68 DEPARTMENT OF TEE XATAL SERVICE 



that a change in condition induced the profuse development. The same may be said 

 regarding light. Darkness did not prevent, but merely delayed the appearance of 

 sporangia; optimum conditions produced them in small numbers; while excess of 

 light acted as a strong stimulus. It may be inferred that they are a normal means 

 of increasing vitality, which may be stimulated by abnormal conditions. 



IV. Artificial Sea Water. — An artificial sea water based on the analysis of Ditt- 

 mar (14) was employed. Gram molecular solutions of the salts to be used were made 

 up and combined in the following proportions: 480.8 c.c. Na CI, 10.28 c.c. K CI, 

 10.86 c.c. CaCla ; 26.70 c.c. MgCl, ; 29.06 c.c. Mg So^ ; 2 c.c. Na H C03. The total was 

 then diluted with distilled water to a volume of one litre. 



Cultures were set up on September 14. Allen (10) has recorded that for the 

 growth of Thalassiosira gravida, in water of a similar composition the presence of a 

 small quantity, 1 per cent to 4 per cent, of natural sea water is essential. To ascer- 

 tain whether a similar condition was necessary in the case of Melosira hyperhorea, 

 all trace of the natural was removed by passing the material through several changes 

 of artificial before finally transferring it to the prepared flask. Two cultures were 

 started, one in artificial sea water, and one in artificial plus Miquel nutrients in the 

 proportion previously employed. These were examined at intervals, and twice during 

 the winter the medium was renewed. Its concentration was maintained by the addi- 

 tion of distilled water. 



Very fair growth resulted, and though it did not equal in quantity that obtained 

 in natural sea water, the material was uniformly healthy. The growth in untreated 

 water was only 25 per cent of that obtained in the treated, but it also was normal 

 in quality. It is worthy of note that in neither culture did sporangial cells appear. 

 It therefore is concluded that the substance whose presence is essential to the 

 development of Thalassiosira gravida is unnecessary to the gi'owth of Melosira 

 hyperhorea. And this seems to support the conclusion that the exhaustion of the 

 mixed cultures recorded above may be due to the loss of some essential nutrient, 

 which the initial growth of some species exhausts ; while the persistence of Melosira is 

 permitted by its lack of dependence on that substance. 



SUMMARY. 



Melosira hyperhorea can endure a great variety of light conditions, but the 

 optimum developrnent will be obtairved in strong diffuse light. Its growth is regulated 

 to some extent by the solution of gases from the air. It can endure a rajige of forty 

 degrees of temperature, and a diminution to forty per cent of natural sea water. It 

 can even exist for a time in tap water. Miquel solutions act as a stimulus to growth 

 in all cases except when added to tap water; they then rapidly prove fatal. Increased 

 concentration of natural sea water is detrimental. Excellent, persistent cultures 

 may be obtained in artificial sea water. A comparison with the work of Allen on 

 Thalassiosira gravida points to fundamental, specific differences in the nutrient 

 requirements of plankton diatoms. 



