40 BACTERIA IN RELATION TO PLANT DISEASES. 
important fact that an unusually large number of poor researches in this field obscured 
the general view and covered the whole subject so to speak, with a wet blanket. 
THE CARRIERS OF INFECTION. 
This subject is now widely investigated and popular, particularly in relation to the 
spread of human diseases, but when the first evidence was obtained showing that bacterial 
diseases of. plants are transmitted by insects almost nothing was known. 
The first exact experiments were by Merton B. Waite in 1891. That year he proved 
conclusively that pear-blight is disseminated by bees in course of their visits to pear blos- 
soms for nectar and pollen. 
In 1893, the writer obtained some evidence that the bacterial wilt of cucurbits is 
transmitted by beetles, and some years later established the fact conclusively. 
In 1895, the writer obtained very typical cases of the bacterial brown rot of the potato 
using the Colorado potato beetle as the agent of transmission. 
In 1897, the writer showed that the bacterial black rot of crucifers could be trans- 
mitted by insect larvae (Plusia) and by molluscs (Agriolimax) and pointed out that there 
was no evidence of transmission of the disease by wind. Brenner confirmed a part of this 
and incriminated aphides. 
More recently gall-forming nematodes were observed by Hunger in Java (in 1901), 
and by the writer in the United States (in 1908), to function as carriers of a bacterial 
disease of tomato, tobacco, etc. 
In 1910, D. H. Jones, in Canada, proved apple-blight (Bacillus amylovorus), to be dis- 
seminated from diseased to healthy shoots by aphides and by bark-boring beetles 
(Scolytus). 
There is therefore every reason to believe that small animals play a large part in the 
dissemination of these destructive diseases. Elsewhere full details are given. 
SPECIFIC DISEASES. 
Admitting the parasitism of bacteria in plants, are there any specific diseases? The 
physician depends to a considerable extent on subjective symptoms for his diagnoses. 
Headaches, pain in various organs, etc., give him many clues. He also has his clinical 
thermometer. There is nothing in plants, however, so far as we know, corresponding to 
the rise in temperature which we call fever. The plant pathologist must depend entirely 
on objective signs—spots, stripes, distortions, enlargements, atrophy, yellowing, sudden 
wilting, etc. Moreover, the plant body being much less highly organized than the body 
of man and the domestic animals, one might expect less differentiation in objective signs 
due to the action of various parasites and to a certain extent this is true. For instance the 
soft-rot bacteria all produce much the same set of phenomena and are capable of attack- 
ing plants belonging to widely separate groups. There are other organisms, however, that 
seem to be restricted to particular families and the morbid phenomena which they origi- 
nate can scarcely be mistaken for diseases due to any other micro-organism. Pear-blight 
is a good example of such a disease. We know only one organism capable of causing this 
train of phenomena. In like manner, so far as we know, only one organism is able to 
cause the bacterial wilt of cucumber, only one is able to cause the olive-tubercle. These 
three diseases are restricted so far as known, to as many families of plants, and there are 
also restrictions within each family, not all genera or species being susceptible. In this 
respect the causes of these three diseases are very different from the soft-rot organisms, 
the action of many species of which overlap, e. g., we may have a soft-rot of the potato or 
cucumber due to half a dozen different organisms, the signs being essentially the same. 
From this point of view these, therefore, are the lowest type of bacterial parasites. 
_A third type of organism is able to produce quite specific over-growth phenomena in a 
