208 PLANT LIFE. 



material. This colony is then weakened in constitution 

 by bad treatment of some kind. It is exposed to tem- 

 peratures which do not suit it, or it is made to develop 

 on substances which do not agree with it. The result 

 is, that each bacterium of the colony is less vigorous and 

 virulent than the original type. If a chicken is inocu- 

 lated with this weak type of the bacterium, it will 

 develop probably only a mild form of the disease. Yet 

 even these weak germs will probably devour all the 

 food which is taken by the chicken-cholera bacterium, 

 and they will pollute the body of the chicken with their 

 excretions. If after this the chicken is accidentally 

 infected by the original wild type, the latter cannot 

 develop, because there is no nourishment in the body of 

 the chicken for it, and because the excretions of the 

 previous forms, already in the body, are poisonous to it. 

 The whole of the theory and the methods of bacteri- 

 ology are quite modern, and already the result has been 

 an extraordinary diminution of human suffering. Thus 

 of 1465 persons treated by Pasteur for Hydrophobia or 

 Rabies, only four died. 



It is, however, also possible to make Bacteria by 

 cultivation twenty times more deadly than they are in 

 nature (Bullock). 



Plants also have their bacterial diseases such as the 

 slime flux of trees and another which attacks the 

 Hyacinth. This has been denied by some authorities, 

 but has recently been abundantly confirmed by many 

 experiments. The distinction between the disease 

 forms and those which are found in decaying material 

 sometimes breaks down. Thus B. colicommunis, which 

 generally lives on dead matter, may become parasitic 

 and produce a disease in living potatoes. 



A very simple example will make it manifest that 



