02 Mendellan Inheritmice and Yelloir Rust in Wlieat 



It has been supposed that increased virulence is gained by a rust 

 after passinj^ one stage of its hfe-history on a congenial host. For example, 

 Pole-Evans (U). working with P. graminis, beUeved that his experiments 

 showed [a) "that the t\ from a cross between a susceptible and an 

 immune wheat is more susceptible to rust attack than its susceptible 

 parent," and (6) "that the rust which passes tiirough a liybrid plant 

 produces a far more severe infection than the rust from the susceptible 

 parent." Neither of these conclusions, however, appear to carry much 

 weight since the wheat employed as the "immune" parent proved on 

 some occasions to be moderately susceptible. As the present investiga- 

 tions have shown, hybrids which result from tlie crossing together of 

 two more or less susceptible varieties are generally more susceptible 

 than either of their parents. The second conclusion referred to has re- 

 cently been negatived by the experiments of Staknian(i:^) who worked 

 with the same rust. Stakman's evidence showed that the patliogenicity 

 of rusts is not easily changed by host-influence. He says "although many 

 attempts were made to increase the virulence of biologic forms on resistant 

 hosts, the results indicated that rust-forms do not gradually adapt them- 

 selves to resistant or semi-congenial hosts." 



Since rust spores probably vary as regards their capacity for infection, 

 an occasional spore may be able to bring about a more or less successful 

 infection on a host plant which is capable of complete resistance in tiie 

 vast majority of cases. In this way ])erhaps the partial or temporary 

 bieakdown of rust-resistance may be partly accounted for. At present, 

 however, the bulk of the experimental evidence indicates that it is to 

 the physiological condition of the liost, rather than to any increased 

 virulence on the part of the parasite, that variations in a ])lant"s sus- 

 ceptibility are due. Changes in the metabohsm of the possible host are 

 brought about in various degrees by the interaction of both inherited 

 and non-inherited factors, and it is suggested that such changes may lead 

 to a diminished resistance by proving unfavourable to the formation 

 of those products upon which inmiunity depends. 



CONCLUSIONS. 



The evidence obtained from the investigations described demonstrates: 

 1. That immunity and susceptibility to Yellow liust attack are 

 inheritable features in wheat. 



"2. That susceptibility and immunity behave as unit-characters, and 



