Oct. 2, I916 
Aspergillus niger Group 
5 
stock culture (No. m). This strain (No. 3534-a) is, however, the lowest 
oxalic-acid producer of all the strains closely related morphologically to 
the typical A . niger . These three forms, therefore, known to be closely 
related to each other and probably also to the Amsterdam culture 
(No. in), differ markedly enough in this one reaction to suggest that the 
mutation which occurred was probably a quantitative readjustment 
among the enzyms. 
In both tables some cultures show much more free acid, as indicated 
by titration, than is shown by the determination of oxalate. This 
difference, as redetermined for certain of these forms in the second experi¬ 
ment, is maintained in approximately the same relative proportion in 
Table II. It has held true also in experiments not included in this paper. 
No. 142, hi, 4020.33, 2657, 3547, 3546, 2580, 3534 -a, 3534--C, and 
3522.30 show slight differences between these determinations at 7 and 
10 days. The other forms show much greater differences. As all the 
cultures become older, the oxalate determination equals, and even be¬ 
comes greater than, the free-acid figure. Unfinished work to be reported 
later shows that citric acid forms part at least of the excess transiently 
found in the determination of free acid. Comparative study of the 
colonies themselves does not correlate these differences in acid production 
with morphology. The difference between the strains used appears, 
therefore, to be one of rate and quantity of reaction rather than a differ¬ 
ence in kind of activity. 
Although most of these forms had previously been studed carefully, a 
microscopic examination of each strain was made, in order to seek in 
morphology a possible basis for separation. The range of morphological 
characters found points to the existence of a series of closely related 
strains in which the differences are in measurement of parts, intensities 
of color, and quantitative differences in the production of particular 
reactions. 
The members of this group grow under a wide range of cultural condi¬ 
tions. When colonies of a particular strain are grown simultaneously 
upon substrata of markedly different composition, distinct differences 
appear. All these strains, however, when grown under the same condi¬ 
tions, have so many common characters and so many intergradations 
that a group characterization upon the lines recently used by one of the 
writers for Penicillium spp. 1 will be more useful than any attempt to 
describe strains separately. This work harmonizes with the conclusion 
of Schiemann 2 that A. niger as commonly understood is an unstable or 
mutating group comparable to Oenothera spp. 
1 Thom, Charles. The Penicillium luteum-purpurogenum group. In Mycologia, v. 7, no. 3, p. 134-142, 
1 fig. 1915. 
2 Schiemann, Elisabeth. Op. cit. 
