Oct. 28. 1918 Plasticity of Biologic Forms of Puccinia graminis 229 



a possible bridging host for P. graminis avenae. Special care was taken 

 to isolate the biologic forms whenever they were mixed, in order to 

 avoid conflicting results and erroneous conclusions. The results of this 

 phase of the work which extended for a period of over three years are 

 given in diagrams i to 10 and in Table II. 



It might have been concluded from the results shown in diagram i that 

 barley acted as a bridge between Agropyron repens and w^heat. It is 

 shown quite clearly, however, by subsequent inoculations that it did not. 

 The rust on the original quack-grass no doubt was mostly of the secalis 

 form. It had probably been contaminated slightly in the field with some 

 of the tritici form. Since there was only a small amount of the latter 

 form, none developed on the wheat in the original inoculations. A very 

 small amount developed on rye and more developed on barley. There- 

 fore, when inoculations were made with the rust from barley, both the 

 tritici and secalis forms were present, but they were separated in the third 

 set of inoculations. A more complicated condition is shown in diagram 2. 



Diagram i. — Results of inoculations with Puccinia graminis from Agropyron repens 

 showing apparent bridging before biologic forms were isolated. 



W-L 



28 



Puccinia graminis from Agropyron repens, St. Paul, Minn., 

 June 22, 1915. 



w- 



. I ' 



Agropyron repens — 



September 19, 1915. 



ISmalluredinia; P. graminis tritici. 



