ARTIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 40^ 



normal to nearly 50°/o concentration, each containing the 

 same amount of Miquel's nutrient solutions. These were 

 inoculated from a mixed culture of Skeleton ema costa- 

 tum, Biddulphia mobiliensis, and Coscinodiscus 

 excentricus. A good growth took place in all except the 

 two with highest concentration. Of these two, the last 

 remained practically sterile and the growth in the other 

 was very poor. The limit of concentration, therefore, seems 

 to lie between 35 and 40 °/q. In the same way series of 

 lowered salinities were prepared, and cultures of the same 

 diatoms were grown in these. Dilution up to 100 ^/^ did 

 not seem to make any difference at all in the quantity 

 or quality of growth. In a series extending the dilution 

 to 200%, even in the cultures of lowest salinity a fair 

 quantity of growth took place. The range of salinities 

 covered by the various series was S = 12 °/qq to S ^ 60 °/qq, 

 and within these limits no effect on growth could be observed, 

 except in the very highest, where a distinct deterioration 

 was noted. 



An attempt to grow Coscinodiscus excentricus in 

 tap-water + Miquel's solutions was tried, and it was thought 

 that some slight multiplication took place, although it was 

 certainly not at all considerable. Inoculating* a culture of 

 normal Mi quel sea- water from this after six weeks gave no 

 growth. 



Light. — Of all the factors controlling the rate of growth 

 of a culture, light seems to be by far the most important. With- 

 out light a culture soon dies off completely, showing marked 

 signs of malnutrition very soon after having been placed in 

 the dark, the brown pigment being the first to go and later 

 the chlorophyll. A culture (Thalassiosira) placed in the 

 dark for five months was found to' be completely killed, the 

 diatoms being quite colourless. In cultures kept in bulbous 

 flasks or in any spherical vessel, the strongest and earliest 

 growth always takes place at the side of the vessel away from 

 the source of light, where the light will be found to be con- 

 centrated owing to the lens effect of a sphere of water. By 



