22 H. L. Russell. 



The bacillus causing this disease produces an hypertrophy of the 

 cambial and extra-cambial tissue, which is, however, quite local- 

 ized. The results of artificial inoculation into young olive trees 

 were evident in 25 days, and in 2 months a well-marked swelling 

 was to be noted at seat of inoculation. Savastano was not able, 

 however, to successfully infect, in equal degree, all varieties of this 

 species, but found a varying degree of susceptibility in different 

 varieties. When the B. olese-tuberculosis was injected into other 

 healthy fruit-bearing trees, such as lemon, bitter orange, pear, apple, 

 quince, etc., no trace of the infection could be noticed. Here is, then, 

 a case of resistance against the organism even though it was a para- 

 sitic organism on other forms of plant life. 



He also infected olive trees in like manner with several other 

 micro-organisms which he found associated in the tuberculous 

 growths with the true cause of the disease, but in no case was 

 any pathological condition brought about, showing that the normal 

 resistance of the plant was able to overcome the accompanying sap- 

 rophytic micro-organisms, although it was not able to withstand the 

 attacks of the parasite in all cases. 



The examples already cited will suffice to show the distinction 

 made between immunity and the normal resistance of the plant, and 

 also that immunity is a varying term itself, sometimes applicable 

 within narrow limits, then again spreading over a greater variety of 

 species. Not only are the limits of immunity ill-defined and vary- 

 ing, but the degree of immunity also varies considerably. This is 

 readily recognized in horticulture, when we say that one variety is 

 more susceptible to the attacks of a certain disease than another. 

 This variation of the susceptibility of different varieties indicates 

 that they possess an immunity to a greater or less extent from the 

 action of a definite specific germ. 



We might consider, also, other phases of immunity which present 

 themselves for consideration, but a mere mention of these will be 

 all that can be given of them in this connection. Thus we have the 

 local immunity, which certain tissues enjoy against the attacks of 

 micro-parasites as they increase in age and consequently become 

 better developed and more resistant. This rests on a purely physical 

 basis, and the importance of it will be considered later more in detail. 

 This local immunity of certain tissues is strongly reinforced, also, 



