176 DISEASE IN PLANTS. 



research is also showing that the various species 

 of Rust-fungi have split up into different varieties 

 or specialised races, according to the particular 

 host plants they inhabit. For instance there are 

 special varieties or races of the particular species 

 known as Puccinia grammis, the wheat rust, each 

 of which grows well on various kinds of grain and 

 grasses but refuses to infect others. Thus, the 

 variety which infects Wheat refuses to infect 

 Barley or Oats, while that variety which grows on 

 Rye will not take on Wheat and so forth. Now it 

 is important to notice that these specialised races 

 are indistinguishable one from another by their 

 visible microscopic characters: they are all botanic- 

 ally of the species Puccinia graniinis which forms 

 its aecida on the Barberry. We must therefore 

 conclude that we have here the same phenomenon 

 as that met with in culture-races of bacteria 

 which, having been fed for several generations 

 on media rich in proteids, refuse to grow on 

 media rich in carbohydrates, or when attenuated 

 races are developed by culture under special 

 conditions. 



Now since such physiological races as I have 

 described are by no means confined to Puccinia 

 but are also known in Melampsora, Gyninosporan- 

 giuni and other fungi, we must conclude from this 

 and from what we know of variation in plants and 

 animals generally, that variation and adaptation 

 are common among parasites, insects as well as 

 fungi. 



These considerations will serve to show more- 



