194 THE ROYAL SOCIETY OF CANADA 



last to be collected, seldom appearing before September. With these 

 exceptions, the experiment has shed little additional light on this 

 problem of the seasonal spread of rust. 



The constancy of behaviour of the biologic forms is one of the 

 striking facts emerging from these investigations. Association of the 

 same form with a great variety of hosts, in widely separated localities, 

 was without apparent effect. Inoculations on the test wheats in- 

 variably gave the same result, whether the inoculum was obtained 

 from the same varieties, or from very different hosts on which the 

 fungus had been cultured for several generations. This experience 

 supports the conclusions of Stakman, Piemeisel and Levine (6), based 

 upon their extended investigations of this point. However, the idea 

 has been frequently expressed that a permanently rust-resistant 

 wheat variety cannot be produced by the plant breeder, owing to the 

 plasticity of the rust, which gives it facility in adapting itself to new 

 conditions, and it is of interest, therefore, to examine one or two ways 

 in which such an erroneous impression may have become current. 



Let us suppose that a person unfamiliar with the exacting techni- 

 que required in the study of biologic forms collects wheat stem rust 

 from Marquis wheat, and that the biologic forms thus fortuitously 

 obtained are III and IX. Suppose now that with this mixture he 

 inoculates Kanred (common wheat), Arnautka (durum) and Emmer. 

 The results, expressed by our formula, would be as follows: 



Form Ma KR Arn Em 



III 3 3 11 



IX 3 3 4 



Thus in every case the variety concerned would show moderate 

 susceptibility, with well formed pustules of one or other of the forms, 

 which, however, would appear identical to the observer. The very 

 small pustules of Form III on Arnautka and Emmer might escape 

 notice, or be set down as poorly developed pustules of the dominant 

 type. Let us suppose, further, that our interested observer transfers 

 material from Kanred to fresh plants of Arnautka and emmer, or 

 from the well developed pustules of Arnautka or emmer to Kanred. 

 In the first case, he would find high resistance, and in the second 

 immunity, and he might easily conclude that the fungus had changed 

 its virulence. This is merely an example of many possible accidental 

 combinations of wheat varieties and mixtures of biologic forms which 

 might give rise to wrong conclusions. 



