6 BULLETIN 629, U. S. DEPARTMENT OF AGRICULTURE. 



leaf and that the pustules developed in these areas were always very 

 small. A further indication of immunity is -said to be the fact that 

 iu the immune forms the incubation period is longer than in sus- 

 ceptible ones. 



In a second paper (13) Stakman reports additional studies of the 

 relation between Puccinia graminis and plants highly resistant to its 

 attack. The occurrence of the same characteristic flecks or areas of 

 killed tissue is again reported, but a new term, " hypersensitiveness, v 

 is used to describe the phenomenon. 



Although no histologic studies have yet been made of the oat 

 material, the external macroscopic evidence is in such close agree- 

 ment with the observed conditions in wheat that there can be little 

 doubt that a struggle between host and parasite of a very similar 

 nature takes place within the tissues of the resistant oat varieties. 



Concerning the indications or signs of resistance which were ob- 

 served in the present study, it may be well to repeat that they are 

 very similar to those in wheat. They are — 



1. The prolonged incubation period. 



2. The formation of flecks (yellow areas of dead host tissue). 



3. The formation of larger blotches of dead tissue and, in extreme cases, 



the premature death of the whole seedling leaf. 



4. Small uredinia, sometimes not completely or promptly rupturing the 



epidermis, and in Puccinia graminis avenae the formation of purple 

 blotches adjacent to the uredinia. 



5. The small number of uredinia (relatively unimportant). 



G. The production of normal telia of the crown rust on seedling leaves of 

 varieties which these other criteria indicate are resistant. 



So far as known to the writer, the occurrence of telia on young 

 seedling leaves of cereals grown in the greenhouse has not been re- 

 corded in literature. Melhus (8) states that in his cultures, which 

 appear to have been on older plants, " teleutospores developed in two 

 to three weeks." 



It is certain that in the hundreds of seedlings described as very 

 susceptible in the present experiments telia were not produced on a 

 single one following a normal and abundant production of uredinia. 

 The fungus on these leaves seems to have finished its life cycle under 

 these conditions by producing the uredinia. After having remained 

 a normal green color for some time after the formation of uredinia, 

 the leaf finally dries up. That part of the life cycle so common 

 to the rust fungus when on ripening grain plants in the field is 

 not completed. On the other hand, quite early in the work it was 

 observed that in a comparatively short time telia were present on the 

 leaves of seedlings which gave other evidences of being resistant and 

 on which no normal uredinia had been produced. The spores from 

 these sori appear in every way normal, so far as determined b}^ micro- 

 scopic observation. 





