PREPARATION OF MEDIA. 71 



IV. Potatoes. 



1. Wash the potatoes thoroughly with water. 



2. By means of a borer cut out cylinders 3-4 inches in 

 length. 



3. Eemove the skin and cut each cylinder into two wedge- 

 shaped pieces. 



4. Place a small piece of cotton in the bottom of each test 

 tube and moisten it. This is to prevent the potato from dry- 

 ing out. 



5. Einse the wedges thoroughly in water and place one in 

 each test tube, 



6. Sterilize in the autoclave at 15 lbs. pressure for ten 

 minutes. 



V. "Mcmnite" Solution for Nitrogens-fixers. 



1. To one liter (1,000 cc.) of distilled water add the fol- 

 lowing substances : 



0.2 gm. K2HPO4. 



0.2 gm. MgSO,. 



0.02 gm. CaCla. 



1 drop 10% sol. Fe^Clo. 



15.0 gms. manuite. 



2. Stir until all the substances are dissolved. 



3. Add a few drops of phenolphthalein and then by means 

 of a pipette add 10% N'aOH, until the solution assumes a 

 very faint pink color. 



4. Sterilize in. the autoclave at 15 lbs. pressure for ten 

 minutes. 



