ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 445 



any conceivable shape. The form of the frustules tends to come Lack 

 to the normal agam, when the culture is well started, and in old stages 

 the majority will be perfectly formed, although small and pale. It 

 was found that the addition of silica (in early experiments as fragments 

 of potassium silicate) was, as far as could be judged, immaterial, which 

 fact led to the conclusion that a sufficiency dissolved out from the glass 

 flasks in which the cultures were kept. During rapid growth, it is 

 possible that the silica does not dissolve out fast enough to supply the 

 demand, although it is also possible that diatoms, during rapid division, 

 cannot absorb silica and form a perfect skeleton, even when the supply 

 is abundant. Eichter (18) has proved the necessity of either CaSiaOj 

 or KgSioO. for the growth of Nitzschia ixdea, grown in pure cultures. 

 We tried the addition of silica in various forms, and in one instance, 

 in a culture of Coscinodiscus excentricus, to which a little precipitated 

 calcium silicate had been added, the uniformity and markings of the 

 valves were much more regular than in the control. The presence of 

 a trace of pure, dialysed silica, also, in one experiment, gave an im- 

 proved regularity of form, but the quantity or rapidity of growth did 

 not seem to be affected. No sign of regeneration could be obtained in 

 exhausted cultures by the addition of silica. 



Organic Infusions. — Miquel recommends the use in culture media of 

 infusions of organic substances such as bran, straw, diatom broth, etc., 

 in addition to the saline solution. He does not make it quite clear 

 if he ever dispensed with them at all. In his general directions, he 

 certainly states that the addition of both saline and organic nutrient 

 material is necessary. As would be expected from the general metabolism 

 of plants, the saline constituents are sufficient for growth. At the same 

 time, excellent cultures have been obtained from dilute organic in- 

 fusions, both with and without the addition of Miquel's sols. A and B. 

 About a square inch of Ulva was boiled in 600 cc. sea-water for half 

 an hour, cooled and filtered. In this medium an excellent growth of 

 Coscinodiscus excentricus in one case, and Biddulphia mohiliensis in 

 another, was obtained, the growth lasting for some considerable time. 



Infusions, made in the same way from a small piece of fresh fish, 

 gave the same results, and although growth was rather slower at first, 

 the final result was, if anything, slightly better. As Miquel points 

 out, these infusions must be made very dilute, otherwise growths of 

 bacteria, moulds, etc., will completely swamp the diatoms. Karsten 

 (7), in some interesting experiments, showed that Nitzschia imlea 

 (Kutz), W.Sm., could be made to alter completely its mode of nutrition. 

 On placing this diatom in organic nutrient solutions, it lost all chloro- 



