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, /. ^., 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, i, 4, 5, 7, and 9). Some- 

 times they spread widely in the intercellular spaces of the parench}'ma, forming 

 numerous cavities (figs. 2, 3, and 6). Sometimes there are striking reactions on the 

 part of the host, c. 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 pi'eserve 

 (properly labeled) in strong alcohol an abundance of typical diseased material for 

 future study, exchange, or reference. Stained co\'er-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 oi fixing plant material containing bacteria much remains 

 to be learned. The writer has had best success with strong alcohol (90 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 a small 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 tliis device will not in every instance insure perfect 

 infiltration. 



