364 [ ASSEMBLY 
nature of the disease, and have shown that it may be easily conveyed 
from tree to tree of apple, pear and quince as already proven by 
Professor Burrill, and even to the English hawthorn, evergreen thorn 
and June-berry, and by inference to any pomaceons plant; that on 
the contrary it is not likely to thrive upon plants outside this group. 
I have shown that inoculations may be made not only in the stem as - 
practiced by Professor Burrill, but also in the leaf and fruit; that 
immature tissues take the disease most readily, and specially those 
of a succulent nature. Tee 
I shall now pass to the reasons for ascribing the disease to the 
agency of bacteria. In the first place it is well to bear in mind that 
bacteria are plants living upon organic matter, and the smallest of all 
living beings; they possess the power of independent motion like 
animalcules, reproduce principally by division, i. e., each divides into 
two, each of these into two, and so on, the parts formed at each divi- 
sion growing to be as large as the parent; that under some circum- 
stances, however, they reproduce by spores. 
If a thin slice of diseased pear wood, especially when taken at an 
active stage of the disease, be placed in a drop of water, a white 
cloud will be seen to emanate from it and spread through the water. 
If this be looked at with a magnifying power of about four hundred, 
diameters, the whiteness is resolved into minute rounded bodies 
somewhat longer than broad, and apparently colorless. These are 
bacteria, and occur in numbers that are absolutely inconceivable. 
They have been found to be a constant accompaniment of the disease, 
being the crucial test of its genuineness. But it may be urged that 
they are only the bacteria of ordinary putrefaction which invade the 
tissues as fast as they become weakened by disease. That the 
disease is inseparable from the bacteria, however, seems almost cer- 
tain from the fact that they are to be found in advance of any visi- 
ble change in the tissues. That they induce an incipient putrefaction 
is evident from the changed color of the tissues, the strong and 
peculiar odor by which an experienced person can detect the disease 
before seeing it, and by the remarkable readiness with which a di- 
seased branch will mould when placed in damp air. They are, how- 
ever, conspicuously smaller than the common putrefactive bacteria 
» (Bacterium Termo); neither can the latter be substituted for them, 
as was demonstrated by inoculating various plants, including pear 
and peach, with an infusion of the bacteria from rotting tomato in 
the same manner as for pear blight, and in every case without results. 
But it may still be urged that in inoculation it is not the bacteria 
which convey the disease but some poisonous principle in the 
mucous surrounding them or in the dead juices of the phan which 
kills the tissues and paves the way for the growth of the bacteria. 
Here it must be confessed is the weak point in the bacterial theory. 
It has been shown that the disease is infectious and always accom- 
panied by a specific bacterium, but it has not been shown that the 
bacteria when completely isolated from the fluids about them will 
convey the disease, or that, on the other hand, the fluids freed from 
