BACTERIA 
gained access to rotting tissues, injured 
by other agents. 
“Saprophytic bacteria can readily make 
their way down the dead hyph (branch) 
of an invading fungus, or into the punc- 
tures made by insects, and aphides have 
peen accredited with the bacterial infec- 
tion of carnations, though more recent 
researches by Woods go to show the cor- 
rectness of his conclusions that aphides 
alone are responsible for the carnation 
disease On the other hand, recent in- 
vestigations have brought to light cases 
in which bacteria are certainly the prim- 
ary agents in the diseases of plants. The 
principal features are the stoppage of the 
vessels, and the consequent wilting of the 
shoots; as a rule, the cut vessels on the 
transverse sections of the shoots appear 
brown and choked with a dark yellowish 
slime in which bacteria may be detected; 
examples in cabbages, cucumbers and po- 
tatoes.” 
A familiar example is that of the com- 
mon blight of apple and pear. See article 
on this subject under Pear. 
Prof. Ward observes further: 
“In the carnation disease and in certain 
diseases of tobacco and other plants the 
seat of bacterial action appears to be the 
parenchyma, and it may be that aphides 
and other piercing insects infect the 
plants, much as insects convey pollen 
from plant to plant, or (though in a dif- 
ferent way) as mosquitoes infect man 
with malaria. If the recent work on 
cabbage diseases may be accepted, the 
bacteria make their entry at the water 
pores at the margins of the leaf, and 
thence by way of the glandular cells of 
the tracheids. Little is known of the 
mode of action of bacteria on these 
plants, but it may be assumed with great 
confidence that they excrete enzymes and 
poisons, (toxins) which diffuse into the 
cells and kill them, and that the effects 
are in principle the same as those of 
parasitic fungi. Support is found for 
this opinion in Beyerinck’s discovery that 
the juices of tobacco plants affected with 
a disease known as ‘leaf mosaic,’ will 
induce this disease after filtration 
through porcelain.” 
565 
Method of Study and Discovery 
How to discover the specific cause of 
any disease, is a question of the greatest 
importance. This is done generally by 
three steps: 
First. The discovery of disease in the 
affected tissues. 
Second, Obtaining the bacterium of 
this disease, in pure culture. 
Third. The production of the disease, 
by inoculation, with a pure culture. 
By means of microscopic examination 
more than one organism may sometimes 
be observed in the tissues, but one single 
organism by its constant presence and 
special relation to tissue changes, can 
usually be selected as the probable cause 
of the disease, and attempts towards its 
cultivation can then be made. 
In cultivating bacteria, outside the 
body from which it is originally taken, 
the food material in which it is cultivat- 
ed, must first of all be sterilized by heat. 
This food material should be as nearly 
like that of the body from which it is 
taken, as possible. The media are used 
either in a fluid or solid condition, and 
are placed in glass tubes or flasks plugged 
with cotton-wool. 
Inoculation 
In testing the effects of bacteria by in- 
oculation, young and vigorous cultures 
must be used, and in the case of plants, 
injected into the sap at some point where 
the cells are vigorous, or in the case of 
animals, by means of a hypodermic 
syringe into the subcutaneous tissue into 
the vein, into one of the serus sacs, or 
more rarely into some special part of the 
body. 
Immunity 
By immunity from disease, we mean, 
non-susceptibility; or not being suscept- 
ible by means of infection, contagion or 
inoculation. The entrance of a bacter- 
ium or any number of bacteria into the 
tissues, does not mean disease, necessari- 
ly. Even though the bacteria be virulent 
or poisonous, the plant or the person may 
have resisting power sufficient to neutral- 
ize its effects. With regard to diseases 
in persons, it has been shown that cer- 
tain races are practically Immune from 
