256 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Voi.. 10 



in addition they represent an extremely strong buffer. Thus if 

 the optimum for a fungus is P H 4.5 and it is inoculated into a 

 medium adjusted to Pu 3.0 the amount of buffering in the solution 

 is likely to be a factor in the end result. On the other hand, if 

 the H-ion concentration of the solution into which the fungus 

 is inoculated is near the optimum of that for the fungus, then the 

 highly buffered solution will maintain an optimum condition 

 longer against the production of acid or alkali than will the un- 

 buffered solution. In the case of organism No. 4 at P H 3.0, the 

 growth became less as the amount of citrate radical was increased, 

 when incubated at 20° C, but at 30° C. the growth increased up 

 to the 2Yl per cent concentration, the fungus not growing at all 

 in the 5 per cent concentration. That this was most likely a 

 case of buffer action maintaining an unfavorable reaction is 

 indicated by the final hydrogen-ion concentrations. At 30° C. 

 this concentration varied from P H 7.5 to P H 3.0. At the former 

 concentration the fungus grew, while at the latter it had not 

 grown and the P H was unchanged. That the amount of citrate 

 radical is not in itself the factor that decides whether the fungus 

 will or will not grow is shown at P H 4.5 where the growth increased 

 progressively with the amount of the citrate radical added. The 

 results for several of the fungi indicated clearly that the ability 

 of a fungus to grow at a certain P H is not based entirely on the 

 hydrogen-ion concentration and nutrients but that temperature 

 and other environmental conditions are vital factors to be con- 

 sidered as well as the case with which the P H of the solution can 

 be shifted. Organism No. 2 grew well in all of the solutions at 

 20° C. and about equally well at P H 3.0 and 4.5, but at 30° C. it 

 grew very little at P H 3.0 and well at 4.5. Organisms 4 and 9 

 were the only ones that grew satisfactorily at 30° C. 



The use of free citric acid. — Further data on the ability of the 

 various fungi to utilize free citric acid were obtained by using a 

 fermentative method. The fungi were grown in 300-cc. flasks 

 until they had formed a substantial mat but had not yet reached 

 the peak of growth. The solutions to be tried— a sugar solution 



and a citric acid 



of mineral 



nutrients, were sterilized in 50-cc. amounts in 120-cc. flasks. 

 After the mat was formed the solution was poured off under aseptic 



