INTEODtrCTION 589 



pears to be nothing other than a^natural, inborn quality, may be de- 

 signated as natural immunity, and it is immunity of this kind which 

 plant breeding for disease resistance has secured. A good illustration 

 of this is to be found in the wilt-resistant water melon of the CaroUnas, 

 which is the result of crossing a naturally susceptible watermelon with a 

 naturally resistant citron. 



Acquired immunity in the plant world is a field yet to be explored. 

 Cases have been cited in which active immunity appears to have followed 

 the disease, but these are extremely rare and the evidence is very 

 questionable. Passive immunity, at the present time, is unknown. 



