384 E. J. ALLKN AND E. W. NELSON. 



The proportionate amounts added to equal volumes of sea- 

 water are : 



Miquel's sol. B. H. Gill's sol. 1. 



NaoHPO^ . . . 10 . . .12 

 CaClo . . . . 10 . . .24 

 FeCl.^ .... 5 ... 3 

 HCl .... 5 ... 6 

 Since cultni'es 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 con- 

 siderable 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; iinder the second, any removal or neutralisation 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- without 

 detriment (cf. p. 369), and that the amount of growth is, 

 within limits, roughly proportional to the amount of KNO^ 

 added, as the following experiment shows : — 



Inoculated from persistent culture of Thalassiosira 

 d e c i p i e n s : 



A. Normal Miquel sea-water. 



Growth as usual. 



B. Ditto, but only half amount of solution A. 



1 Loeb, ' The Dynamics of Living Matter,' New York, 1906, p. 77. 



- For the sake of convenience the expression solution A will be used 

 throughout the rest of this paper to indicate a simple solution of potas- 

 sium nitrate (5 per cent.), and solution B to indicate Miqiiel's phospho- 

 ferri-calcic solution. Unless otherwise stated ihe amounts of each 

 added to 1000 c.c. sea-water will be normal, i.e. 2 c.c. solution A and 

 1 c.c. solution B. 



