i6o 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
acidity was developed by Penicillium when sugar was added to the decoction, 
but that alkalinity was developed by the same organism when no sugar was 
added. Working with four wood-rotting fungi, Meacham (7) found that 
these organisms grew over a large range of acidity and that a rather high 
concentration of the hydrogen ion is required to inhibit their growth. He 
places the limiting acidity at a pn of about 1.7. His graph shows that in 
proceeding towards a more acid reaction, growth is fairly uniform until a 
Ph of about 2.6 is reached, when growth begins to decrease very rapidly to 
Ph 1.9. It then decreases more gradually to pn 1.7. He states that fre- 
quently there is a maximum of growth which occurs at about pn 3. 
Brightman, Meacham, and Acree (8) in their investigations of "salt 
effects" and phenolsulphonphthalein indicators illustrate graphically some 
data obtained by Meacham on the growth of Endothia parasitica at various 
hydrogen-ion concentrations on bean decoction buffered with dipotassium 
hydrogen phosphate and acetic acid. As drawn, their curve for the initial 
Ph of the medium plotted against growth increases to a maximum at about 
Ph 4.5, falls off to about pH 5.0, and then continues practically horizontally 
to Ph 8.0 where it descends rapidly to pn 8.5. If this curve were redrawn 
through the points as plotted following the points more closely between 
Ph 4-0 and pn 6, it would be seen that the maximum at pn 4.5 is followed by 
a distinct minimum and a second maximum. Similarly, the curve of the 
final Ph of the medium plotted against the growth shows slight evidence of a 
minimum. Attention is drawn to this minimum, apparently overlooked by 
these workers, because of certain data presented in this paper which show 
a minimum in the growth of Gibberella Sauhinetii as plotted against hydro- 
gen-ion concentration. 
Meacham, Hopfield, and Acree (9) grew Endothia parasitica on various 
media in which the hydrogen-ion concentration was regulated by means of 
phosphate-acetate and phosphate-phthalate buffer mixtures. They found 
that the organism grew well at a pn of about 5.7. 
The above named organisms all apparently have a rather wide range of 
acidity at which they will grow. In the case of the potato-scab organisms, 
however, Gillespie (10) found that the growth is inhibited by a relatively 
low acidity. All the strains he used showed inhibition at pn 5.1, and 
usually there was no growth at all at pn 4.8. Waksman (11) found that 
the limiting concentrations for the genus Actinomyces as a whole are about 
Ph 5-0 and pn 9.0, although several species are able to grow at higher hydro- 
gen-ion concentrations (pn 4.6-4.8). The optimum reaction is placed at 
Ph 7-0-7.8. 
Zeller, Schmitz, and Duggar (12) conclude from their work on wood- 
destroying fungi on liquid media that the hydrogen-ion concentration is 
not a limiting factor in the growth of these organisms. Webb (13) presents 
some interesting data on the relation of hydrogen-ion concentration to the 
germination of the spores of certain fungi. He determined the maximum 
