1180 Pear Blight and its Cause. [December, 
full name of M. amyhovorus Burrill. Members of this genus are 
globular, or somewhat longer than broad, either single or in 
short chains of a few individuals ; they multiply by a transverse 
division, each half growing to the size of the original from which 
it was derived, and finally becoming an independent cell. The 
formation of spores does not occur, or at least has not been dis- 
covered, in this genus; no transformation takes place. 
The pear blight Micrococcus is oval in outline and measures 
1 by 1%», which is .o0004 by .00006 inch. This is extremely small, 
so smail, in fact, that it would take more than a thousand placed 
end to end to reach around the period at the close of this line. 
They are considerably smaller than the bacteria of common 
- putrefaction (Bacterium termo), the hay bacteria (Bacillus subtilis) 
which usually appear in all sorts of vegetable infusions, and 
many others. On the other hand they are by no means so small 
as the germs of diphtheria (Streptococcus aiphtheriticus), of small- 
pox (S. vaccine), and others. When in active growth they are 
single or attached to each other in pairs, dumb-bell form; when _ 
growing more slowly there is a larger proportion of the dumb- 
bells, and in addition chains of four to six cells each. 
Although these bacteria are very small, yet there is no diffi- 
culty in demonstrating them in the tissues of the pear tree on 
account of their extraordinary abundance. The slightest frag- 
ment taken in midsummer from a blighted pear limb and placed 
in a drop of water will readily enough show the presence of bac- 
teria to the unaided eye by the white cloud of them which spreads 
from the bit of wood throughout the water. Slice up some 
blighted wood into a small quantity of water and it will be ren- 
dered milky by the enormous outpouring of the bacteria. Under 
the microscope these are found to be all of one kind, and not in- 
_ termixed with ordinary putrefactive or other bacteria, a circum- 
_ stance which may yet be turned to account in studying problems 
-~ _ in which an admixture of forms is detrimental. The exudation 
already referred to is found by the microscope to be composed 
_ solely of blight bacteria and a soluble substance which holds 
m together. 
~ 
ien in very vigorous growth the blight bacteria are active, 
present an animated appearance under the microscope, but 
ily pe molecular movement common to all 
fluid. Their progress rough the 
