Some Concepts in Mycology. William B. Brierley. 231 



omitted than for what it produced; and in the Hght of what has 

 been said, and particularly of the minimal experimental 

 conditions, fulfilment of which is obligatory in studies designed 

 to elucidate this issue, this research possesses now only historical 

 interest. 



Evidence of much greater weight is the well-known work of 

 Salmon (37) on the adaptive parasitism of Erysiphc graminis 

 and the parallel results obtained for certain rust fungi by 

 Ward(4«), Freeman(i2), Freeman and Johnson(i3), Johnson(2o), 

 Pole Evans(33) and others. Until however this work can be 

 carried out under conditions much more rigidly controlled, and 

 with the nicety of technique which has been indicated as 

 imperative to the present issue, these most interesting results 

 must be placed on one side, and the verdict on the present 

 (question be regarded as "non-proven." Moreover it may be 

 noted that the more recent work of Stakman(4i), Stakman and 

 Piemeisel(42), Stakman, Parker and Piemeisel(43) and Stakman, 

 Piemeisel and Levine(43a) has produced results which directly 

 negative the conclusions previously reached. As a result of their 

 investigations theystate(43): "Theresultsof theexperimentswith 

 P. graminis tritici-compacti shew that barley which both theoreti- 

 cally and from the results obtained by previous investigators 

 might be expected to increase the infection range does not do so. 

 Even susceptible varieties of wheat do not change the parasitic 

 capabilities of the rust so as to enable it to attack a normally 

 resistant variety. Furthermore the rust does not acquire ad- 

 ditional virulence when associated for a long time with a given 

 host. Barley is moderately susceptible to the rust but the rela- 

 tions between host and rust are apparently the same regardless 

 of the length of their association with each other. Wheats 

 resistant to the rust remain resistant regardless of the previous 

 history of the rust." 



"In no case, however, was there the slightest evidence of 

 any change in the virulence of the parasite, nor any indication 

 that a short sojourn on a susceptible hybrid had given it any 

 peculiar ability to cause normal infection on a heretofore 

 resistant variety or to cause a more than usually virulent 

 infection on a susceptible variety." 



And again (43a) "Differential hosts must be used to isolate 

 biologic forms from mixtures before conclusive experiments can 

 be made with bridging hosts." "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." "The writers have not been able 

 to detect any mutation nor to induce perceptible evolutionary 

 changes experimentally." 



