198 THE ROYAL SOCIETY OF CANADA 



whenever a germ tube of any rust fungus enters any plant but its 

 own proper host, a struggle goes on resulting in the death both of the 

 host, locally, and of the parasite. The more closely related the host 

 is to the proper host of the fungus, the longer and more extensive will 

 be the struggle. 



Miss Marryat (43) found that P. glumarum manages to make 

 good its entry into semi-immune wheats, to produce comparatively 

 large and numerous hyphae, and even in rare cases to form small or 

 abortive pustules, but that, sooner or later, it is starved to death 

 by the breaking down and death of the host tissue in its vicinity. 



Stakman (52), working with P. graminis, observed that in a 

 resistant host a limited number of cells adjoining the point of infection 

 are killed, and the fungus fails to develop normally; while, in a sus- 

 ceptible host, the fungus grows vigorously without immediate serious 

 injury to the host tissue. To explain this he advanced his "hyper- 

 sensitive" theory, which assumes that in resistant forms the host 

 cells are hypersensitive to the fungus; that is, when the infecting 

 hypha enters the cells of a resistant form the cells immediately begin 

 to disintegrate. From this point of view the meaning of the terms 

 "resistance" and "susceptibility" could perhaps be more clearly 

 expressed, respectively, by the terms "intolerance" and "tolerance." 

 The immediate death of intolerant cells on penetration by the fungus 

 leads to the starvation and death, in its turn, of the parasite. The 

 net result is a failure of the infection, a demonstration of "resistance." 



This local killing of intolerant tissue may be clearly seen in our 

 Plates I, II and III, in the form of chlorotic areas on the resistant 

 varieties. 



Since the publication of the papers reviewed above several 

 wheats have been discovered which display a considerably greater 

 immunity than those described by these authors. It is a matter of 

 common observation that immune wheat varieties, when inoculated 

 with P. graminis iritici, show characteristic flecks. The lesions pro- 

 duced are not identical on all resistant varieties, but the presence 

 of larger or smaller dead areas, with small uredinia, or even no uredinia, 

 is characteristic of them all. In extreme cases of incompatibility the 

 leaf area involved is usually so small that no indication of it can be 

 seen with the unaided eye. Such is the case of Kanred wheat when 

 inoculated by Forms I, IX, XVII, XIX, XXI, XXIV, XXIX and 

 XXX, forms to which it is extremely resistant. As flecks can rarely 

 be found upon this wheat the question has naturally been raised as 

 to whether the rust fungus actually enters this variety. 



