Oct. 28. 1918 Plasticity of Biologic Forms of Puccinia graminis 243 



In Table III the results of a large number of attempts to increase the 

 virulence of the tritici strain on rye by means of constant association 

 with this host are given. It will be noticed that the rust was obtained 

 from a number of different sources and from a number of different 

 localities. The experiments covered a period of about three years. 

 Whenever normal infection occurred on a rye plant the rust was trans- 

 ferred to other rye plants in the hope that it might prove to be a mutation 

 or a tendency toward the differentiation of a strain of tritici capable of 

 infecting rye normally. It will be seen, however, that this was not 

 accompHshed. The rust invariably gradually died out. Sometimes it 

 seemed to increase in virulence as a result of successive transfers, often 

 due to the abundance of infective material. At other times it apparently 

 increased in virulence, owing merely to a change in experimental con- 

 ditions. In several trials the rust had previously been on intermediary 

 hosts for a number of generations, but this made no difference in the 

 results. It so happened that some of the best results were obtained 

 when the rust was taken directly from wheat. It was never possible 

 to keep the rust on rye more than six generations. It simply became 

 unthrifty and eventually failed to produce any uredinia, although 

 excellent conditions for infection and subsequent development of rust 

 were maintained. 



Table IV. — Results of attempts to increase the virulence of Puccinia graminis avenae 

 by successive transfers to uncongenial hosts 



Ht^ Brsmus iectoruni. 



Hg— Dactylis glomerata. 



Ph pT—Phlejim pralcnse. 



The avenae strains used in the inoculations represented in Table IV 

 were not able to develop increased virulence as a result of successive 

 transfers on barley, Phleuni pratense, or Bromtis tectorum. The rust 

 was kept on B. tectorum longer because this host is the most congenial 

 of the three. However, it gradually died. 



