Oct. 28. 1918 Plasticity of Biologic Forms of Puccinia graminis 245 



Table VI.— Results of successive transfers of Puccinia graminis agrosiis to barley, oats, 



and rye 



As seen from Table VI, P. graminis agrostis transfers with difficulty to 

 barley, oats, and rye. The uredinia are always few in number and are 

 practically always very small. It was impossible to increase the 

 virulence of this rust by successive transfers to any of those hosts, 

 although not a great number of experiments were made 



GENERAL DISCUSSION 



From the foregoing results it seems perfectly safe to conclude that if 

 bridging and adaptation do occur, they occur rarely. Although all of 

 the possibilities have not been exhausted it would seem that the experi- 

 ments have been extensive enough practically to eliminate any idea of 

 the possibility of sudden or even gradual changes in the rust under 

 experimental conditions. Furthermore, as a result of extensive inocula- 

 tions with biologic forms of P. graminis from a large number of hosts 

 from widely separated localities {21), it seems that observational evi- 

 dence corroborates the experimental evidence which the writers have 

 obtained. The biologic forms obtained have remained pure and fixed 

 after having once been isolated. It is true that when experiments are 

 carried on for a short period of time only, there may appear to be 

 distinct differences in the different strains of the same biologic form, 

 but when the experiments are carried over a period of years it becomes 

 quite evident that these differences are often due to experimental 

 conditions. It seems quite probable that plus and minus fluctuations 

 may occur, but that there is always a tendency to return to the normal. 

 These plus and minus fluctuations may be induced by host influence 

 or by environmental influence, but with an obligate parasite like P. 

 graminis which can not be grown on a standardized medium but must 

 be grown on living plants they are to be expected and the limits must 

 be determined by extensive work. 



