BREEDING DISHASH-RESISTANT PLANTS 405 
yet been definitely answered. According to Ravaz, a chemist has 
_ thought to measure resistance by the amount of resinous principles in 
the roots; a physician by the relative duration of the roots; an anatomist 
by the relative thickness of the medullary rays; but all these explanations 
have failed to withstand investigation. Foéx states that resistance was 
first thought to be due to great vigor, large root development and ease 
of production of new roots but that this was insufficient since some 
vines of small vigor, like Vitts monticola, are resistant while others of 
great vigor are susceptible. Foéx, himself, traces a relation between 
the thickness and succulence of the bark of the root and susceptibility. 
There is also a theory, which originated in Italy, that resistance is due to 
acidity of the sap and the degree of acidity is highest in seedling plants 
and in clones which have recently come from seedlings, the acidity 
decreasing with the age of the variety. But this is contradicted by the 
fact that vinzfera seedlings are quite as susceptible as their parents. 
Variability in resistance of several varieties of grape when grown in 
different infested localities is accepted by Grassi as evidence of the 
existence of “benignant”’ and ‘‘malignant” races of phylloxera. But 
this does not explain the high resistance or immunity of some 
American species. Having in mind the fact that the phylloxera sucks 
its nourishment from the leaf or root by inserting its prolonged rostrum 
into the living tissue, 1t seems most probable that resistance is to be 
explained as absence of response to a specific stimulus. The many 
remarkable instances of hypertrophy in vegetative tissues due to wounds 
inflicted by insects can be explained satisfactorily only by assuming that 
the insect injects something into the wound which causes abnormal 
functioning of the affected parts. If this occurs in the case of phylloxera 
then resistance consists in failure of the wounded tissue to respond to 
the foreign element injected by the insect. Such failure of response 
might be due either to the absence of a particular substance which reacts 
so as to stimulate growth or to the presence of a specific anti-body which 
counteracts the effect of the insect’s poison. The latter seems the more 
probable condition in view of what is now known concerning immunity 
in general. The complete susceptibility of V. vinifera would then be due 
to absence of the anti-body. But the absolute resistance or complete 
immunity of V. rotundifolia may be caused by the presence of a sub- 
stance which is actually repellent to the insect itself. At any rate, the 
fact that we are dealing here with distinct natural species makes it rea- 
sonably certain that resistance and susceptibility to phylloxera infesta- 
tion are somatic expressions of genotypic diversity. 
Another important case of variation in disease resistance among 
species of the same genus is found in the relation of various chestnuts 
to the very destructive bark disease caused by the fungus, Endothia 
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