350 
ROBERT AARON STEINBERG 
Assuming that the H-ion concentration is a source of "stimulation," 
it should of necessity follow that a modification of the hydrogen-ion con- 
centration of the Pfeffer nutrient solution would result in the modification 
.» 
of the optimum concentration of other "stimulants" for growth, and in 
such a sense that increased acidity brings about a decrease in the optimum 
of the "stimulant" and a decrease in the acidity an increase in the optimum 
of the "stimulant." Some evidence bearing on this question is furnished 
in the experiments of Richards (57), who has already pointed out that in 
the presence of the optimum iron concentration, the addition of zinc 
causes no further increase in growth. In the case of manganese and zinc, 
Bertrand and Javillier (5) found that even a further increase in yield took 
place in optimum concentration of either if the other was added. This 
modification of the optimum is easily demonstrated, as reference to the 
following experiments (25, 26) will show. See also experiment 23. 
25. The effect of an increase or of a decrease in acidity of the culture medium on the growth 
accelerations obtained by addition of zinc. 
strain W, 8 
+ 0.2 CC. NH4OH 
+ 0.2 CC. H3PO4 
0.136 g. 
0.015 g. 
0.329 g. 
0.200 
0.020 
0.313 
0.139 
0.025 
0.334 
0.135 
0.018 
0.318 
0.095 
0.024 
0.299 
0.I4I 
0.020 
0.319 
Mg. Zn/L 
-f 0.2 Cc. NH4OH 
+ 0.2 Cc. H3PO4 
0.025 g. 
0.358 g. 
0.060 g. 
0.614 g- 
0.05 
0.519 
O.I 1 1 
0.787 
O.I 
0.820 
0.241 
0.879 
I.O 
0.929 
0.651 
0.903 
10. 0 
0.945 
0.661 
0.866 
25.0 
0.950 
0.767 
0.866 
50.0 
0.957 
0.704 
0.852 
100.0 
0.972 
0.598 
0.605 
Acidities of the Culture Nutrient Solutions at Time 
OF HAR^■EST 
Mg. Zn/L 
+ 4 Cc. NH4OH/L 
+ 4 Cc. H3PO4/L 
. Ph = 3-4 
Ph - 3-4 
Ph = 2-3 
0.025 
^3 
^3 
2-3 
0.05 
2-3 
^3 
2-3 
O.I 
1-2 
2-3 
2 
1.0 
1-2 
2-3 
1-2 
lO.O 
1-2 
2-3 
1-2 
25.0 
1-2 
2-3 
1-2 
50.0 
1-2 
^3 
1-2 
100.0 
1-2 
= 3 
1-2 
The curves obtained by plotting yield against zinc concentration make 
somewhat more evident the variation of the zinc optimum as a result of 
