106 BACTERIAL DISEASES OF PLANTS 



climates. The student, therefore, must learn how to prepare it. 

 For details of preparation of both agar and gelatin, consult 

 ''Bacteria in Relation to Plant Diseases," Vol. I, pp. 29 to 36. 

 Our Standard agar is +15 and our Standard gelatin +10 on 

 Fuller's scale, or 1.5 per cent and 1 per cent respectively, 

 if reckoned on 100-cc. portions. It is better to keep to Fuller's 

 scale since we make up media in liters not in 100 cc. portions. 



We still use Nelson's photographic shredded gelatin for our 

 gelatin culture media and for our agar media a powdered agar 

 made by Kahlbaum in Berlin, and have no trouble in filtering 

 either one or in obtaining a uniform and clear product. Diffi- 

 culties of agar filtration may usually be traced to the fact that 

 the medium has not been cooked sufficiently. The work must 

 then be done over. It is a lazy bacteriologist who is content 

 with a clouded culture medium, either fluid or solid. 



Peptone-water Media. — For the study of the behavior of 

 organisms in contact with various sugars and alcohols we use 

 distilled or river water to which has been added 1 or 2 per cent 

 Witte's peptone and the desired amount of the carbon food, 

 seldom more than 2 per cent. It is steamed, filtered, filled into 

 fermentation tubes and sterilized discontinuously as in the case 

 of other substances. Comparison should be made using other 

 peptones, Merck's, Difco, etc. 



Excess of acidity or alkalinity of medium should be corrected 

 by determining the actual acidity or alkalinity by titration, 

 using phenolphthalein and N/20 sodium hydrate or N/20 hydro- 

 chloric acid, as the case may be, and then adding calculated 

 quantities of a much stronger acid or alkali and re-titrating. 

 The latter must not be neglected. Without titration it is impos- 

 sible to have any two batches of media alike, but when it is regu- 

 larly employed there is great uniformity in the product of the 

 laboratory, and in the results obtained. Our method is to put 

 50 cc. of boiling distilled water in a white capsule with >2 cc. of 

 the phenolphthalein solution and 5 cc. of the substance to be 

 tested and run in barely enough alkali to give a trace of pink color 

 which is the neutral point. Do not re-boil, nor make the solution 

 red. The slightest pink that can be seen is the place to stop. 



Synthetic Media. — Certain synthetic media have great use 

 in the bacteriological laboratory because micro-organisms be- 



