90 THE AMERICAN MONTHLY [May, 
aspect of the brain, but at first better described as mulberry-like. Whatever 
variation may take place, this peculiar cerebric surface is an unfailing char- 
acter, and not known to occur in the case of any other bacteria. 
The range of fluid culture-media is very great. An infusion of almost - 
any vegetable substance containing a fair amount of soluble carbo-hydrate is 
likely to furnish sufficient nourishment for growth. The potato has proved 
most satisfactory. The infusion is prepared by slicing in 3 or 4 times its 
bulk of water, warmed to 70° C. and kept so for a couple of hours with oc- 
casional stirring, then filtered, and placed in culture vessels for sterilizing and 
use. The resulting liquid is clear and watery ; iodine gives a blue coloration, 
proving the presence of starch; corn-meal similarly treated gives an almost 
equally good culture fluid. Media which are not well suited for the culture 
of the bacteria interfere with the growth of the pellicle, which may not form. 
The presence of 3 per cent. of malic acid prevented the growth of zooglaa 
and the pellicle, but gave a sediment formed of groups of blight bacteria 
of the brilliantly refractive kind found normally on the borders of the pellicle. 
Two per cent. malic acid gave similar results with less abundant sediment. 
In test tube gelatin cultures the most characteristic results have been obtained 
by adding bacteria to liquid gelatin, then thoroughly shaking the tube. In 
from 2 to 4 days the gelatin contains numerous small white dots, which, on 
examination, prove to be masses of bacteria. The dots are globular or oval, 
and increase to about .5 mm. No further growth takes place, and they remain 
for weeks without liquefying the gelatin. When sown on the surface of gela- 
tin the growth is feeble and does not amount to more than a slight shining 
appearance. No success was obtained by the use of agar-agar. The most 
successful opaque solid cultures have been upon freshly-gathered unripe pears. 
Slices of these placed under a moist bell-jar and infected by touching with 
a needle which had been dipped into some substance containing the bacteria. 
In 2 or 3 days fine milky drops like beads of dew appear scattered over the 
surface for 5 mm. or more about the infected spot. In slices of baked or 
boiled potato the bacteria do not grow readily. The opaque solid cultures 
prove very distinctly that 17. amylovorus requires a large supply of water for 
its best development, a fact which has an economic bearing. 
The most successful stainings have been made with a watery solution of 
Bismarck-brown in cover-glass preparations, but no staining has been dis- 
covered by which to distinguish this bacteria from other micrococci. 
The chemical changes brought about by the agency of the bacteria are not 
entirely understood. The most obvious product is carbon dioxide, which often 
passes off so freely as to produce slow effervescence. Butyric acid and alco- 
hol are formed in very small quantities, if at all. Glucose does not seem to 
be formed, and there is a diminution in the amount of sugar. Tests made to 
detect the presence of a poisonous ptomaine, the early assumed cause of the 
disease, gave negative results in every instance. 
3. Action on the living plant. 
The bacteria have the power of growth and multiplication in the presence 
of the living cells of the pear, and in this one important respect differ essen- 
tially from other species of bacteria, as has been proven by experiment. It 
is upon this that the rapid progress of the disease depends—the blight batte- 
rium extending far out among the living cells gradually kills them, and then 
other forms begin their work and escape destruction. One property which 
enables this species to so successfully penetrate the pear tree is its indifference 
to acids which prevent most other forms from making any growth ; the juices 
of the pear give a strong acid reaction with test-paper. .The chemical changes 
brought about in the plant by its activity cannot be definitely stated, further 
than to say that a mucilage or gum, which is soluble in water, is produced in 
an soi 
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