CULTURE OF PLANKTON DIATOM THALASSIOSIRA GRAVIDA CLEVE. 427 



cally no signs of gro\\'tli at all, and remain quite clear. Since the above 

 conclusions were reached it has been my practice in critical experiments 

 always to inoculate from a culture in the artificial medium, so as to reduce 

 the amount of natural water carried over on inoculation to a minimum. 



From what has been said it seems clear that there is in natural sea- 

 water some substance (or substances) not contained in the artificial water 

 treated with Miquel's solutions, minute traces of which are essential to 

 the growth of Thalassiosira. That the quantity present in the culture 

 flasks after the addition of even 4 per cent of natural sea-water must 

 be extremely minute is obvious from the fact that all substuices which are 

 present in natural sea-water in quantities beyond a mere trace are con- 

 tained in the artificial culture medium. It becomes a matter of great 

 interest and perhaps also of great importance to endeavour to find out 

 what this substance may be, of which such exceedingly minute traces 

 make all the difference between practically no growth at all and a vigorous 

 and continued development of the diatoms, for the growths once started 

 may go on increasing rapidly and healthily for several months. 



The addition of many substances, both organic and inorganic, to the 

 artificial miqueled water has been tried, generally in several concentra- 

 tions, but up to the present no definite chemical compound has been 

 found which can take the place of the 1 per cent of natural sea- water. 



Of inorganic substances the following have been tried in different 

 concentrations without result : Potassium bromide, potassium iodide 

 (alone and with bromide), gold chloride, potassium nitrite, aluminium 

 chloride, strontium chloride, lithium chloride and lithium carbonate. 



It may be suggested that silica is the missing substance, but this seems 

 precluded from the fact that all the experiments have been carried out 

 in glass vessels, and the amount of silica which would go into solution 

 from the glass would certainly be greater than that contained in the 

 added 1 per cent of natural sea-water. Richter * has shown that dia- 

 toms grown in glass vessels obtain the sihca they require from the glass. 



In the course of the experiments it was found that the addition to the 

 artificial miqueled water of a small percentage of sea-water from the 

 tanks of the Plymouth Laboratory gave distinctly better cultures than 

 the addition of the same percentage of natural sea-water brought in 

 from outside. This comparison has been repeated a great many times, 

 and the difference has been so marked and constant that I am compelled 

 to regard it as firmly established. Different samples of sea- water brought 



* Richter, 0., Verh. d. Gesell. dent. Xatnrf. u. Ardc., Breslaii, II, 1904, and S.B.K. 

 Akad. Wiss. fVien., CXY, 1906, 



