CHAPTER III 



DISEASE AND DEATH OF TREES— GENERALI- 

 TIES 



OR normal development trees require, as we have 

 seen, light, heat, water, air, and nutritive ele- 

 ments in certain amounts. If any of these 

 conditions or elements are temporarily deficient 

 (or sometimes when in excess) the normal functions are 

 interfered with, and the tree may be ailing, although not 

 necessarily sick, for it ma}^ recover its normal condition as 

 soon as the deficiency is corrected. 



We speak of disease only when li\'e parts fail to perform 

 their normal functions and begin to die before their time, 

 that is to say, when buds, rootlets, or cambium die at any 

 time, and lea^'es change color, become dry, and fall before 

 the end of the season. 



Since, as we have seen, the li\ing parts of the tree lying 

 on its periphery rejuvenate themselves every year by the 

 formation of new shoots, buds, cambium and rootlets, there 

 can be actually no natural death from old age in the same 

 sense as in animals. Even death from internal causes as a 

 direct result of disease, at least sudden death, is rare. 



Trees die mostly as a result of unfa\'orable external con- 

 ditions, whicli interfere with tlieir nutrition, and which are 

 generally capable of control. As a rule, they succumb so 

 gradually that they actually die by inches; it may take many 

 years before all fife is gone, and hence there is time for 

 recuperative measures. 



29 



