ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 453 



occurred in our experimental work, could be explained by unforeseen 

 changes in " alkalinity," can only be answered by future research. 



Saliiiity. — The salinity (or amount of salts dissolved in a litre of 

 sea-water) of the outside water used in these experiments only varied 

 between small limits, S = 34-5 - 35-57oo. The salinity of " tank- water " 

 is also fairly constant, the average being about S = 34-97co \ water is 

 only pumped up into the reservoirs at high water, spring tides, and 

 unless the salinity on analysis is well above S = 34-57c, no water is 

 taken. Experiments to show what effect salinity pure and simple had 

 on the growth of diatoms were undertaken. Samples of sea-water of 

 various salinities were prepared by diluting down "outside water" 

 with double-distilled water, and by concentrating " outside water " by 

 slow evaporation. Two litres of "outside water" S = 34-9 were 

 evaporated down to the bulk of one litre, giving a 50%* concentra- 

 tion. Miquel solutions 4 cc. A, 2 cc. B, were now added, and the 

 solution was divided into ten culture vessels, 20 cc. in each. Double- 

 distilled water was added, 2 cc. to the first, 4 cc. to the second, 20 cc. 

 to the last, so that a series of media were obtained, varying in salinity 

 from normal to nearly 50% concentration, each containing the same 

 amount of Miquel's nutrient solutions. These were inoculated from a 

 mixed culture of Sheletonema costatum, Biddulphia mohiliensis, and 

 Coscinodiscus caxentricus. A good growth took place in all except the 

 two with highest concentration. Of these two, the last remained 

 practically sterile and tlie growth in the other was very poor. The 

 limit of concentration, therefore, seems to lie between 35 and 40%. 

 In the same way series of lowered salinities were prepared, and 

 cultures of the same diatoms were grown in these. Dilution up to 

 100% did not seem to make any diff'erence at all in the quantity or 

 quality of growth. In a series extending the dilution to 200% even in 

 the cultures of lowest salinity, a fair quantity of growth took place. 

 The range of salinities covered by the various series was S = 127oo to 

 S = 607oc ^i^d within these limits no eff'ect on growth could be 

 observed, except in the very highest, where a distinct deterioration was 

 noted. 



An attempt to grow Coscinodiscus cxccntricus in tap water + Miquel's 

 solutions was tried, and it was thought that some slight multiplication 

 took place, although it was certainly not at all considerable. In- 

 oculating a culture of normal Miquel sea- water from this after six 

 weeks gave no growth. 



Lifiht. Of all the factors controlling tlie rate of growth of a culture, 



* i.e. from every 100 cc. Sfa-watei' .^jO cc. H^G liad lieen siilitracted. 



