




247 
gelatine about it as is usual with other kinds, and the colony did not 
long continue to grow. It is not, however, necessary to go into the 
details of these researches in this connection, the main economie 
deduction to be drawn is that the germs will grow in dead organic 
matter outside the tree. This is a more important fact than would 
appear at first thought. So lohg as it was not recognized, the mode 
of transmission of the disease remained obscure. Observation and 
experiment alike showed that direct infection from a neighboring 
tree or branch, even though the diseased branches touched the 
healthy ones, rarely, if ever, occurred ; the bacteria are imprisoned 
by the bark, or escape in a viscid substance that dries into a hard 
gum, and hence do not pass directly from the tree into the air; 
lastly, it has been shown that the germs enter the tree in early spring 
when but little living pear blight is to be met with. But, knowing 
that the germs may be propagated in hay infusion, and so far as ex- 
periments yet go, without losing any of their virulency, makes it 
clear that a marsh or any wet land rich in organic matter may sus- 
tain the disease, presumably for years, and aided by favorable winds 
and other conditions be the origin of an epidemic. 
The artificial cultures do not develop any transformations in the 
bacteria other than the formation of zoogloa, which are usually lob- 
ulated masses of definite outline. If a pellicle is formed on the 
liquid, which happens infrequently, these masses occur in it, but 
they are also scattered through the liquid. Apparently no spores 
are ever formed. 
Some attention has been given to the nature of the chemical 
changes which the bacteria bring about in the plant. The first and 
most obvious of these is the disengagement of carbon dioxide (car- 
bonic acid gas). Slicing up freshly blighted pear wood or fruit into 
a narrow vessel and just covering with water, bubbles of gas will 
begin to accumulate in course of an hour and gradually rise to the 
surface. If after atime the gas which has collected above the liquid 
be tested with a drop of lime water it will give the characteristic 
white film produced by carbon dioxide. 
The change has been said to give rise to butyric acid, but the most 
careful tests failed to disclose any trace of it. They were made by 
cutting up a portion of freshly blighted pear limb or fruit, adding 
alcohol and an excess of concentrated sulphuric acid, and heating. 
Although a peculiar fruity odor was developed, it assuredly was 
not that of butyric ether, as would have been the case had any 
quantity of butyric acid been present. A sound pear under the 
same treatment gave lke results. 
It is frequently called a putrefactive fermentation, especially by 
those who look upon the bacteria as accompanying and not causing 
the disease, and hence regard their presence asa mere matter of 
course. ‘There is, however, no good evidence to’support the pre- 
sumption. <A weak, not disagreeable but peculiar odor, is percepti- 
ble, but it is not the odor of rotting substances. Neither the bulk 
or time of keeping changes the quality of the odor until other bacte- 
ria have taken,possession of the mass. The taste of a blighted 
fruit is not offensive, nor is it peculiarly sweet or sour, but bland and 
indifferent. This indicates that it is not a putrefactive or even an 
