FUNGAL PARASITES 181 



there are a number of species in Britain, are 

 liable to the attacks of a parasitic rust fungus 

 known as Puccinia dispersa. Now when the 

 parasite has been growing for a while on one 

 species of brome, it loses the power of infecting 

 some of the others. And yet the puccinia 

 is found flourishing on these apparently 

 resistant species also. The clue to the puzzle 

 lies in the fact that although the puccinia 

 thus develops " races " which preferably 

 attack single species of brome, they can be 

 induced to recover their powers of infecting 

 others by the simple device of cultivating 

 them on other species which are only inter- 

 mediate in ttheir powers of resistance. Thus 

 a race which will thrive on a species A, but 

 cannot attack another species C, will never- 

 theless recover the power of doing so if it be 

 grown on a third species B. This remarkable 

 occurrence of " bridging species " of plants is 

 of obvious importance in connection with the 

 sudden appearance of parasitic epidemics. 

 It is not confined to the rust fungus, but is 

 known to extend to others ; amongst them is 

 Erysiphe graminis, which also infests the 

 brome grasses 



