8. F. Ahmstkong 71 



attack had been nioditied or partially broken down. This greater or le.s.s 

 degree of predisposition to attack was probably due to the interaction 

 of other causes or "factors '" such as are discussed later under section III. 



The last two cultures in Table III (25/9 and 22/27) were attacked 

 considerably beyond the average, thougli not nearly so severely as the 

 pure susceptible cultures were. They were probably the progeny of 

 heterozygous susceptible plants that escaped infection in 1918. 



(c) Twelve cultures (Table IV) were raised from F.^^ plants which 

 showed only traces of attack. Three of these proved highly resistant 

 and compared closely in this respect with the iirst fifteen cultures given 

 in Table III. One of these (82/9) was further tested on a large scale; 

 forty-seven F^ cultures were grown containing some 850 plants, all of 

 which proved highly resistant in 1920. It is therefore reasonable to 

 conclude that the two cultures, 31/23 and 70/13, were also the offspring 

 of homozygous "immune" plants, although these plants showed traces 

 of attack in 1918. 



Table IV. Results of amdysis of F-^ riiUares (1919) grown from F^ plants 

 whicJi had onlij traces of rust attack in the previous season. 



Extent of the nist attaek at final examination 

 Plants (28th July to 11th August) 



The other nine cultures gave evidence of segregation into shghtly, 

 moderately, and badly rusted types, and were clearly the offspring of 

 heterozygotes for rust resistance. In eight of these the plants were fully 

 graded and included 99 individuals with a shght attack, or none, and 

 305 with a moderate or bad attack. 



(d) Of the cultures raised from only shghtly rusted -^2'^' ^^ gave 

 distinct evidence of segregation (Table V). Twenty-two of these plants 



