8 TECHNICAL BULLETIN 17 



measuring 60 by 20 microns, and containing from 4 to 7 septa. Both 

 of the latter are described as elliptical in shape. 



Stevens (12) makes the following statement regarding the form 

 causing the foot-rot of wheat: "The spores, observed as grown on 

 autoclaved wheat leaves or stems in humid air, are from 24 to 122 

 microns long, the majority of them falling within the limits 80 to 90 

 microns, with septa or pseudo-septa varying from o to 13, usually 

 5 to 10. The spores are usually typically thickest in the region about 

 midway between the base and the middle point of the spore, approach- 

 ing a narrow or broadly elliptical shape, tapering somewhat toward 

 each end. They possess an outer dark wall that is thin and extremely 

 fragile and an inner, colorless, thick wall that is frequently soft and 

 gelatinous . . . The spores usually, perhaps always, germinate 

 either from one or both ends, not laterally, and are functionally only 

 one-celled." 



After making a large number of isolations from Helminthosporium 

 lesions on barley, wheat, and rye, great variations were found in the 

 size of the spores of the various cultures, altho they resembled each 

 other more or less in shape and color. In order to find out just what 

 variations might be expected in one strain, as a guide to the interpreta- 

 tion of the species, a single spore was again isolated from culture 82a 

 and a biometric study was made of the spores produced under various 

 conditions. 



The single spore was planted on a potato dextrose agar slant and 

 incubated at 24 C. for ten days. Transfers were then made to potato 

 dextrose agar and to ripe autoclaved barley heads. Agar cultures were 

 incubated at the following temperatures: 5, 14, 18, 24, 28, 32, 

 and 36 C. The cultures grown at 5 and 36 did not produce spores. 

 The barley head cultures were incubated at 24 C. Fresh barley 

 leaves were taken from the greenhouse, placed in moist chambers, 

 inoculated with spores of the same culture, and incubated at 24. The 

 length of time required for the cultures to sporulate at the different 

 temperatures varied considerably; those at 24, 28, and 32 were 

 ready for measurement in 16 days, while those at 14 required 37 days. 



