CULTURE OF PLANKTON DIATOM THALASSIOSIRA GRAVIDA CLEVE. 435 



(6) \ normal amount, (c) \ normal amount, {d) f normal amount, (e) the 

 normal amount, i.e. 10-86 c.c. M.sol. CaClg per litre, (/) \\ times the 

 normal amount, {g) \\ times normal. 



(a) During the first week showed little sign of growth and was far 

 behind the others. At the end of a month, however, there was 

 quite a good growth, still very healthy, but the quantity was far 

 below that in (c), {d), (e), (/) and {q). 



(b) Small growth during the first week and remained always better 

 than (a), but never equal to (c), {d), etc. 



(c) Fair growth during first week and went on well, though the quantity 

 was never up to (d), (e), etc. 



(d) The growth was nearly equal to the normal (e) throughout, and at 

 the end of a month it was not possible to distinguish between 

 the two. 



(e) A fine healthy growth with long chains. 

 (/) About the same as (d) throughout. 



(g) About the same as (d) and (/) throughout. 



A repetition of (a) to (e) gave just the same result. In connection with 

 this series it must be noted that Miquel's B solution contains CaClg, so 

 that the amount of Ca present in {a) will be that contained in the 

 4 per cent of natural sea- water, plus that contained in the Miquel B. 



Varying the Amount of Potassium Chloride. An exactly similar series 

 was set up in which the potassium chloride was varied from to 1| times 

 the normal. All these gave very fine growths, of which the last two 

 (1| and 1| times normal) were the best during the first week. Subse- 

 quently it was not possible to distinguish between the amounts in the 

 different flasks. This result was also confirmed by a second experiment. 



It should be remembered that potassium was added as nitrate in this 

 as in the other experiments (2 c.c. of a 2 M.sol. KNO3 per htre). 



Variations in Salinity. It was shown in our previous paper * that in 

 the case of Skeletonema costatum, Biddulphia mohiliensis and Coscinodiscus 

 excenfricus, plankton diatoms of very similar habit and distribution to the 

 species Thalassiosira gravida chiefly used in the present experiments, the 

 salinity of the culture medium could be varied within wide limits without 

 greatly affecting the growth of the diatoms. Thus between 35 and 40 

 per cent of the water could be evaporated from a culture medium having 

 natural sea-water as its basis without seriously affecting the growth of 

 the diatoms, whilst dilution of the culture medium up to 100 per cent 



* Allen and Nelson, loc. cil., p. 453 [Q.J. M.S., p. 402], 



