DIMORPHISM IN CONIOTHYRIUM PIRINUM SHELDON 457 
be raised that these cultures were impure, i. e., mixtures of two strains. 
In anticipation of such an idea it seems desirable to state that frequent 
pourings of dilution cultures were used to preclude such a possibility. 
Progeny were then selected only from well-isolated plants, micro- 
scopic examination of which showed that each was derived from a 
single spore. By consulting the chart, the frequency of these pourings 
may be considered. In each instance the poured plate is preceded 
by the symbol =. It will be evident from this that the possibility 
of either of the strains being constantly a mixture is eliminated. 
Both strains have repeatedly arisen from the progeny of a single 
Fig. 8. Ill + and III — , 8 days old. These plates were inoculated from the 
plus and minus sectors respectively of the colonies shown in Fig. 7. 
plus spore. When once purified the minus strains remain constant 
. from generation to generation. The variation apparently is occurring 
in only one direction. 
Attempts to determine the factor which disturbs the stability 
of the plus strains and causes the liberation of new minus strains 
have proved fruitless up to the present time. 
Attempts have been made to develop a minus strain from a plus 
strain by artificial selection alone. The following methods have 
been employed: 
