THE STUDY OF MICROORGANISMS IN THE SOIL 109 



bacteria produce a strong acid reaction from glucose, medium 

 acid from sucrose and little or no acid from xylose. Soybean 

 and cowpea bacteria produce a strong alkaline reaction in these 

 culture media. 



Prepare the following stain: 



Barlow Stain for Root Nodule Bacteria 



Glucose 50 . gm. 



Glycerol 50 . cc. 



Distilled water 50 . cc. 



Gentian violet 3.0 gm. 



Dissolve the glucose in the glycerol-water 

 solution by heating and then add the 

 Gentian violet. Bring this mixture to a 

 boil and allow to cool. 



Negative Stain. — Place a loopful of the 

 gum from an agar slant culture on a clean 

 slide and rapidly whip out into long thin 

 streaks. Dry in the air. 



Flood with the Barlow stain, let stand for 

 30 seconds to 1 minute and wash off in run- 

 ning water. Blot off the excess water and 

 dry quickly with gentle heat. 



Positive Stain. — Prepare the slide as 

 above. 



Flood the slide with water and run the 

 stain into this water at one end of the slide. 

 As soon as the stain has diffused through 

 the water, wash in running water and dry 

 quickly. 



Exercise 20 



The Formation of Root Nodules in 

 Agar and Sand Cultures 



Wash thoroughly the seed of alfalfa, 

 clover, and similar plants, which are to be 

 grown in agar under aseptic conditions, in 



, ,. . ^ . 1, ., Fig. 7.— Growing plants 



water and immerse m mercuric chloride free of bacteria. A Pyrex 

 solution (1:500) for 2 to 3 minutes. To ^l^^^ cylinder 4 inches in 



,^ , , 1, i , ,1 1 .1 diameter and 24 inches tall 



secure the best results, treat the seeds with covered with a beaker. 



