334 
ROBERT AARON STEINBERG 
Evidence for the action of the hydrogen ion on growth and reproduction 
has been many times presented, and a direct correlation of this effect with 
that of the ions exercising a ''stimulative effect" is perhaps possible. 
Rauhn (55, p. 162) pointed out quite clearly that a definite optimum 
acidity or alkalinity exists for the growth of each organism, and further that 
the optimum reaction varies with the organism. The "Mucedinees" and 
the " Mycodermes," he stated, prefer an acid medium; the "vegetaux- 
ferments" a neutral; and the "Infusoires" an alkaline medium. Raulin 
himself utilized tartaric acid in his nutrient solution in order to favor the 
growth of Aspergillus niger and to exclude " Infusoires." With high con- 
centrations he reports a retardation or suppression of growth and spore 
formation. 
It has come to be a commonplace that fungi as a rule prefer an acid, 
and bacteria an alkaline, medium. This is the basis of the conception of 
many of the early students of plant diseases that only fungi and not bacteria 
could cause plant diseases; the bacterial diseases being confined to animal 
organisms whose protoplasm is alkaline (see Smith, 63). Similar statements 
with regard to the acidity requirements of fungi and bacteria have been 
repeated with great frequency, as for example by v. Nageli (46, p. 63), 
Pfeffer (53, p. 384), and Jost (23, p. 205), without as a rule any definite 
statements as to the actual effects of acidity on the growth of any one 
organism. 
As respects the algae, we may note that Migula (43) observed that 
very slight increases in acidity cause an acceleration of the growth in length. 
Cell division, however, seems not to have been accelerated. 
For the higher plants as grown in water culture, A. Mayer (39, p. 273), 
Pfeffer (53, p. 415), Jost (23, p. 92), and even Russell (60, p. 49) in 1917, 
refer to the effects of acidity only vaguely. 
Within comparatively recent times quantitative experimental evidence 
on the relation of acidity to growth has begun to accumulate. The dis- 
covery of Kahlenberg and True (24) that the toxicity of acids depends on 
their ionization {i.e., the resulting H-ion concentration) has been of great 
importance and significance. We need only consult Michaelis' (41) sum- 
mary of the literature to realize the fundamental importance of the H-ion 
concentration for enzymatic action, etc. 
Clark (9) states that for a number of acids the growth of A . niger varies 
with the acidity: . . 3^ to J/g the lethal concentration has a strong 
stimulating influence on the mycelial development, and tends to suppress 
or at least retard fructification" (p. 294). Unfortunately no yields are 
given, and the acidities are calculated from the ionization of the acids in 
water though added in this case to a sugar-beet decoction. Fred and Loomis 
(17) have studied the influence of the H-ion concentration of the medium 
on the reproduction of alfalfa bacteria. The optimum acidity they note 
corresponds to pH = 7.2; at pn = 3.10 no growth occurs, [pn = — logio 
