ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 459 



are killed off. Wliy is it that, although species of the second class are 

 always present in small numbers in plankton taken from the sea, they 

 are there altogether outnumbered by the true Plankton forms, whereas 

 under conditions such as those of our experiments they invariably 

 succeed in gaining the upper hand I What are the factors which 

 determine the difference in beliaviour of these two sets of organisms in 

 the sea and in the culture vessels ? The whole question offers a very 

 fruitful field for further exi)eriment. The evidence at present available 

 is so slight that further discussion of it here is not likely to be of 

 much service. 



The details of two experiments which we have made, bearing on the 

 subject of mixed cultures, may, however, be recorded. 



A flask, containing about 1000 cc. of sea-water treated with Miquel's 

 solutions, was inoculated with approximately equal amounts of certain 

 persistent cultures of diatoms, which we possessed at the time. The 

 following diatoms were in this way introduced : — Cliactocerasconstrictum, 

 Biddulphia mobilicnsis, Skeldoncma codatum, Coscinodiscus cxcentriciis, 

 Strcjototlicca thamcnsis. The liagellate (Chilonionas sp.) was also intro- 

 duced, since it was present in the culture of Coscinodiscus. The experi- 

 ment was started on August 26th, 1907. On September 6th (11 days) 

 Biddnlpkia, Coscinodiscus and Chactoceras were increasing rapidly and 

 were very healthy. Skeletonema. was not so good, and no Strejjtothecct 

 was fo.nid. 



On October 2nd (37 days) Biddulphia was numerous and healthy, 

 Coscinodiscus was healthy but not so numerous. iSlccJctoncma was poor, 

 and Chactoceras was not seen. Flagellates {Chilomonas) had become 

 very numerous. 



On October olst (66 days) all the diatoms were in very poor 

 condition, Coscinodiscus being slightly better than the others. The 

 flagellates (Chi/oinon.as) were extremely thick, giving the water a deep 

 red colour. 



Subsequently a small green alga {Flcurococcus mucosus) appeared, 

 having probably been derived from the Coscinodiscus cnlture. This 

 increased very greatly in quantity, whilst the flagellates became 

 inconspicuous. 



On July 28th, 1909 (1 year 11 months), some Coscinodiscus, which 

 were still in a healthy condition, were seen in a sample examined from 

 the flask. A great (juantity of Pkurococcus, in a healthy condition, 

 was also present, but no other organisms were noted. On this date a 

 subculture was made from the flask in normal outside Miquel. The 

 subculture gave a considerable growth of Slrlctonema, the cells being, 

 however, of a very abnormal cliaracter, and a good many n(jrmal and 



