8 BACTERIA IN RELATION TO PLANT DISEASES. 
When all is said, the signs of many plant diseases, it must be admitted, are 
much alike, and this is particularly true of the bacterial soft rots. This is an added 
reason for studying them in each case as critically as possible. The captious reader 
might also remember that while an enormous amount of painstaking labor has been 
devoted to animal pathology, including twenty centuries in case of human medicine, 
we are only in the beginning, so to speak, of our knowledge of the minute pathology 
of plant diseases, and especially of those due to bacteria. 
PATHOLOGICAL HISTOLOGY. 
The relation of the parasite to the tissues of the host should be studied both in 
fresh material and in stained microtome sections made from material properly fixed 
and infiltrated with paraffin. ‘The organism may be a wound-parasite, or it may be 
able to enter through uninjured parts, z. e., in the absence of visible wounds. Often it 
affects special tissues or systems of tissues. Sometimes the bacteria are quite closely 
restricted to the vascular system, forming occlusions (figs. 1, 4, 5, 7,and 9). Some- 
times they spread widely in the intercellular spaces of the parenchyma, forming 
numerous cavities (figs. 2, 3, and 6). Sometimes there are striking reactions on the 
part of the host, e. g., an enormous multiplication of cells resulting in cankers or 
tumors (plate 2). The habits of the parasite and the behavior of the tissues of the 
host are best learned from serial sections. The student should not fail to preserve 
(properly labeled) in strong alcohol an abundance of typical diseased material for 
future study, exchange, or reference. Stained cover-glass preparations and stained 
sections should also be mounted in xylol-balsam, carefully labeled, and filed away. 
Neglect of these precautions prevents the experimenter from furnishing the con- 
vincing proofs in case his printed or oral statements are called in question. 
As to the best methods of f.xzug plant material containing bacteria much remains 
to be learned. ‘The writer has had best success with strong alcohol (go per cent to 
absolute) and with picric acid dissolved to saturation in absolute alchohol and used 
boiling hot. In general the watery fixatives can not be used because they do 
not hold the bacteria in place; even alcohol as strong as 70 per cent allows many 
kinds of bacteria to diffuse out into the fluid. Boiling absolute alcohol saturated 
with mercuric chloride is sometimes useful. The alcohol may be boiled in an open 
Erlenmeyer flask set on wire gauze on an iron tripod over asmall Bunsen flame. 
The alcohol is first brought to a boil. ‘The pieces of tissue are then thrown in and 
allowed to remain 3 to 5 minutes. It is better to divide the material into pieces 
suitable for embedding before fixing rather than after. Usually such a piece should 
not measure more than one-half square centimeter or one-half cubic centimeter. 
As far as possible only fresh material should be used for this purpose. Old material 
has often absorbed air in quantity sufficient to render infiltration with paraffin impos- 
sible or at least very difficult. In such cases infiltration in vacuo will often render 
good service. The writer uses a specially devised air-tight paraffin bath connected 
to the vacuum-pump. Even this device will not in every instance insure perfect 
infiltration. 
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