i 



however, may be neglected in this case since it was shown by- 

 special tests that variations, even larger than those which 

 actually accurred in the experimental cultures had no ap- 

 preciable influence on the rate of growth of the fungi. 



Two other aerial conditions standing in direct re- 

 lation to the effective conditions within the medium are 

 oxygen concentration and carbon-dioxiue concentration. 

 Tne growing fungus tends to decrease the oxygen content of 

 the medium and to increase the carbon-dioxide content. At 

 the same ti.ue gas exchange between the agar and the air-space 

 above it tends to offset these two effects of the fungus upon 

 the medium, since oxygen would tend to enter the medium from 

 the air space while carbon dioxide would tend to excape from 

 the medium into the air. The air of the culture dish being 

 in direct connection with the external atmosphere it is im- 

 probable that it became markedly different from the latter 

 in regard to these gase3 in the short time employed. It 

 was not, however, experimentally considered in the present 

 work. Whatever variation in the oxygen and carbon-dioxide 

 concentration of the medium that may have occurred in the 

 various cultures, are here neglected as far as the analysis 

 of environmental conditions is concerned. Such variations, 

 if effective, appear in the results as evidence of internal 

 variations in the organism. Looked at in this way, if the 

 growing fungus alters its own chemical surroundings (and it 

 may alter the medium with respect to many other substances 

 besides oxygen and carbon-dioxide) this effect may, for the 



15 



