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‘blight rests, are grasped by this part of the public. This is 
brought forcibly to mind by an item in the last number 
that has come to hand of a leading horticultural journal, 
‘which says ‘‘the germs of these low orders of vegetation 
exist everywhere; . . . inoculation of a healthy sub- 
ject is not demonstration; . . . if any other demon- 
stration has been offered it has not come under notice,” etc. 
If the leaders among horticulturists can come so far short 
of appreciating the bearing of the facts which demonstrate 
that specific bacteria cause pear blight, it cannot be amiss 
‘to briefly go over the argument again. 
Proof that specific bacteria cause pear blight.—(a) Bac- 
teria are found in great abundance in actively blighting 
tissues, so as to be easily demonstrable to the naked eye, 
and occur in less abundance in proportion as the disease is 
less active. (b) The disease may be introduced into healthy 
tissue by inoculation with germs from diseased tissue. (c) 
It is communicated with equal certainty when the germs 
are separated from all accompanying juices of the diseased 
tissue by a series of fractional cultures. (ad) Per contra, it 
is not communicated by the juices of the disease, after the 
germs are removed by filtration. (e) The germs connected 
with the disease constitute a single species, which is essen. 
tial to successful inoculation. (/) Per contra, the numerous 
other species of earth, air and water are not found toa 
noticeable extent in connection with the disease, and can- 
not be made to originate it by inoculation, or otherwise. 
The experimental data, on which these several proposi- 
tions are founded, have been abundantly set forth in the 
two reports preceding the present, and repetition is unneces- 
sary. Ifthereisa weak point in this argument, it is in con- 
nection with the third statement (c). It is clear that frac- 
tional cultures (which in this case extended over a period of 
three months) do after a time rid the bacteria of all extra- 
neous matter, both diffusible and non-diffusible, originally 
accompanying the germs used to infect the first culture of 
the series. Butit is possible some product may subsequent- 
ly be formed in the cultures by the activity of the bacteria, 
which, introduced into healthy tissue with the inoculating 
drop from the last culture, acts as the real inciting agent, 
instead of the bacteria. Several bacterial diseases of ani- 
mals have been shown to beaggravated or possibly induced 
‘by alkaloid poisons which result from the growth of the 
bacteria. If this were true in pear blight, the third proposi- 
tion, taken by itself, would lose its force. Some plausibility 
that this is the case, is found in the oft repeated expression 
of borticultural writers that the disease is due to “ poison ”’ 
or ‘poisonous sap” in the tree. If such poisons really ex 
ist, they would belong, so far as present knowledge indi- 
