262 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



be governed bv genetic factors in the same way as morphologic 

 characteristics are governed. Nevertheless the idea of adaptation 

 is maintained to be operative, and it has the support of a certain 

 oroup who believe in evolution through adaptative modifications. 

 Attention was directed to this point of view bv the results of 

 Ward (1903) on Puccinia dispersa on brome grasses and of Sal- 

 mon (1904) on various powdery mildews, especially Erysiphe 

 graminis on grasses. Ward maintained that this rust could acquire 

 the ability to attack a resistant host if it were first transferred to 

 another host with lesser resistance. After repeated transfers on 

 this less resistant host, it acquired the ability to attack the resist- 

 ant one. The less resistant variety, therefore, served as a "bridge." 

 Similarly, Salmon maintained that E. graminis from barley could 

 not infect wheat unless the leaves were injured. When grown 

 on injured wheat leaves for several transfers, it acquired the ability 

 to infect intact ones. From this type of results he concluded, 

 "The restriction in power of infection characteristic of biologic 

 forms breaks down if the vitality of the leaf on which the conidia 

 are sown is interfered with in certain ways." He also noted that 

 the powdery mildew from Brovnis racemosus did not infect B. 

 comrnutatus in 12 trials, but that it infected B. hordeaceus in each 

 of 36 trials. Furthermore failure of infection resulted in 36 at- 

 tempts when conidia from B. comrnutatus were applied to B. race- 

 7/iosus, and infection occurred in 40 out of 49 trials in which 

 conidia from B. hordeaceus were applied to B. comrnutatus. From 

 these experiments B. hordeaceus was concluded to act as a bridg- 

 ing species for powdery mildews on B. racemosus and B. com- 

 rnutatus. 



Hammarlund (1925) attempted to repeat Salmon's work, using 

 Erysiphe comimniis tritici cultivated for 37 generations on 

 wounded leaves of Hordeum europaeuvu with the result that 

 there was no increasing tendency to become adapted to barley. 

 He also employed E. graminis tritici cultivated on wounded leaves 

 of Hordeum vulgare for 128 generations. In this experiment like- 

 wise there was no evidence at the end that the powdery mildew 

 had acquired the ability to infect intact barley leaves. 



Stakman and his associates (1926) attempted to adapt Erysiphe 

 graminis tritici to grow on barley, rye, and oats. They subjected 

 the plants to "every conceivable form of torture," but all refused 

 to become infected. 



