454 ON THE AETIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



light seems to be by far the most important. Without 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 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 concentrated owing to the lens 

 effect of a sphere of water. By painting a tlask black on the outside 

 up to the water-line of the medium, a very marked diminution 

 in the rate of growth was obtained. The total growth was not affected, 

 but depends on the available quantity of food-stuffs present. 



Experiments on the reaction of cultures to different rays of the 

 spectrum, obtained by coloured glass, were tried, but no results 

 obtained. Miquel obtained marked results with yellow light, but 

 in our experience, with plankton diatoms, satisfactory cultures could 

 not be obtained under these conditions. 



Tcmjjeraturc. Tlie highest temperature which diatoms and allied 

 forms can stand was about uniform for all the species tested, and 

 lay between 35°-40° C. Cultures of the following species, viz. 

 Asterionella japonica, Nitzschin clostcrium, minute naviculoid diatom, 

 Pleurococcus vuicosus, Chilomonas sp., were slowly heated in a water bath, 

 and at every rise of 5° C. from 15° C. to 45° C, a few drops of the culture 

 were pipetted out and a fresh flask inoculated. In all the flasks 

 cultures were obtained where the heating process had not been 

 carried above 35" C, but none in those where the temperature had 

 exceeded this. 



In the earlier stages of experimentation the cultures of diatoms 

 were kept in various places about the Laboratory, and so were under 

 quite different temperature conditions. Those placed in the warmer 

 situations, i.e. near hot-water pipes, as a rule gave the most satis- 

 factory results. In all the later work the cultures have been kept 

 in one room, and an attempt has been made to keep the temperature 

 of this room as nearly as possible constant at 15° C. A continuous 

 record of its temperature has been kept by means of a recording 

 thermograph, and no very great change of temperature has been 

 noted. In a few isolated cases the temperature has dropped as low 

 as 9° C, and in hot weather has risen just above 20° C, but these have 

 only been for very short periods, the average temperature having kept 

 remarkably constant. An apparatus in which flasks could be kept at 



