H. Gr. Thornton 



265 



which developed from a single diluted suspension of Rothamsted soil on 

 media containing 0-05 per cent., 0-1 per cent, and 0-2 per cent, mannitol. 

 Eight parallel platings of each medium were poured and incubated for 

 seven days at 20° C, No significant differences in the number of colonies, 

 on the different media, were found. It was decided to include 0-1 per 

 cent, mannitol in the medium as this gives the maximum energy supply 

 without increasing "spreading." 



Attention was also turned to the effect of phosphate supply on the 

 spreading of B. dendroides. The increase in area of surface growth of this 

 organism was measured on media containing 0-2 per cent., 0-1 per cent., 

 0-05 per cent, and 0-025 per cent. K2HPO4 respectively. Variation of 

 the concentration of K2HPO4 between these limits was found to be 

 without significant influence on the "spreading." 



601- 



0«2%KNO3_ 



Days gi'owth. 

 Fig. 11. B. dendroides. Effect of nitrate content of medium on spreading. 



3. Maintenance of Standard Ee action. 



One of the features necessary in a medium to be used in quantitative 

 work is that its reaction should not vary sufficiently to afiect the colony 

 development. 



The reaction is commonly standardised just before sterilisation. It 

 is during sterilisation that changes occur which are not always constant, 

 so that uniformity in the sterile medium may be lost. In developing a 

 medium, it is therefore important to consider the change in hydrogen 

 ion concentration which will occur during sterihsation. If possible, the 

 medium should be so constituted that this change shall not be sufficient 

 to affect colony development. In this portion of the work, therefore, it 

 was necessary, firstly, to ascertain the limits of change in reaction which 

 might occur in the medium without affecting the number of colonies that 



