410 E. .1. aij;kx and e. w. nelson. 



■wliicli they supersede, not merely by robbing them of their 

 food supply, but perhaps, also, l)y tlie production of toxic 

 substances. This suggestion does not, however, give an 

 adequate explanation of the essential facts concerning these 

 organisms. We have to consider two sets of species — (1) the 

 true plankton forms, which flourish in the open sea and can 

 be grown quite easily in the laboratory, provided the cultures 

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

 "bottom forms," which under experimental conditions invari- 

 ably take possession, when present in mixed cultures, whilst 

 the plankton forms are killed off. AVliy is it that, although 

 species of the second class are always present in small 

 numbers in plankton taken from the sea, they are there alto- 

 gether outnumbered by the true plankton forms, whereas 

 under conditions such as those of our experiments they 

 invariably succeed in gaining the upper hand? What are 

 the factors which determine the difference in behaviour of 

 these two sets of organisms in the sea and in the culture 

 vessels ? The whole question offers a very fruitful field for 

 further experiment. 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 c.c. of sea-water treated 

 with Miquel's solutions, Avas 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: CliEetoceras constrictum, Bid- 

 dulphia mobiliensis, Skeletonema costatum, Cos- 

 cinodiscus excentricus, Streptotheca thamensis. 

 The flagellate (Chilomouas sp.) was also introduced, since 

 it was present in the culture of Coscinodiscus. The ex- 

 periment was started on August 26th, 1907. On September 

 6th (11 days) Biddulphia, Coscinodiscus and Chteto- 

 ceras were increasing rapidly and were very healthy. 



