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. : 
(4) 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. 
