260 BACTERIOLOGY. 



number of bacterial and fungous diseases of plants are 

 known at the present day. The various species of the 

 vegetable kingdom, just as man and the lower animals, 

 are subject to their peculiar infectious diseases. 



Methods of Infection. 



As indicated in the preceding part, the detection of 

 an organism in every case of a given disease does not, in 

 itself, constitute a proof that it is the cause of that disease. 

 It is necessary to carry the investigation farther in order to 

 furnish an indubitable proof. The second rule of Koch 

 requires that the suspected organism shall be isolated, in 

 other words, obtained in a condition of absolute purity. If 

 the pure culture of the suspected organism, isolated accord- 

 ing to the methods heretofore described, on inoculation into 

 susceptible animals reproduces the characteristic symptoms 

 and pathological lesions of that disease there can be no 

 escape from the conclusion, that the organism tested is the 

 causal factor of the disease in question. 



Extreme care must be taken in order to obtain an abso- 

 lutely pure culture. Likewise, the utmost care must be 

 employed, when making the inoculations of animals, to 

 prevent the accidental introduction of foreign organisms. 

 Careless inoculation may give wholly misleading results. 

 All operations on animals, even the most trivial, should be 

 carried out under as nearly sterile conditions as possible. 

 Every instrument employed should be sterile, and, while it 

 is not possible to render the hands of the operator and the 

 skin of the animal as free from organisms as in the case of 

 an instrument, yet nothing should be left undone to decrease 

 the number of bacteria present and to lessen thus the danger 

 of mixed infection. 



The experimental methods of infection are . numerous. 

 ;Some are exceedingly simple, whereas others entail a more 



