INTRODUCTION. 491 



the large-leaf kinds. Resistance of this sort, which appears to be nothing 

 other than a natural, inborn quality, may be designated 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 Carolinas, 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 ques- 

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



