— 10 — 
1,000 diameters. It is only men trained in the use of the 
microscope that can carry. on observations under these con- 
ditions. The growth of an organism in a culture fluid is 
readily observed by the naked eye, by reason of its action 
on the fluid, and the results obtained by inoculation are 
easily seen. These two points must serve to inspire confi- 
dence in the statements of the microscopist regarding what 
takes place beyond the range of natural vision. The specific 
name, amylovorus, given by Professor Burrill, to this organ- ^ 
ism, means starch-devouring, and was given because the 
removal of starch from the cells appears to be the work 
they perform. In the process, which is a true fermentation, 
carbon-dioxide is given off, and butyric acid is formed. 
EPIDEMIC NATURE OE THE DISEASE. 
Like all diseases which have been traced to an origin 
in low forms of life, pear blight is epidemic in its character. 
During certain seasons it is very destructive; this extreme 
virulence may last two, three, or four years, then the dis- 
ease will decrease, or possibly pass away entirely, to appear 
again after a long interval. 
Charles Downing says, in speaking of his locality: “Pear 
blight has appeared at intervals of about twenty years, and 
the duration of each has been from three to five years. I 
have passed through three of these periods, and with each 
additional visit the attack is very much lighter ; and like 
many other diseases it may run itself out in time.” Mr. 
Downing’s statement was made before the true cause of the 
disease was known. There does seem to be a periodicity 
connected with the disease, and while we are likely to have 
intervals of immunity, I have no faith in its finally running 
out. 
xMEANS ON DISSEMINATION. 
1 low does the disease get into the tree, and how is it 
carried from one tree to another? First as to its dissemina- 
tion. Whether the germs of the disease are carried in the 
air or not has not yet been satisfactorily demonstrated, but 
it is well known that insects carry the disease, and that in 
them we have the chief means of its dissemination. The 
gummy exudation already alluded to, which is commonly 
present in cases of attack upon the trunk or larger branches, 
is shown by examination to consist of myriads of the living 
organisms, held together by the viscid secretion which seems 
so characteristic of their work. This exudation is most 
abundant in the spring after the tree has started into full 
