288 Marine Microbiology 



the end of a run were consequently higher than at the start. 



When Z. eistla had been shown to have a Plwma type re- 

 sponse to varied temperature and sahnity, other substances were 

 substituted for the constituents of sea water in order to determine 

 whether control of the growth rate at a given temperature is a 

 function of salinity specifically or of osmotic pressure in general. 



Sodium chloride was chosen for one series of experiments. 

 Media were prepared containing 0.17 M, 0.48 M, 0.58 M, 0.95 M, 

 1.38 M, and 1.74 M NaCl. These concentrations provide an 

 amount of salt, as NaCl, equal to the total amount of mixed 

 salts present in the sea water experiment. Other salts, yeast 

 extract, and glucose were added in the same amounts as in the 

 basic medium while in other experiments, other compounds re- 

 placed NaCl. In one series we used glucose as an example of 

 a non-ionized, metabolically active compound capable of pro- 

 viding an osmotic gradient. 



Glycerol, when incorporated in the medium, produced no 

 better growth than controls containing 0.1 per cent yeast extract 

 as the only other carbon source. It was consequently used in one 

 experiment as an example of a non-ionic, non-metabolizable, 

 osmotically active compound. Media were prepared with gly- 

 cerol in the same molar concentrations as NaCl in the experiment 

 described above, with glucose, yeast extract, and salts, solidified 

 with 0.85 per cent Oxoid lonagar. In the last three experiments 

 described here, cultures were incubated at the full range of 

 temperatures. 



An incomplete experiment was performed using pentaery- 

 thritol as a non-ionic, non-metabolizable, osmotically active 

 compound to which cell membranes are impermeable ( 1 ) . It 

 was added to the usual glucose-yeast-extract medium in con- 

 centrations of 0.07 M, 0.18 M, 0.25 M, and 0.40 M. Pentaery- 

 thritol, being insoluble in water beyond 0.40 M, did not allow a 

 complete series for comparison. 



Growth was plotted against osmotic pressures in atmospheres. 

 Figures for the osmotic pressures of sea water are from Pearse 

 and Gunter (5); glucose equivalents are from Morse et al. (4). 

 Experimental values for glycerol and erythritol were not avail- 

 able, but as both are non-electrolytes, we worked on the assump- 



