H. G. Thornton 



261 



acid. This explains the fact, observed by Conn (3), that on Lipman and 

 Brown's peptone agar, "overgrowths are often so abundant... as to inter- 

 fere with counting and prevent the isolation of pure cultures from the 

 colonies," and also that these overgrowths are reduced on his medium 

 containing ammonium phosphate and sodium asparaginate. 



On the tyrosine medium scarcely any growth took place. It appears 

 that this is due, not to any inhibiting action of the tyrosine, but to the 

 inability of the organism to make full use of the tyrosine molecule in its 

 nutrition. This is shown in the following experiment, in which the 



80 



70 



60 



^ 50 



a 40 



30 



20 



10 - 



Tyrosine + Asparagine 

 Asparagine 



• Tyrosine 



2 3 4 5 6 

 Days growth. 



Fig. 8. B. dendroides. Effect on tyi'osine and asparagine on the spreading growth. 



growth was compared on media similar to those tested in the last 

 experiment but having organic nitrogen supplied as follows: 



A. 0-12 per cent, tyrosine. 



B. 0-12 per cent, tyrosine + 0-05 per cent, asparagine. 



C. 0-05 per cent, asparagine. 



The test was conducted in a manner similar to the last experiment 

 except that the plates were incubated at 25° C. to accelerate the growth. 



The curves (Fig. 8) show the mean areas of growth, measured at 

 intervals, of ten parallel platings in the case of media B and C and of 

 seven parallel platings in the case of medium A. It will be seen that in 

 the presence of asparagine, tyrosine does not check but slightly stimu- 

 lates the growth, whereas with tyrosine alone the growth is very slight. 

 Tests made with the tyrosine medium, however, showed that it was 



