DIMORPHISM IN CONIOTHYRIUM PIRINUM SHELDON 459 
of the two strains. Since C. pirinum reproduces asexually segre- 
gation from heterozygous parents cannot explain the origin of the 
two strains. The only explanation which remains is that the minus 
strain is a sport or mutant arising from the plus strain at irregular 
and unprognosticable intervals. What makes it arise and what are 
the controlling factors of such mutation are worthy of speculation and 
experiment. 
The writer^ has reported a somewhat similar mutation in a fungus 
belonging apparently t the genuso Phyllosticta. 
3 and III — 3, 6 days old. 
Development of Plus and Minus Sectors 
A single isolated plant of Coniothyrium pirinum like most fungi 
grows radially. A germinating spore may produce one or two hyphae 
which, by a continuous dendritic branching, may give rise to large 
numbers of branches all of which however are nearly equal in diameter 
for their entire lengths and many of which end at the growing margin. 
Each hypha with its many branches then covers an area shaped like 
the sector of a circle. Therefore if something should happen to a 
hypha which would change its growth characters, its branches would 
^ Crabill, C. H. A mutation in Phyllosticta. Phytopathology 4: 396. 1914. 
