1887.] MICROSCOPICAL JOURNAL. 89 
and from the Rocky Mountains to the Atlantic Ocean, but is usually rare upon 
the sea-coast. The disease is not recorded in Europe, and it is only within 
a year that European authorities have become acquainted with it. In the 
years of 1826 and ’32 the blight became a wide-spread epidemic, and 
in 1844 few orchards escaped without some loss, and many were entirely 
destroyed. Early conjectures regarding the cause of the disease were not 
satisfactory. One, that of Cox,-supposed it due ‘ to the hot rays of the sun 
acting through a misty atmosphere deranging the vital activities of the plant.’ 
Another assigned it to the work of insects, founded on the discovery of a 
small brown beetle, Xyleborus pyrz, still known as the blight-beetle, which 
penetrated the branch and caused the part beyond it to die. Another theory 
is talled the frozen-sap theory ; it held that the freezing of unripe wood gen- 
erated a poison, which was distributed the next spring and summer, causing 
death. This was first published by the late Rev. Henry Ward Beecher, in 
Flovey’s Magazine,in 1844. In 1863, Dr. J. H. Salisbury figured a fungus 
which he decided was the specific cause of the blight. The last hypothesis of 
importance is that of Prof. T. J. Burrill, who, in 1878, stated his belief that 
the disease was bacterial, and the presence of which in affected parts he ob- 
served in great numbers. He made a series of experiments by inoculation, 
and presented the results of his investigations at the Boston meeting of the 
American Association, and extended and confirmed his result in 1884. 
The question of the cause of pear-blight was finally removed from the 
domain of speculation, in 1885, by the writer by a series of crucial experi- 
ments. These consisted in showing that the bacteria, when removed from the 
tree and passed througha series of artificial cultures, would generate the dis- 
ease when again introduced into the tree, but the juices accompanying blight 
will not produce the disease if inoculated after the bacteria have been removed 
by filtration. 
2. Life history. 
Prof. Burrill first recognized the bacteria of blight in 1877; in 1882 he 
characterized the organism as A/zcrococcus amylovorus. Its form under 
varying conditions is very constant. Single cells are from oval to roundish- 
ovoid, and only yary by slight changes in the ratio between their length and 
breadth. Length 1 to 14», breadth 4 to 2»; quite colorless. They exist 
usually as single independent cells, but often are in pairs, especially when 
multiplying, or even in series of 4 or 5, but never extend into chains. 
During rapid growth in rich media they exhibit rapid swarming movements, 
never moving in a straight line for any great distance. ‘These movements 
become slower as the rate of growth decreases. 
The most characteristic feature in the life-history of AZ. amylovorus is the 
formation of the zoogloea, which occurs with regularity in the fluid cultures, 
though it has never been observed in the tree or upon solid culture-media. 
They are produced through the fluid, but are most abundant in the thin pel- 
licle which forms on the surface within about 48 hours from the beginning 
of the culture. The substance of the pellicle is a colorless matrix filled with 
motionless bacteria, and against this the zooglaa are sharply defined. They 
are brought out even more distinctly by the surrounding colorless layer free 
from bacteria, which is doubtless an extension of the ground substance of the 
zoogloea mass. 
The masses are far more dense than the pellicle, and compactly filled with 
bacteria, which have become highly refractive. They are oblong or globular in 
form, and may be single or united into chains of two or more placed end to end. 
When the length of the mass reaches 20y, the mass, from a smooth surface, 
acquires a wrinkled, folded surface eventually, something like the external 
