RULES OF PROOF. 9 



DIRECT-INFECTION EXPERIMENTS. 



Direct-infection experiments will frequently separate out a parasite which is 

 overwhelmed by some saprophyte and thus furnish better material for plate-cultures, 

 and they are also sometimes very useful when one is remote from laboratories and 

 so situated that it is impossible to obtain pure cultures. It is, however, a crude 

 method and only to be employed when more exact methods can not be used or 

 would not serve as well. By "direct " infection is meant the transfer of fluids or 

 solids from the diseased plant directly into the tissues of the healthy plant, an effort 

 being made to include some of the supposed parasites in this transfer. It is a con- 

 venient expression and will be used often in this book. 



THE ORGANISM. 



This may be considered under three heads its ability to produce disease, its 

 form, and its physiological peculiarities. Many of the latter might equally well be 

 denominated cultural characters, and the pathogenic properties really belong under 

 physiology, but are kept distinct for sake of convenience and because they constitute 

 not only the most important attributes of the organism, economically speaking, but 

 also a distinct and peculiar phase of the investigation. 



PATHOGENESIS. 



What constitutes proof of the pathogenic nature of any organism ? Upon the 

 ability of the student to give a proper answer to this question depends very largely 

 his success or failure as an investigator. Henle perceived clearly what was neces- 

 sary as long ago as 1840, and Koch's rules are still fresh in the minds of all. There 

 is consequently now so good an understanding of this subject among animal patholo- 

 gists and professional bacteriologists that if this book were designed principally for 

 such persons no comment would be necessary. A glance, however, at the literature 

 of plant diseases shows that many of the writers on bacterial diseases of plants have 

 not had this professional training. The four cardinal requirements, as understood 

 by the writer, are as follows : 



RULES OF PROOF. 



(a) Constant association of the organism with the disease. 

 (b~) Isolation of the organism from the diseased tissues and careful study of the 

 same in pure cultures on various media. 



(c) Production of the characteristic signs and lesions of the disease by inocu- 



lations from pure cultures into healthy plants. 



(d) Discovery of the organism in the inoculated, diseased plants, re-isolation of 



the same, and growth on various media until it is determined beyond 

 doubt that the bacteria in question are identical with the organism 

 which was inoculated. 



