Sibyl T, Jewson and F. Tatterspield 235 



This result corresponds with that obtained by Henderson Smith (4) in 

 his work on the toxic action of phenol on Botrytis spores and it seems 

 probable that a similar explanation should be given in this case, i.e. one 

 based upon the variation in resistance of the fungus spores. If it is 

 assumed that this variation is normal and that the spores could be 

 graded according to resistance, then successive grades would contain 

 numbers of spores rising to a maximum in the middle grades and falhng 

 again in the last grades. Thus with each successive dose of Pyridine suc- 

 cessive grades of spores would fail to germinate. It follows that the 

 middle of the curve is steepest, since with these doses the largest numbers 

 of spores are either killed or their growth inhibited. The toxic effect of 

 the addition of small doses of Pyridine is therefore at first slight then 

 rises to a maximum and falls again as higher concentrations are reached^. 



Table X. 



Comparison of Effect of Pyridine and Caustic Soda on 

 growth of Aspergillus at the same pff values. 



^ As the medium used was sUghtly acid (pH 4^45^-75) it might be considered that 

 at the lower concentrations of Pyridine its toxicity might be seriously lessened by partial 

 neutralisation. Mr E. M. Crowther kindly determined for us the effect upon the pK value 

 of our medium of progressively increasing additions of Pyridine and so the amounts of 

 undissociated base present. With an addition of •Ol gm. of Pyridine to 100 c.c. of medium 

 53 per cent, was present as free base, while additions from •! to ^9 gm. to 100 c.c. of medium 

 gave amounts of free undissociated Pyridine ranging from 82-92 per cent, of the amounts 

 added. The effect upon the curve is to displace it slightly to the left, but not fundamentally 

 to alter its character. We are aware of the fact that Aspergillus niger may during growth 

 give rise to notable amounts of acid and that in tracing out an accurate graph of the 

 toxicity of Pyridine to this fungus the amounts of free base before and after the experi- 

 ment should be determined. This, however, was outside the scope of these preliminary 

 experiments, which were intended to ascertain to what extent a common fungus could 

 tolerate this base when added to a synthetic medium. 



