PLUS AND MINUS STRAINS IN THE GENUS GLOMERELLA 25 1 
only the minus strain seemed to be present though the colonies were 
so numerous in these that they could not be told with certainty. 
There was some objection to this method, however, and a slight un- 
certainty because no matter how careful one might be in transferring 
from a single droplet of ascospores, there was a slight chance of other 
loose ascospores being present. 
In order to eliminate all of the uncertainty in regard to the strains 
present in a single perithecium, it was decided to isolate single asci 
from the boundary line of perithecia and let the different ascospores in 
them germinate and see if the different ones from a single ascus would 
develop one or both strains. The isolation of an ascus from a culture 
of this genus is a difficult operation on account of the fact that the 
spores are shed as soon as the ascus is mature. In examining a mount 
made from perithecia, there will be a few scattering asci with mature 
ascospores present, a great many immature asci generally held in 
clumps though often loose, and a multitude of loose ascospores. It 
was found impossible to plate these out dilute enough and then find 
an ascus in the plate. The only way that seemed possible to isolate 
the ascus was by some method to pick it up from the mass of 
spores and transfer it to a marked place in a plate. After many 
unsuccessful attempts this was found to be possible by attaching 
a very fine capillary tube which was sealed at the large end to the 
substage mechanism of the microscope. The capillary tube was 
firmly held and could be moved in any direction by the various 
screws which regulate the position of the condenser of the microscope. 
By means of this capillary tube, the ascus was picked up and then 
transferred to a sterilized cover slip. The small drop of water on the 
cover slip was then examined and if only the ascus was found to be 
present, the cover slip was pushed down into a plate of sterile agar. 
If other spores were transferred with the ascus, the latter was again 
picked up and transferred to another cover slip in the same manner 
or else it was discarded. This was a slow process but it insured the 
isolation of the ascus. After the ascus was placed in the culture 
medium, its position was marked on the bottom of the plate with a 
blue pencil. It was then watched while the spores in it were germinat- 
ing to make absolutely sure that no other spores were present. The 
ascospores would germinate in a few hours and finally they would 
Edgerton, C. W. A Method of Picking up Single Spores. Phytopathology 
4: 115-117- 1914- 
