Oct. 28. 1918 Plasticity of Biologic Forms of Puccinia graminis 247 



onto them, all sorts of puzzling results may be obtained and erroneous 

 conclusions may be drawn. 



Biologic forms must have originated in some way and the forces which 

 induced their origin may still be operative but it seems probable that 

 these forces operate so slowly that they do not affect the practical prob- 

 lems of controlling rusts of economic importance. It is highly important, 

 however, that the geographic distribution of biologic forms be ascer- 

 tained and their relation to the varieties grown or bred for those regions 

 be intensively studied. When this has been done many of the appar- 

 ently strange and inexplicable phenomena of rapid change in rust resist- 

 ance will undoubtedly be explained. 



SUMMARY 



(i) Barberry does not increase the host range of biologic forms; nor 

 does it act as a reinvigorator of the rust. The biologic specialization in 

 the aecial stage is apparently the same as that in the uredinial sta,ge. 



(2) Differential hosts must be used to isolate biologic forms from 

 mixtures before conclusive experiments can be made with bridging hosts. 



(3) In experiments with a small number of biologic forms and extend- 

 ing over a short period of time there is danger of erroneously concluding 

 that bridging has occurred. 



(4) Many hosts equally susceptible to P. graminis secalis and P. 

 graminis tritici were used as bridging hosts in attempts to change the 

 parasitism of the two forms. Barley, various species of Elymus, Agro- 

 pyron, Hordeum, and Bromus were used most. 



(5) Pv€cinia graminis secalis, which does not attack wheat, but does 

 infect barley readily was cultured on barley and other theoretical bridg- 

 ing hosts continuously for three years during which time more than 

 2,000 wheat plants were inoculated. The rust acquired no new parasitic 

 capability on account of its association with barley. 



(6) Puccinia graminis tritici attacks wheat readily, but can attack rye 

 only weakly. Barley is easily attacked. The rust was confined to 

 barley for about 32 months but it never acquired the power of attacking 

 rye more readily than rust taken directly from wheat. 



(7) Several species of Elymus, Agropyron, Hordeum, and Bromus 

 were used as bridging hosts for both the secalis and tritici forms ; but no 

 bridging resulted. 



(8) Attempts to change the parasitism of P. graminis avenae by means 

 of bridging hosts were also unsuccessful. 



(9) The taxonomic relationship theory of bridging was tried. If plant 

 C can not be attacked by the rust from a taxonoraically distant host plant 

 A, it can not be attacked after the rust has been grown on a form B inter- 

 mediate taxonomically between A and C. 



(10) No one so-called bridging host nor any combination of such hosts 

 enabled any biologic form tried to infect naturally immune plants nor to 

 infect a highly resistant plant more readily. 



