CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 355 
the existence of a distinct action of the "stimulative" ion apart from its 
effecting an increase in acidity. 
Another aspect of the difference in growth acceleration attained by 
increased acidity and by addition of the zinc ion lies in the probability that 
there is a variation in optimum acidity for A. niger at different stages of 
development (9). The growth in the Pfeffer solution (pH = 3-4), I have 
repeatedly noticed, is greater than in the same solution to which phosphoric 
acid is added (0.2 cc.) for the first twenty-four hours. Nevertheless, the 
growth is as one to two at the end of seven days. The initial retardation is 
more than compensated for later. Experiments were therefore performed 
with the idea of ascertaining if a still greater growth acceleration with the 
hydrogen ion could be obtained through the gradual addition of free acid 
from day to day in comparison with that previously obtained by adding 
acid only before inoculation (experiments 28, 29, 30, 31). 
All the culture flasks in experiments 28, 29, 30, and 31 were provided 
with a short glass tube inserted through the cotton plug of the flask so 
that the lower end almost reached to the bottom of the flask, while the 
upper end (plugged with cotton) projected above the cotton plug of the flaks. 
28. The effect of a daily increase in acidity of the culture medium on the growth. 
H3P04/Flask 
Strain W, 11 
Initial 
I Day 
2 Days 
Total 
Yield 
0 CC. 
0 CC. 
0 CC. 
0 CC. 
0.156 g. 
0 
O.I 
0.1 
0.2 
0.460 
0.05 
0.1 
0.1 
0.25 
0.558 
O.I 
C.I 
O.I 
0.3 
0.318 
29. The effect of a daily increase in acidity of the culture medium on the growth. 
Strain W, ii 
H3P04/Flask 
Initial 
First Day 
2 
3 
4 
Total 
Yield 
0 CC. 
0 CC. 
0 cc. 
0 cc. 
0 cc. 
0 CC. 
0.203 g- 
0.2 
0 
0 
0 
0 
0.2 
0.374 
0.3 
0 
0 
0 
0 
0.3 
0.443 
0.4 
0 
0 
0 
0 
0.4 
0.400 
0 
0.05 
O.I 
O.I 
0 
0.25 
0.328 
0 
0.05 
O.I 
O.I 
O.I 
0-35 
0.564 
0.05 
O.I 
O.I 
0 
0 
0.25 
0.398 
0.05 
O.I 
O.I 
O.I 
0 
0.35 
0.371 
O.I 
O.I 
O.I 
0 
0 
0.3 
0.520 
O.I 
O.I 
O.I 
O.I 
0 
0.4 
0.504 
+ 0.15 mg. Zn/L 0.910 g. 
