BIOLOGIC FORMS 9 



that the specialization has taken a different course in Sweden from 

 that it has followed in the United States. The most widely grown 

 crops would naturally be the ones on which the particular biologic 

 forms adapted to them. would attain their highest development. There- 

 fore, the fact that a rust shows particular relationships in one country 

 does not by any means preclude the possibility of a quite different set 

 of relationships in another country. 



The idea that the fungus changes its habits as a result of environ- 

 ment is substantiated by many observations. Perhaps an extreme case 

 of such a tendency is found in the life history of Puccinia graminis in 

 Australia. McAlpine (1906, p. 21) states that the teleutospores seem 

 unable to infect the barberry in Australia, and, according to his obser- 

 vations, the fungus is quite rapidly being reduced to a reproduction 

 by uredospores only. This is accounted for by the absence of the bar- 

 berry. 



Eriksson (1896, p. 339) shows that closely related host forms 

 are somewhat similar in their relation to rust. He says, however, that 

 the taxonomic relationship of host plants does not entirely determine 

 the specialization of the rust form. Ward (1901) in his work on the 

 rust (Puccinia dispersa) of the bromes states that the closeness of re- 

 lationship of hosts is the determining factor in the ability of the rust 

 to pass successfully from one host plant to another. Freeman (1902) 

 also concluded that the farther removed a species of Bromus was tax- 

 onomically from the plant serving as a host for the rust the less prob- 

 ability there was of infection. Ward showed further (1903) that some 

 forms of bromes might act as bridging species in enabling the rust to 

 pass indirectly from one group of bromes to another, although direct 

 transfer was impossible. Salmon (1904) showed that the same thing 

 was true of Erysiphe graminis D. C. Freeman and Johnson (1911) 

 have found that barley can act as a bridging form enabling Puccinia 

 graminis to increase its range of infection power. Salmon (1904 and 

 1905) showed that the range of infection possibility of Erysiphe 

 graminis forms may also be increased under certain cultural conditions. 

 By injuring leaves and subjecting plants to heat and anesthetics he 

 was able to infect normally immune forms. 



The conception of a biologic form, then, is that it represents a 

 tendency toward adaptation. This tendency may be due to various 

 causes, the evidence being that it depends largely on the availability 

 of host species. Hitchcock and Carleton (1894), Carleton (1899), and 

 Freeman and Johnson (1911) investigated quite thoroughly the mat- 

 ter of biologic forms of Puccinia graminis in the United States. Free- 

 man and Johnson (1911, p. 27) give the following as the biologic 

 forms of this rust in the United States : 



P. graminis tritici (stem rust of wheat) on wheat and barley. 



