CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 349 
The addition of acid to the Pfefifer nutrient solution resulted (experi- 
ments 19 to 24 inclusive) in an acceleration of growth. The maximum 
acceleration of growth, no matter which of the acids was used, was but 
slightly more than double that of the control. The effect of the addition 
of alkali to the Pfeffer solution was a diminution of the yield. Further- 
more, the effect of the increased acidities of the cultures was to cause a 
retardation or suppression of spore formation as distinguished from the 
effects of decreased acidity which did not cause the exhibition of the same 
phenomenon. 
All seven acids used in the above described experiments gave results 
essentially similar; an indication that these results are due to the hydrogen 
ion primarily and not to the anions. The acceleration of growth due to 
tartaric acid, however, needs to be verified, since I am inclined to attribute 
its action, at least in the sample used, to the presence of impurities. As a 
result of increased acidity, it will be noted, the yield can be doubled; while 
a decrease in the acidity of the Pfeffer solution acts in an opposite manner. 
My experiments serve, therefore, to confirm the long recognized fact that 
a fluctuation in the acidity brings about a corresponding variation in the 
yields; and moreover, that the acidity of the Pfeffer solution is sub-optimal 
for the growth of A. niger. 
I have studied the appearance of the mycelia and of the modification 
of sporulation in cultures on Pfeffer solution of varying acidity. In all 
respects the agreement between the effects of increased acidity and the 
effects of ions like zinc and iron appears to be complete. The effects of 
decreased acidity and of increased acidity of the Pfeffer nutrient solution 
are distinct and opposite. Excessive acidity or alkalinity may, it is true, 
entirely prevent growth. Within these limits, however, an acidity lower 
than that of the Pfeffer solution (pn = 3-4) results in a decreased yield, 
the hyphae are dark and watery in appearance — translucent — and the 
thin, flat membranes are black with spores. , Increased acidity causes in- 
creased growth, the hyphae are white or light tan in color and heavy- 
walled — opaque — and the thickened and wrinkled membranes may show a 
retardation of spore formation. 
The appearance and yields of the cultures in Pfeffer solution of higher 
acidity (greater than pH = 3-4) are entirely similar to those containing 
zinc and iron. Cultures of lower acidity than that of the Pfeffer solution 
are similar to those from which zinc, etc., ions have been excluded. More- 
over, a regular transition in the appearance and yields of the cultures is 
apparent from those of maximum alkalinity to those of maximum acidity. 
The phenomena of chemical "stimulation" of A. niger: (a) increased 
growth, (b) increased respiration, (c) increased fermentative activity, (d) 
modification of the character of the cell walls, and (e) retardation or sup- 
pression of sporulation, are quite the same as those associated with the 
action of the hydrogen ion on the fungus. 
