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 their 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 
