ARTIFICIAL CULTURE OP MARINE PLANKTON ORGANISMS. 407 



whicli our cultures have been grown, i.e. in glass flasks, 

 the question of silica does not seem to enter into the problems 

 which we have discussed. A few words must, however, be 

 said as to the dissolved gases. Whipple (62) and Baldwin (44) 

 have drawn attention to the observed relations, which are 

 found in natural waters, between algal growths and the 

 amounts of dissolved oxygen and carbonic acid. That these 

 factors are of great importance cannot be doubted, but in our 

 cultures it seems reasonable to suppose that the conditions of 

 saturation of these gases are the same in all, since series of 

 cultures in standard media, such as Miquel sea-water or 

 Berkefeld water, can be set up with the certainty that, if not 

 every one, at least a very high percentage, Avill give normal 

 results. 



Of the purely physical factors, light is by far the most 

 important. Within limits, the rate of growth in a suitable 

 medium seems to depend directly on the intensity of the 

 light (Whipple [60]). Absence of light, as would be 

 expected, soon completely kills the diatoms. 



Temperature also seems to affect the rate of growth to a 

 certain extent, but for those temperatures at which we have 

 experimented it does not appear to alter the quantity of 

 growth. 



Salinity, apart from the quantities of available nutrient 

 materials, can be varied within large limits without appreci- 

 able effect on the diatoms. 



II. Mixed Cultures. 



In what has been said up to the present, we have been 

 dealing with persistent cultures containing a single species of 

 diatom, which are comparatively, if not entirely, free from 

 admixture of other organisms. The study of cultures which 

 contain a considerable mixture of organisms is not without 

 interest. 



A number of experiments have been made on the following 

 lines : About 10,000 c.c. of water, taken at some distance 



