332 
ROBERT AARON STEINBERG 
The necessity of iron for the fungus has been a much disputed question. 
That iron is an "essential" participant in the metabohsm of the fungus 
is the opinion of Mohsch (45). Benecke's (3) experimental results, as he 
himself points out, are inconclusive. More recently Currie (11) has con- 
cluded that the exclusion of iron from the nutrient solution is without 
effect on the growth and reproduction of A. niger. 
Von Nageli (46) concluded that certain substitutions are possible. 
Thus, he thought, rubidium or caesium could be used in place of potassium; 
and calcium, barium, or strontium in place of magnesium. That Nageli 
was in error has been shown by Benecke (3), inasmuch as in the entire 
absence of either potassium or magnesium he could obtain no growth. 
Benecke found, however, that partial replacement was possible: greater 
growth took place but the cultures did not fruit. This non-sporulation is, 
at least in the case of rubidium and caesium, not to be confused with an 
analogous phenomenon accompanying ''stimulation" (see below) however, 
but is simply due according to Sauton (61) to insufficient potassium and 
not to a ''sterilizing action" of the caesium and rubidium. 
The nutrient solution utilized by Pfeffer (52), which is quite similar to 
that worked out by Mayer, is being increasingly made use of in nutritional 
studies. It is perhaps the simplest of the nutrient solutions in use. 
As respects the action of the components of the nutrient solution, it 
was early partially recognized by v. Nageli (46, 47) that the action of a 
compound on the growth of an organism depends on and increases with its 
concentration, and that the presence of other compounds can cause a 
modification of this action. Each compound, according to him, exercises 
a "specific action" which is apart from and without reference to its nutritive 
value. Every compound has an "optimum" concentration for growth; 
and a "schadliche" (toxic) concentration. The latter is higher in value 
than the "optimum." Both the "optimum" and the "toxic" concentra- 
tion vary with the compound and with the particular organism used. In 
other words, the "optimum" concentration of any compound in a nutrient 
solution varies with the composition and concentration of each of the other 
compounds present and with the organism. 
Much evidence has accumulated showing the specific effect of "poisons," 
"stimulants," etc., on the growth of organisms. That in sufficiently high 
dilution many "poisons" are not harmful, but may cause an acceleration 
of the life activities of the organism was apparently known to Braconnot (6) 
Pfeffer Solution 
Water 
Cane sugar 
Ammonium nitrate 
Monopotassium phosphate 
Magnesium sulphate 
Ferrous sulphate 
1000.0 g. 
50.0 g. 
lo.o g. 
5-0 g. 
2.5 g. 
trace 
