BREEDING DISEASE-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. Foe'x 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 Vitis monticola, are resistant while others of 

 great vigor are susceptible. Foe'x, 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 vinifera 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, it 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 

 wi 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 



