154 THE ROYAL SOCIETY OF CANADA 



Part II. The Development of the Parasite within the Tissues 

 OF Resistant and Susceptible Hosts 



Historical Introduction 197 



Histological Material and Methods 199 



Normal Infection of a Susceptible Host 200 



Infection of a Resistant Host 202 



Summary of Part II 204 



Bibliography 205 



Explanation of Plates I to VI 209 



PART I 



Biologic Forms of Wheat Stem Rust (Puccinia Graminis Tritici) 



IN Western Canada 



Introduction 



The control of wheat stem rust, caused by Puccinia graminis 

 tritici Erikss, and Henn., is one of the major problems in Canadian 

 agriculture. The problem in Canada is somewhat different from that 

 in the United States because there are not sufficient barberry bushes 

 to account for the great rust epidemics which sweep across the country 

 from time to time. We have, moreover, no evidence to show that 

 urediniospores, after pasing through the long, cold Canadian winter, 

 with the alternate thawing and freezing in the spring, can cause 

 successful infection on the young wheat plants. The generally 

 accepted hypothesis is that the rust moves northward from the 

 United States. If this is true, then the breeding of resistant wheats 

 is the only practical way of solving the problem. That resistant 

 varieties of wheat can be secured has been conclusively shown by 

 Biften, in England, who succeeded in developing a variety which was 

 not only highly resistant to Puccinia glumarum Schm., but which was 

 also commercially desirable. It would seem, then, that the problem 

 is primarily one for the plant breeder. Possible complications and 

 difficulties in the breeding of resistant varieties were indicated by 

 the common experience that a variety resistant in one locality may 

 be susceptible in another. The explanation of this seeming anomaly 

 was made evident by the discovery by Stakman and his co-workers 

 (56 et seq.) that there are several biologic forms of Puccinia graminis 

 tritici, and that a variety resistant to one form may be quite sus- 

 ceptible to another. 



