438 ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



We have seen that if we compare " tank-water," i.e. water from the 

 closed circulation of the Plymouth Aquarium, with off-shore sea-water 

 ill situ, a most obvious difference is the much increased density of the 

 larger forms of animal life in the former, combined with the almost 

 complete absence of plant life. Hence the concentration of excretory 

 products in the tank-water must be very much higher tlian in outside 

 water. Other factors, such as increased bacterial action, artificial 

 aeration, etc., in tank-water, must also be taken into account (cf. 

 Vernon, 58 ; Smith, 56). There seems to be direct evidence to show 

 that the concentration of nitrates, possibly due to the action of nitri- 

 fying bacteria on the products of excretion, such as urea, ammonia, 

 etc., is considerably higher in the tank-water, and the presence of soluble 

 organic matter, in concentrations never met with in the sea, can 

 almost certainly be assumed. It is probably due to the presence of 

 these nitrates and soluble organic substances that sterilized tank-water 

 is a much better medium in which to grow diatoms than sterilized out- 

 side water (see p. 435). 



The constituents of MiqiieVs solutions. It has already been stated that 

 no better medium for the culture of plankton diatoms has been found 

 by us than the solutions recommended by Miquel, although these solu- 

 tions may be modified and simplified in various ways with equally 

 good results. The formulae recommended by Houghton Gill give very 

 similar cultures. The essential features of Miquel's and Houghton 

 Gill's methods, when adapted to sea-water, are the same. Miquel's 

 solution A, and Gill's solution 2, can both be replaced by a solution of 

 potassium nitrate (p. 427). Again, Miquel's solution B and Gill's solu- 

 tion 1 only differ in the proportionate amounts in which the various 

 constituents are prescribed. The formula? are : — 



The proportionate amounts added to equal volumes of sea-water 

 are : — 



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

 Na., HPO4 .... 10 .12 



CaClo 10 .24 



FeClg . . . . 5 .3 



HCl 5 .6 



