28 



BACTERIA IN RELATION TO PLANT DISEASES. 



on by severe reagents, when they may be stained in mordanted solutions or in dyes 

 which have been preceded by a mordant. The outer wall of the bacterium generally 

 reacts to stains in the same way as the flagella, /. e., it usually remains unstained. 



Staining media may be roughly divided into four groups : (a) Simple stains 

 dissolved in water, e. g., fuchsin (basic), gentian violet, methylene blue ; (b} alcoholic 

 solutions and various complex stains, c. g., saturated alcoholic solutions of anilin 

 dyes, alcohol-iodine, iodine potassium iodide, Russow's cellulose test, Ziehl's carbol- 

 fuchsin, L/oeffler's alkaline methylene blue, Ehrlich's anilin-water gentian violet, 

 Gabbett's stain, Gram's method, Delafield's hsematoxylin, Ehrlich's acid hsema- 

 toxylin, Heidenhain's iron-hsematoxylin, Fleming's triple stain; (r) flagella and 

 capsule stains, c. g., Loeffler's stain, Moore's modification, Fischer's modification, 

 Bunge's stain, Lowit's stain, van Ermengem's nitrate of silver method, Zettnow's 

 gold method, etc.; (</) stains for spores, e. g., prolonged exposure to simple stains 



dissolved in water (which should 

 have little effect), steaming carbol- 

 fuchsin with methylene blue for 

 contrast, etc. (see " Formuke " and 

 Bibliography of General Literature 

 under "Flagella," "Spores," etc., for 

 various observations on staining). 



Griibler's stains are preferred. 

 Cover-glasses should be clean and 

 free from fat, traces of which are 

 easily removed in a Bunsen flame. 

 A little experience is necessary in 

 flaming thin covers in order not to 

 crack them. They may be passed 

 through the flame three times, wait- 

 ing a moment or two after each pass 

 for them to cool. The student should 

 see that the water used in making 

 F>g- 27.' the cover-glass preparations or the 



stains does not itself contain bacteria. It is usually wise first to dry a drop of 

 the water on the cover and stain without addition of the bacteria. Eternal vigi- 

 lance is the price of trustworthy results. It is best to make all mounts on cover- 

 glasses of a known and uniform thickness (o. 15 mm.). Many a good preparation has 

 been spoiled for examination with lenses of a short-working distance by mounting 

 under a thick cover-glass, and sometimes the lens itself has been ruined in the 

 attempt to focus. The thickness of covers often varies greatly from the statements 

 of dealers, and they should not be accepted until tested with a reliable cover-glass 

 measurer (fig. 27). 



-T- Zi/iss cover-glass measurer. The cover in place shows a registered thickness of o.lS 

 mm. Fractions of an inch are also registered on this instrument. 



