458 ON THE AKTIFICIAL CULTURE OF MAEINE PLANK'ION ORGANISMS. 



kinds were to be found amongst the precipitate at the Lottoni of the 

 jar, AsterioneUa japonica and Coscinodisciis excentricvs Leing often 

 numerous here. During tlie course of the next week, however, 

 Nitzschia clostcrivm rapidly increased in quantity until, not only the 

 sides of the jar were coated with it, but tlie whole mass of the water 

 became thick and opaque. By this time the plankton diatoms had all 

 disappeared, with the exception of those which may survive for a con- 

 siderable period amongst the precipitate at tlie bottom of the jar. 

 Bottom diatoms (JVaviaila, etc.) had begun to grow on the sides of tlie 

 jar, and small green and brown algcC {Plcurococcus mucosus, Ectocarinii^, 

 etc.) also appeared. Infusoria (Uuplotcs and other smaller forms) then 

 became numerous, and as the Nitzschia and bottom diatoms increased 

 on the glass, large numbers of Araoebcc made their appearance among 

 them. The jars continued in this condition for many months, the alga^ 

 becoming more and more predominant. 



From these experiments, as well as from instances of mixed cultures 

 detained in the courser of our attempts to secure persistent cultures of 

 single species of diatoms, it seems usual that, in a culture obtained by 

 inoculating Miquel sea-water with plankton taken freshly from tlie 

 sea, the true plankton diatoms are the first to develop in considerable 

 numbers. Subsequently bottom diatoms and algte of various kinds 

 become abundant, and the true plankton forms die out. 



A complete explanation of this sequence of events would probably 

 be of a very complicated character, and we have practically no evidence 

 from our experiments which bears very directly on the question. It 

 would seem, however, that the early predominance of the plankton 

 forms in the cultures would naturally follow from the fact that, in the 

 plankton material used for inoculation, these plankton forms are 

 numerous, whilst bottom diatoms and spores of algte are rare. The 

 subsequent very great predominance of such a species as Nitzschia clos- 

 teriu/ii may be due simply to a very much more rapid growth rate, 

 though it is difficult to avoid the impression that the organisms which 

 finally take possession of the cultures, are in some way directly inimical 

 to those which they supersede, not merely by robbing them of their 

 food-supply, but perhaps, also, by the production of toxic substances. 

 This suggestion does not, however, give an adequate explanation of the 

 essential facts concernins; these oro'anisms. We have to consider two 

 sets of species: (1) the tiue Plankton form-, which flourish in the open 

 sea and can be grown quite easily in the laboratory, ])rovided the 

 cultures remain pure ; and (2) what we may call " aquarium " or 

 " bottom forms," which under experimental conditions invariably take 

 possession, when present in mixed cultures, whilst the plankton forms 



