On Evil in Small Doses 



touch with a hot razor has the same effect, or, if the 

 leaf is slightly injured by anaesthetics, it loses its power 

 to keep oft the enemy.""-^ 



The fact that it is enfeebled larches which fall a prey 

 to the disease has been recently insisted upon. It does 

 not matter, apparently, what brings about this depressed 

 state of health, but whether it be injuries due to a 

 roebuck or frost or bad soil, the result is that the fungus 

 is the conqueror. One might almost say that it is a 

 general rule that healthy plants resist when weaker 

 ones are invaded by parasitic fungi.^^ 



These mildew and rust-fungi are specialists ; there 

 seems to be a particular form of most of them which 

 confines its attacks to one kind of host plant. They 

 cannot infect other species, unless by some accidental 

 weakness in the particular individual. 



But this is not always the case, for the South African 

 Nemesia, but lately introduced into our gardens, has 

 been successfully invaded by the common rust-fungus 

 of the Scotch fir. 



When wheat was first introduced in America and in 

 our other colonies, the destruction of the crops by rust 

 seems to have been very great, and indeed seriously 

 influenced their prosperity during the first few years. 

 Can this be explained by the feebleness of the wheat 

 which found itself in a new and strange climate ? 



Once a fungus has established itself on a wounded 

 leaf, it is able to infect healthy leaves of the same plant 

 without any difficulty whatever. It was found that old 

 and probably rather infirm blueish-green leaves (of 

 Galium silvaticum) can be infected by a certain fungus, 

 whilst young fresh green leaves vanquished it.^^ Not 

 all fungi are parasitic, for many kinds live entirely upon 

 dead and decaying vegetation and are unable to attack 

 live plants. But it may be possible to train one of the 



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