— 92 — . 



less predisposed to disease. This is due to the unna- 

 tural and excessive development of particular structures 

 as substances caused by high cultivation, and so pro- 

 ducing a general morbid condition of the plant, pre- 

 disposing it to disease whenever the conditions of 

 cultivation are too strongly or too suddenly opposed 

 to those of nature ; making exciting causes act with 

 great intensity whenever the predisposition exists. 



Modern investigations in vegetable anatomy and 

 physiology all point to a close analogy between vege- 

 table and animal life, and to a similar analogy between 

 many of the diseases which affect both of them, at 

 least in so far as such diseases produce disorganization 

 or destruction of the tissues. 



Just now I experienced by a microscopical inves- 

 tigation upom avocat blight {Persea gratissima) a 

 disease that is analogous to melanotic or black cancer. 

 The black knot in Zizyphus Jujuha (Masson) and Dico- 

 tyledone dulcis (Letchy) trees is certainly analogous 

 to a gangrenous ulcer. The disease known as the 

 " yellows " in peach trees is so similar in many of its 

 symptoms to syphilis that it may be called vegetable 

 syphilis. The deleterious effects of the gases escaping 

 into the atmosphere from chemical works in manu- 

 facturing certain chemicals is as injurious to vegetable 

 life as it is to animal life, and sometimes even more 

 so ; the liquid waste from our sugar manufactures 

 escaping into rivers or ponds is as destructive to the 

 acquatic plants therein as it is to the fish. 



