FUNGAL PARASITES 177 



plant. The hyphae 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 



