16 USTILAGO AVENAE AND USTILAGO LEVIS 



caryopsis which would account for 60.72 per cent of the infected plants, 

 are not necessarily restricted to the stigma end. 



These preliminary studies gave data which indicate only the position 

 of the pathogene organs which are accountable for seedling invasion. 

 In order to determine ( 1 ) the nature of these structures — that is, whether 

 they are mycelium or spores — and (2) whether they are external 

 to the caryopsis or within its tissues, more exacting experiments were 

 undertaken. 



Nature and relative importance of the various pathogene structures in 

 the oat flower and grain 



Scour ces of, and methods of selecting, experimental materials 



Ustilago avenae. — As has already been stated, the writer considered 

 it absolutely necessary to have seed of known history to work with. 

 While the foregoing preliminary studies were being carried out, such 

 seed was obtained in the following manner : 



Supplies of Swedish Select, a glumed variety of oats, and Selection 

 202, a glumeless variety, were obtained from the Department of Plant 

 Breeding at Cornell University. The Swedish Select oats were known 

 to have come from an infested field. The lot of the glumed variety was 

 divided into two parts, one of which was untreated and sown in plots at 

 East Ithaca in order to guarantee a supply of spores for inoculation 

 purposes. The other part, and the supply of the glumeless variety, were 

 treated with formalin and sown in plots near Forest Home, which is 

 about a mile from East Ithaca. These were to be used for producing 

 plants for inoculation. 



Shortly before the panicles emerged from the leaf sheaths of the plant 

 at Forest Home, some of both varieties were covered with wax-paper 

 bags to insure a supply of each kind against natural or accidental inocu- 

 lation. They were harvested later, bags and all. Thus, clean uninocu- 

 lated seed was obtained. 



At blossoming time spores of U. avenue were obtained from the un- 

 treated plots at East Ithaca. These were examined microscopically to 

 make sure of their rough-spored character, since covered smut also was 

 present in the plots. The so-called "loose" character of the lesions 

 caused by U. avenae does not seem to be consistent, since gradations may 

 be found that correspond more to the "covered" character of the U. 

 levis lesion. 



When the blossoms in the plots at Forest Home were fully developed 

 and undergoing pollination, some of both varieties were artificially inoc- 

 ulated. This was accomplished much after the method employed by 

 Zade, namely, by tapping a small camel 's-hair brush, previously dipped 

 in spores, against the finger, thus introducing spore material into the 



