ARTIFICIAL CULTUKE OF MARINE PLANKTON ORGANISMS. 383 



off-shore sea-water in 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 than 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 nitrifying bacteria on the products ol 

 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 

 ahnost certainly be assumed. It is probably due to the 

 presence of these nitrates and soluble organic substances that 

 sterilised tank-water is a much better medium in which to 

 grow diatoms than sterilised outside water (see p. 379). 



The Constituents of Miquel's Solutions. — It has 

 been already stated that no better medium for the culture of 

 plankton diatoms has been found by us than the solutions 

 recommended by Miquel, although these solutions 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, ai-e the 

 same. Miquel's solution A and Gill's solution 2, can both be 

 replaced by a solution of potassium nitrate (p. 369). Again,. 

 Miquel's solution B and Gill's solution 1 only differ in the 

 proportionate amounts in which the various constituents are 

 prescribed. The formula3 are : 



Use 1 c.c. per 1000. Use 3 c.c. per 1000. 



