FUNGAL PARASITES 177 
plant. The hyphe secrete a poison which 
kills the cells in advance of its track, and thus 
the fungus succeeds in completely destroying 
the plant. Critically regarded, the change 
from saprophytism to parasitism in this 
instance is a somewhat imperfect one because 
the fungus, inasmuch as it kills in advance, 
is really living on dead tissues. But it shares 
this property with the majority of the de- 
structive parasites. It is only the more 
specialised forms that tax but do not destroy. 
It represents a transitional phase, and one of 
interest, inasmuch as it shows how appropriate 
nourishment may accelerate, and increase 
to an effective degree, physiological powers 
already present, but normally inadequate, 
for purposes of direct application. 
As regards the susceptibility of the host 
plant to fungal attacks, it is a matter of the 
commonest experience that some individuals 
of a race are more liable to contract disease 
from these causes than others. Every year 
sees the introduction of new varieties of 
potatoes which are claimed to be immune 
towards the disease (Phytophthora) that often 
does so much damage to the crop. Sometimes 
these varieties are resistant in certain districts 
and less so in others, and it may happen that 
their immunity gradually disappears after 
some years of cultivation. It is evident, then, 
that immunity in such instances is not a 
simple matter. Whilst it may partially depend 
on those properties which together make up 
M 
