Immunity 



open to infection by a parasite which attacks one 

 variety of it. For example, the spores of a certata 

 mildew which attacks grasses will not attack a 

 grass " A " if they have been taken from mildew on 

 a grass " C," although spores from mildew on grass 

 " A " will readUy attack the leaves of other plants 

 of the same variety. They will, however, attack 

 grass " B," and spores produced by the mildew 

 growing on grass " B " from the spores obtained 

 from grass " G " will attack grass " A." This fact 

 seems to show that one and the same fimgus may 

 behave differently according to the source from which 

 it is obtained, and it has been found possible to train 

 a fungus which usually lives on dead plants to 

 attack living ones ! Further, one stage of a fungus 

 may grow on a living plant, and another slightly 

 different stage, only on the dead remains of the 

 plant. A fimgus may enter a plant (generally 

 through a wound) and grow but fail to fruit imtil the 

 plant it has attacked is dead — such are the fungi 

 that produce silver leaf in plums and " coral-spot " 

 on the dead shoots of currants. 



A further remarkable phenomenon shared with 

 certain animal parasites is afforded by some of the 

 rust fungi. These fimgi form a distinct group and 

 in their most highly developed forms produce a 

 variety of different spores duriag their Ufe-history. 

 The early stage when present produces numerous 

 dust-like spores in minute circular cups with tumed- 



53 



