364 
ROBERT AARON STEINBERG 
iron and the zinc salt used in these experiments are of a purity sufficient 
for analytical work but hardly come up to the required standard as respects 
physiological work of this character. Enough iron is still probably present 
in the treated solution together with that added with the zinc salt and in 
the spores to enable a limited (0.654 g.), but still marked, increase in growth 
on addition of zinc to take place. Aso (i) has reported that in the case of 
Aspergillus Oryzae the spores contain appreciable amounts of Fe203 (0.14 
percent). With a similar iron content in the spores of A. niger it is quite 
evident that enough iron might be added to influence the results in view of 
the heavy inoculations necessar}/ in these experiments for uniform results.^ 
The increased acidity, since its action is to aid that of the zinc, makes its 
effect evident in the concentrations employed. This effect of increased 
acidity of the nutrient solution has already been noted in connection with 
the increased effectiveness of zinc in the untreated Pfeffer solution as well 
as in connection with the increased effectiveness both of iron and of zinc 
in the treated Pfeffer solution. This relation between acidity and the 
action of heavy metal appears to be of general significance, since, as already 
noted, it seems to hold also for the higher plants. 
Inspection of the values obtained in the presence of calcium ion, in the 
Pfeffer nutrient solution of about the same acidity as the treated solution, 
shows that the influence of these two factors on the results obtained in the 
treated Pfeffer solution can be disregarded. Even in the presence of much 
more calcium than is present in the treated solution as well as of a decreased 
acidity corresponding approximately to the treated solution (slightly acid 
to litmus), the customary accelerations of growth with small amounts of 
zinc are obtained. 
The next experiment (experiment 36) was planned to test the value to 
be placed on the accelerations of growth obtained in the presence of iron 
and zinc salt alone. In this experiment two Kahlbaum "Zur anal." 
compounds [FeP04 and Fe3(P04)2] were used, the phosphates being selected 
for the very simple reason that neither the nitrates nor sulphates in a 
sufficient degree of purity were available. The relative insolubility of these 
salts is hardly a factor, since the addition of iron in soluble form results in 
the precipitation of the phosphates in the nutrient solutions.^ 
The spores were brown in many of the zinc-plus-iron cultures. Accom- 
panying the brown spores, many papilla-like protuberances on the upper 
surfaces of the membranes were visible. These papillae have never been 
observed in zinc-free cultures, and in zinc cultures containing sufficient 
iron have been observed only at irregular intervals. Both the iron salts 
are the Kahlbaum "Zur. anal."; the zinc salt, the Baker's "Analysed." 
2 Approximately 4 mg. dry weight of spores, etc., per flask. 
3 I have estimated that the amount of material (dry-weight) added at the time of 
inoculation is on the average approximately 3-4 mg. to each flask. The actual amount of 
course probably differed widely from flask to flask. 
