404 K. J. A1;LEN AND E. W. NELSON. 



painting a flask black on the outside up to tlie 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 experiments, with plankton diatoms, satisfac- 

 tory cultures could not be obtained under these conditions. 



Temperature. — The 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, Nitzschia 

 closterium, minute naviculoid diatom, Pleurococcus 

 mucosus, Chile monas 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 whei'e 

 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 abi)ut 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 satisfactory 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 pos- 

 sible constant at 15° C. A continuous record of its temjjerature 

 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 Avhich 

 flasks could be kept at different uniform temperatures from 

 10° to 25° C, by means of hot air, was used, but no really 

 satisfactory result could be obtained. About 17° C. seemed 



