CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 339 
paper referred to above (65). We may note that both the zinc optimum and 
the maximum yield of the two strains differed (experiments 4, 5, and 6). 
The above noted cultures indicate also the nature of the results obtained 
by the addition of zinc sulphate to the Pfeffer nutrient solution. We note 
that with increasing concentrations of zinc the yield increased and spore 
formation decreased. The maximum acceleration of growth occurred in 
the case of both strains in a zinc concentration of o.i mg. Zn/L or less, 
higher concentrations of zinc still resulting in the maximum growth or in 
a slight decrease. The optimum concentration of zinc for growth as well 
as the maximum growth obtained is greater for the W than for the Y strain. 
Two explanations have been suggested for the method of action of these 
ions on growth. Javillier (22) is inclined to believe that the increased 
growth on addition of zinc is due essentially to the increased elaboration of 
an enzyme — sucrase. The evidence offered does not, however, settle the 
question as to whether the increased production of sucrase by the organism 
is the result or the cause of the accelerated growth. 
Further there is the suggestion of Loeb (35) that the phenomenon de- 
pends on the balancing of the solution. Of this, however, I could find no 
evidence in my studies. Variation of the concentration and proportions 
of the constituents of the Pfeffer solution (except iron sulphate) in no case 
led to the exhibition of phenomena at all comparable to those accompanying 
the action of zinc. That is, no indication could be obtained that either 
ammonium, potassium, hydrogen, or magnesium ions were present in 
toxic concentration. Experiments 7 to 16 inclusive will make this clear. 
The Pfeffer nutrient solution contains NH4NO3, KH2PO4, and MgS04. 
7H2O in amounts of 10 .-5 : 2.5 grams respectively per liter. The composi- 
tions of the following solutions have been indicated in the same manner. 
7. The effect of a decrease in concentration of the magnesium sulphate. 
4:2:1* 
4:2: 0.2 
4:2: 0.04 
4:2: 0.008 
0-139 g- 
0.127 g. 
0.093 g. 
0.038 g. 
0.142 
0.118 
0.074 
0.035 
0.151 
O.IIO 
0.083 
0.035 
0.131 
0.II3 
0.092 
0.032 
0.169 
0.II5 
0.091 
0.036 
0.146 
0.II7 
0.087 
0.035 
NH4NO3 KH2PO4 MgS04-7H20 Sporulation 
4:2 : I Very good. 
4:2 : 0.2 Good. 
4:2 : 0.04 Fair. 
4:2 : 0.008 Very sparse, very delicate mycelium. 
* MgS04.7H20 (Kahlbaum). 
