194 



Journal of Agricultural Research 



Vol. IV, No. 3 



the host tissue and fails to develop normally, in sharp contrast to the 

 conditions in normal infection. Where extreme incompatibility exists 

 between host and parasite, there is often no externally visible evidence 

 that the fungus has even gained entrance. In fact, it is hardly accu- 

 rate to speak of the inoculated plant as the host, since the fungus is 

 unable to attain any considerable degree of development in it (7). 

 The work reported in this paper was with such forms and was under- 

 taken in order to determine whether the phenomenon was one of real 

 resistance or an extreme case of hypersensitiveness. Hypersensitive- 

 ness is used here to indicate the abnormally rapid death of the host 

 plant cells when attacked by rust hyphae. It is used in this sense with- 

 out any implication as to the exact physiological nature of the phe- 

 nomenon, referring, therefore, only to the facts substantiated by visual 



evidence. 



FORMS OF THE FUNGUS INVESTIGATED 



The forms used for study were the following: Oats {Avena sativa) 

 inoculated with Puccinia graminis tritici; oats inoculated with P. gram- 

 inis hordei; rye {Secale cereale) inoculated with P. graminis from Dactylis 

 glomerata after three generations on oats; wheat {Triticum spp.) inoculated 

 with P. graminis from Dactylis glomerata after three generations on 

 oats; wheat inoculated with P. graminis avenae; and barley (Hordeum 

 spp.) inoculated with P. gram-inis from Dactylis glom£rata after three 

 generations on oats. The rusts used, except that from Dactylis glom- 

 erata, had been confined to the particular host from which they were 

 taken for at least 20 " generations" — i. e., 20 successive transfers had been 

 made in the greenhouse. The rust from Dactylis glomerata was taken 

 from rusted plants in the field, inoculated on oats, and then transferred 

 to oats two successive times. 



The forms selected represent extreme cases of incompatibility, as is 

 shown by the following table : 



Results of inoculations with strains of Puccinia graminis 



Form of rust used. 



Puccinia graminis tritici 



Puccinia graminis hordei 



Puccinia graminis from Dactylis glomerata . 



Do 



Do 



Puccinia graminis avenae 



Plant 

 inoculated. 



Oats. . 

 ..do... 

 Rye.... 

 Wheat. 

 Barley. 

 Wheat. 



Number of 



plants 

 inoculated. 



Numberof 

 plants 

 infected. 



115 



183 



114 



86 



58 



In the case of the rust from Dactylis glomerata, some of the inoculations 

 were made directly from the grass and some after one to three genera- 

 tions on oats. They are all grouped in the table, since the rust appar- 

 ently was not changed after having grown for some time on oats. 



