CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 333 
in 1845. Schulz's (62) studies on yeast demonstrated that the presence of 
such substances may result in increased respiration. The role of zinc in 
the nutrition of A. niger and other fungi was apparently first considered 
as an analogous phenomenon by Pfeffer (52). Richards (57) on the basis 
of a broad comparative study concluded that the action of zinc is shared 
by many other substances, such as iron, lithium, cobalt, nickel, fluorides, 
etc., and even alkaloids; and that zinc therefore cannot be regarded as 
more ''essential" than many other elements. 
Since the initial research of Raulin (55), by which was indicated the 
increased yield resultant upon the addition of zinc to cultures of Aspergillus 
niger, various other (approximately seventeen) ions have been shown to 
act in a similar manner. ''The increased growth," to quote Pfeffer (53, 
p. 146), "appears to be due to a general power of reacting against injurious 
influences possessed by living organisms, for similar results are produced by 
ether, alkaloids, etc., not only upon growth, but also upon respiration and 
fermentative activity." Further: "A very strong poison produces its 
optimal stimulating eft'ect when extremely dilute and growth may be re- 
tarded by doses above the optimum, while substances which act as poisons 
only when highly concentrated produce no perceptible result at all." 
The acceleration of growth of Aspergillus niger may appear in the pres- 
ence of very diverse carbon (57, 56) and nitrogen (57, 22) sources. The 
increase in respiration has been studied by Schulz (62) with yeasts; and 
Richards (58), Ono (49), and others have pointed out that the "economic 
total acidity of the culture solution, it has been found by Currie (11), 
increases more rapidly in the "stimulated" cultures than in the controls. 
This condition is reversed after some time. 
The increased growth involves (Fred, 16) an increased number of cell 
divisions. 
We may next note the evidence from the literature for increased growth 
as dependent on the acidity of the culture medium. Attention has already 
been drawn by Mathews (37) to the fact that the hydrolytic dissociation 
of the salts of the heavy metals is an important factor in their action, as 
shown by their precipitating power. Similarly, Mines (44) has pointed out 
that as a result of hydrolytic dissociation the action of an electrolyte may 
be either increased or decreased. Of interest in this connection are certain 
experiments by Pantanelli (50), who, in his studies on the selective absorp- 
tion of ions from single salt solutions by Cystosira amentacca, found that in 
practically every case the H-ion concentration of the solution increased 
considerably after an immersion of two hours. The action of the salts of 
the heavy metals, which upon hydrolytic dissociation give an increased 
acidity of the solution (see for example Denham, 13), will probably be found 
to have been over- or underestimated on this account. 
is increased in the presence of zinc. The 
