CHAPTER IX 



PATHOLOGY 



EVERY living organism is liable to have the balance of 

 its delicate mechanism disturbed by some cause or 

 another, and plants, no less than animals, suffer from 

 a variety of such causes which destroy utterly, or merely 

 locally affect their lives. The diseases of plants have 

 not yet been studied so elaborately as those of animals, 

 and " doctors " generally confine their attention to 

 the higher vertebrates, but, nevertheless, there is a 

 great mass of facts which have been -accumulated about 

 the various parasites and diseases which attack the 

 vegetable world. 



Accidents, like broken limbs or wounds caused by 

 stones or sharp instruments, happen to plants as they 

 do to animals. In such a case, if the individual to 

 whom the accident happens is normally healthy, the 

 tissues respond and attempt to heal the gap or to mend 

 the fracture. In the case of trees such wounds arise 

 oftenest by the felling of a trunk or by the snapping of 

 a branch in a gale. The broken surface exposes inner 

 tissues to the atmosphere, laden, even in the woods, 

 with germs and microbes of disease, and the first essential 

 is that the broken surface shall be covered. The plant 

 makes an effort to do this by the growth of " callus." 

 In the neighbourhood of the wound the cells are stimu- 

 lated to divide and grow rapidly, and they attempt to 

 form a healing tissue across the surface of the wound. 



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