On Evil in Small Doses 
touch with a hot razor has the same effect, or, if the 
leaf is slightly injured by anesthetics, it loses its power 
to keep off 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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