CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 35 1 
the modification in acidity of the PfefTer nutrient solution. Figure i, 
including all the zinc concentrations used, is given for its general interest 
in depicting the types of curves obtained. 
1.0 
Fig. I. For explanation see text. 
Figure 2 shows more clearly the portions of these curves concerned 
with sub-optimal values. 
We see that for any particular zinc concentration up to about 0.6 mg. 
Zn/L the amount of growth is least in the presence of NH4OH and greatest 
in the presence of H3PO4. The zinc optimum also, as closely as these 
(.0 
c 
' Oi5 
ll 
Yield 
1 
+ MH40H__^_- ' 
0.Z 0.4- 0.6 
o.a 
1.0 
Fig. 2. For explanation see text. 
values permit of the location in each case, is about 0.06 mg. Zn/L in the 
presence of H3PO4; about 0.08 mg. Zn/L in the Pfeffer solution without 
any addition; while o.i mg. Zn/L may be taken as the optimum in the 
presence of NH4OH. The value 0.08 mg. Zn/L for the nutrient solution 
