CHAPTER X. 



HEALTH AND DISEASE. 



Variation Disease Comparison to a top. Health 

 Extinction of species Natural demise. Examples of 

 complex interactiotis in health Interference, and 

 tende?icies to ill- health. 



When we come to enquire into the causes of 

 disease, it appears at first an obvious and easy 

 plan to subdivide them into groups of factors 

 which interfere with the normal physiology of the 

 plant. Scientific experience shows, however, that 

 the easy and the obvious are here, as elsewhere 

 in nature, only apparent, for disease, like health, 

 is an extremely complex phenomenon, involving 

 many reactions and interactions between the plant 

 and its environment. If we agree that a living 

 plant in a state of health is not a fixed and 

 unaltering thing, but is ever varying and under- 

 going adaptive changes as its life works out its 

 labyrinthine course through the vicissitudes of the 

 also ever-varying environment, then we cannot 



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