PLUS AND MINUS STRAINS IN THE GENUS GLOMERELLA 249 
all cases on a suitable medium this line of perithecia has developed 
when the plus and minus strains from the same host were used. The 
four cultures from cottonwood (Plate I, fig. 2), okra (Plate I, fig. 3), 
morning glory (Plate II, fig. 2), and beggarweed always worked in 
the same way. 
There seemed to be two possible explanations of the formation of 
the ridge of perithecia on the boundary line between the two strains. 
It was possible that these two were really different sexual strains of 
the same fungus or else that the perithecial development was due to 
some chemical or mechanical stimulus due to the two cultures coming 
in contact. The latter explanation has been offered by Shear and 
Wood^° in their studies on this group. In order to test out this possi- 
bility some cultural work was carried on with closely related forms of 
the genus Glomerella. As is well known, species or races of Gloeo- 
sporium and Colletotrichum are very widespread in nature being 
found on an extremely large number of host plants and many of these 
are very similar and possibly identical. Cultural work shows the 
forms to be extremely variable and it is impossible from a morpho- 
logical study to differentiate between many of them. If the formation 
of a boundary line of perithecia is due to a chemical or mechanical 
stimulus, why will not this line develop when one of the strains is 
grown with other forms? To test this out, the plus and minus strains 
of the cottonwood and morning glory fungi were grown in the same 
plate with non-ascogenous cultures of the Gloeosporiums and Colleto- 
trichums isolated from Mains sylvestris, Gossypium hirsutum, Capsicum 
annuum, Manihot sp., Melilotus indica, Ficus carica, and Hibiscus 
esculentus. These cultures came in contact with both the plus and 
minus strains. The plus and minus strains would develop the ridge 
of perithecia between themselves but in no case would either of the 
strains develop any perithecia in contact with any of the colonies from 
the other hovSts. Finally to test this out further, another culture was 
isolated from leaves of Populus deltoides in 19 12. This proved to be a 
non-ascogenous culture. This was grown in the same plate with the 
ascogenous plus and minus strains from the same host but the line of 
perithecia would not develop between it and either of the strains. 
Having been unable to develop a boundary line of perithecia 
between either the plus or minus strain and any of the other non- 
^° Shear, C. L., and Wood, Anna K. Studies of Fungous Parasites belonging to 
the Genus Glomerella. Bur. PI. Ind. Bui. 252: 74. 1913. 
