CHAPTER XVIII. 



VARIATION AND DISEASE. 



Predisposition and immunity Pathological cottditions 

 ^ary Hardy varieties ^^ Disease-proof " varieties 

 Disease dodging Thick skitis Ifidian wheats, etc. 

 Cell-contents vary Citrus, Cinchona, Almotids, etc. 

 Double ideals in selection Cultivation of pest and host- 

 plant Variations of fungi Bacteria Specialised 

 races Difficulties Experiment only will solve the 

 problems. 



The numerous and often expensive failures in the 

 application of any prophylactic treatment, have 

 proved an acute stimulus to the research for other 

 ways of combating the ravages of plant diseases. 

 It is a matter of every-day experience that par- 

 ticular varieties of cultivated plants may suffer less 

 from a given disease than others in the same 

 district ; also that one and the same species may 

 suffer badly in one country and not in another 

 e.g. the Larch in the lowlands of Europe as 

 contrasted with the same tree in its Alpine home, 



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