ON THE ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 429 



In from three to four months the culture begins to show signs of 

 exhaustion and the frustules lose colour, but they do not, as in the case 

 of sterilized outside and tank water, completely die off. A great 

 number certainly do die, but some remain in a resting condition, and 

 often, after a period of six months or so, these begin to multiply again 

 and the culture regains its former vigour. This is probably due to the 

 food-stuffs contained in the dead frustules going into solution again, 

 possibly by means of bacterial action. This periodicity in cultures is 

 interesting in that it resembles what takes place in the ocean. Cultures 

 in this medium will persist indefinitely, so far as our experience goes. 

 The oldest culture in our possession is one of Skeletoncma costahmi, 

 made at the very commencement of this work, dated April, 1905. 

 Although the frustules in this culture are quite unrecognizable as 

 any diatom now, on making a subculture in fresh Miquel a normal and 

 healthy growth can always be obtained. 



In old cultures the diatoms are nearly always found to be very 

 much deformed, and often appear to be only a mass of broken-down 

 chromatophores. Whether regeneration can be successfully obtained 

 from a single chromatophore, which must presumably be contained 

 within a cell-wall of some kind, has not been definitely decided, but 

 results seem to point in this direction. 



At the start of a culture a tendency to teratological forms is often 

 exhibited, but when the growth is well advanced, the shape of the 

 frustules is usually quite normal. 



(li) English Channel Water {" Outside Water "). — In a large 

 number of our experiments sea-water brought in from outside the 

 Plymouth breakwater, and therefore taken at some distance from the 

 shore, has been used. This is referred to as " outside water." It has 

 an average salinity of about 35"0 °l^^, and the temperature range for the 

 year is from 8° C. to 16° C. 



If a sample of " outside water " is inoculated from a persistent 

 culture of a plankton diatom, a small growth is obtained in from five 

 to fifteen days. But soon minute bottom forms of diatoms, other 

 alga;, flagellates, infusoria, etc., appear, and the inoculated species is 

 lost. The total growth of any form is never large. If the growth of 

 these foreign forms is prevented by sterilizing the water before 

 inoculation, a considerably better growth of the plankton form is 

 obtained. The water was, as a rule, sterilized by simply heating to 

 70° C, which temperature was found to be quite adequate. Boiling 

 gave equally good results, but the former was preferred, as less con- 

 centration due to evaporation took place. Even under these conditions 



