ON THE ARTIFICIAL CULTURE OF JIARINE PLANKTON ORGANISMS. 437 



by side, under exactly identical conditions, as far as can be recognized, 

 quite different degrees of growth will be observed. All experiments 

 must therefore be frequently repeated before entire confidence can be 

 felt in any conclusions which they seem to indicate. 



It must be remembered, also, that in all the persistent cultures of 

 diatoms that we have used, bacteria have probably been present, and 

 this fact has probably had some influence on the result. Unfortu- 

 nately our attempts to obtain absolutely pure cultures have not met 

 with success. 



Methods. In carrying out the experiments to be described in this 

 section the procedure has been as follows : — All media have been pre- 

 pared from sterile sea-water, and sterile vessels and instruments have 

 always been used. The cultures have usually been made in 60 cc. of 

 liquid, in short-necked, wide-mouthed flasks of 125 cc. capacity. When 

 a number of cultures were to be compared, the flasks were kept stand- 

 ing in a row together in such a way as to keep the physical conditions 

 as similar as possible. Control cultures in standard media were in- 

 cluded in each series, so that results from different series could be com- 

 pared by reference to the controls. The various media were inoculated 

 from a persistent culture of a species of plankton diatom, which in the 

 great majority of cases was Thalassiosira decipicns (p. 460). When pre- 

 paring the different media the methods used were, as far as possible, 

 identical, and although only about 60 cc. was needed for a culture, a 

 litre was made up, so that errors due to measuring very minute quanti- 

 ties might be avoided. The media were all freshly prepared for each 

 comparative series of cultures, the same sample of sea-water being used, 

 when the basis of any two or more was the same. Comparative esti- 

 mates of the amount of growth in the different cultures were made by 

 eye. alone. Any difference between amounts of growth that has been 

 described here as appreciable has always been accompanied by a marked 

 difference in appearance to the eye on holding the cultures up to the 

 light. A few drops from each culture were also, from time to time, 

 examined microscopically as a test of the quality and purity of the 

 growth. 



The sea-ivater employed. The sea- water employed as a basis for the 

 culture media has been either (1) " outside water " or (2) " tank-water." 

 A general description of these will be found on pp. 429-431. An 

 accurate chemical analysis of both types of water would probably 

 make clear many difficult points, but, as already pointed out, no 

 chemical methods of sufficient delicacy have yet been devised. 



