160 
studied these strains, and records the belief that the culture mentioned 
above “‘as a possible mutation . . . . was really the minus strain 
of the bitter-rot fungus.” 
In 1909 I reported with Dr. Hall (Stevens and Hall, 118) for the 
genus Ascochyta variant sectors in Petri-dish cultures quite like those 
reported in this paper; but since the study was not made from single- 
conidium isolations it is possible, though not probable, that I had merely 
a segregation of elementary species. 
Shear and Wood (105) in 1913 reported that in cultures of Glomerella 
started from a single ascospore ‘“‘an important variation or mutation sud- 
denly occurred in the fourth generation and was transmitted through 
three following generations.”’ They cite other variations, less permanent, 
which they regard as fluctuations. 
Burgeff (31) in 1914 reported results from an extensive study of 
asexual variation in the genus Fhycomyces. Working from single-spore 
isolations he got great diversity in many characters. 
Crabill (36) in 1915 described two strains of Coniothyrium pirinum 
which he designated as plus and minus strains that differ markedly in 
several characters, particularly in size and abundance of pycnidia (verg- 
ing on complete sterility), and in color of colony. He says: 
“The cultural studies show that minus strains may arise from plus 
strains by a sudden sporting or mutation. An objection might be raised 
that these cultures were impure, i.e., mixtures of two strains. In antici- 
pation of such an idea it seems desirable to state that freqyuent pourings 
of dilution cultures were used to preclude such a possibility. Progeny 
were then selected only from well-isolated plant, microscopic examination 
of which showed that each was derived from a single spore. : 
Both strains have repeatedly arisen from the progeny of a single pine 
spore. When once purified the minus strain remains constant from gen- 
eration to generation. The variation is apparently occurring in only 
one direction. . . . . The only explanation which remains is that 
the minus strain is a sport or mutant arising from the plus strain at irreg- 
ular and unprognosticable intervals.” 
The plus strain by sudden sporting gives rise to the minus strain, but 
minus strains were not seen to give rise to plus strains, that is to say, the 
saltation is orthogenetic. He states also that the variation apparently 
occurs in the spore and not in the mycelium, which is quite the contrary 
to my findings. He finds, in agreement with my work, also with Kleb’s 
law (74), that the minus strain, that is the sporiferous one, grows faster 
