CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 367 
fundamental rearrangements, the chemistry of the process as used for the 
removal of iron and zinc is well understood and is utilized as a standard 
procedure for the separation of the trivalent from the bivalent metals of the 
ammonium sulphide group. 
In the light of these experiments, I am therefore inclined to the opinion 
that the increased growth of A. niger which I have obtained in Pfeffer 
solution on addition of zinc salt, and which other workers (see review of the 
literature) have also obtained in this manner, is due not alone to the zinc 
added but to the fact that iron is present, in the same manner that the 
increased growth obtained in the presence of an iron salt is due not alone to 
the iron added but to the fact that zinc is present. 
Lipman and Gericke (34) in a carefully made series of experiments find 
that when a sodium salt in toxic amount is added to the soil in pot experi- 
ments the injurious effects can be eliminated by the addition of copper 
and zinc salts. The thirteen tables of data submitted indicate that, as 
compared with the controls with alkali salt in toxic concentration, the 
presence of copper or zinc salt results as a rule in a greater dry weight of 
straw, grain, and roots. The importance of the practical application of 
these results as a means for the utilization of "alkali" soils is obvious. 
The marked increases in yield obtained by these authors previously (33), 
under the same conditions except that no alkali salts were added, would 
indicate the possibility that in these as in their present investigations the 
acceleration in growth may be due to the action of copper, zinc, etc., 
primarily as "stimulants." That is, an acceleration in growth occurs in 
the presence of both non-toxic and toxic concentrations of "alkali" salts 
when zinc, copper, etc., are added. The effects of CuCOs in the presence 
of toxic concentrations of Na2C03 is, as compared to that obtained with 
NaCl or Na2S04, not marked according to the authors. In cultures of 
Aspergillus niger the effect of the addition of substances causing a decrease 
in acidity is, as shown in experiments 23, 25, 26, etc., markedly to decrease 
the "stimulative" action of zinc. Zinc salts and acidity have a mutual 
effect on growth, reproduction, etc., in cultures of A. niger. Acidity within 
certain limits markedly favors the growth of A. niger. Acidity of the soil is 
supposed to be detrimental to cereal crops, but here again probably only 
when it exceeds certain limits. If, as I believe to be the case, the facts 
determined for the nutrient solution with Aspergillus niger hold also in 
general for the higher plants, then maximum growth will be best assured by 
a relatively (compared to the optimum of the particular plant) lower acidity 
and the presence of heavy metals (iron, zinc, manganese, etc.) in predeter- 
mined amount. The practical application of these facts to diminish the 
action of heavy metal salts in toxic concentrations is well shown, even if 
unconsciously, by the practise of applying a good dressing of lime as an 
effective antidote (60, p. 51) whenever under special conditions infertility 
is traced to any of these metallic salts. The decreased acidity, perhaps 
