterwards was 

 better than A or B and lasted longer. 



D. Ditto, but five times amount of sol. A. 



As C, but in greater degree. 



Considering the nature of the substance added, and its already well- 

 known action in plant metabolism, these results, coupled with the fact 

 that exhausted cultures can often be regenerated by the simple addi- 

 tion of nitrates (see below, p. 444), are quite consistent with the 

 assumption that sol. A is simply nutritive in action. The concentra- 

 tion of nitrates in natural sea-water is so low (Brandt, 47) that the 

 amount available in a culture of untreated water very soon becomes 

 completely exhausted, and it is this deficiency that sol. A probably 

 corrects. 



Considering now the action of sol. B, it must first be observed that 

 increased concentration of nitrates alone will not explain the whole 



^ * Loeb, The Dynamics of Living Matter (New York, 1906), p. 77. 



t For the sake of conveuience, the expression sol. A will be used throughout the rest of 

 this paper to indicate a simple solution of potassium nitrate (5 per cent) and sol. B to 

 indicate Miquel's phosi)ho-ferricalcic solution (p. 423). Unless otherwise stated, the amounts 

 of each added to 1000 cc. sea-water will be normal, i.e. 2 cc. sol. A and 1 cc. sol. B. 



NEW SERIES. — VOL. VIII, NO. 5. MaKCH, 1910. 2 H 



