CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 36 1 
hyphae were opaque and the membranes wrinkled. Spore formation had 
been partially suppressed under these conditions. The small increase in 
growth on increasing the acidity indicates that the decreased acidity as a 
result of treatment cannot account entirely for the results observed. 
With respect, therefore, to the marked acceleration of growth observed 
with very high concentrations of magnesium sulphate, I am of the opinion 
that the results observed are due to the presence of minute amounts of 
substances (probably zinc, iron, etc.) as impurities in the magnesium sul- 
phate. 
As I have already had occasion to state in a former publication (65) 
"it is perhaps questionable whether tHus far anyone has grown A. niger 
in the complete absence of zinc." Accidental factors are exceedingly 
difficult to control, e.g., variation in purity of a compound — even of different 
samples from the same bottle; contamination with dust, etc., from the air 
and from the cotton plugs of the flasks, etc. 
The use of the hydrolytic purification method is well adapted for the 
study of the influence of zinc and iron on the growth of A . niger. Further- 
more, by the use of this method I have been able to obtain a culture medium 
with which the non-addition of either iron or zinc markedly decreases the 
acceleration of growth by the other. This relation between iron and zinc 
is brought out in experiments 34 to 37 inclusive. 
34. The effect of the addition of iron and zinc to the treated solution. 
Each value for one culture. 
Strain W, 11 
Treated Pfefifer Solution 
Composition of Solution 
Yield 
Ph 
.0.018 g. 
4 
Control 
.0.013 
4 
+ I mg. FeaCSOOs/flask 
0.044 
2-3 
5 " 
4 
20 " 
0.099 
2-3 
+ I mg. Zn/L 
0.040 
4 
10 " 
.0.045 
4 
50 " 
.0.056 
4 
-t- I mg. Fe2(S04)3/flask + i mg. Zn/L . 
.0.731 
1-2 
I ** " " + 10 mg. Zn/L 
.0.787 
1-2 
The evidence shows that neither ferric sulphate even in 0.4 percent 
concentration nor zinc in a concentration of even 50 mg. Zn/L added alone 
can cause any but very slight increases in growth. If we add, however, 
but 0.002 percent Fe2(S04)3 and i mg. Zn/L together, we obtain the char- 
acteristic phenomenon of ''stimulation." The high yields obtained by the 
combined action of zinc and iron have not been demonstrable by any other 
method of experimentation, neither has the markedly limited growth in the 
absence of zinc or of iron from the culture medium. I have further tested 
the possibility that the results with the treated solution are due to the 
calcium ion and the much lower acidity. The influence of the latter 
