PREPARATION OF POTATOES 



119 



the correct proportion of the ingredients, is their reaction. 

 It must be neutral or very slightly alkaline to litmus. (See 

 remarks on Neutralization of Media.) Only a few organisms 

 develop well on media of an acid reaction. 



FIG. 18 



PREPARATION OF POTATOES. 



With an ordinary cork borer punch out from sound 

 potatoes cylindrical bits that will slip easily into the test- 

 tubes to be used. Cut away all particles of the skin. Then 

 cut on each cylinder a slanting surface ex- 

 tending from about the middle diagonally 

 to the end. Leave the cylinders in run- 

 ning water over night to prevent them 

 from becoming discolored when they are 

 sterilized. 



One potato cylinder thus prepared is 

 then to be placed in each of the already 

 cleaned, plugged and sterilized test-tubes, 

 after which they are sterilized by either 

 the intermittent method with streaming 

 steam or by steam under pressure in the 

 autoclave. In the latter event one atmos- 

 phere of pressure should be continued for 

 twenty minutes. (See Fig. 18.) 



For some purposes potatoes may be ad- 

 vantageously peeled, sliced into disks of 

 about 1 cm. in thickness, and placed in small Potato in test-tube, 

 glass dishes provided with covers, similar 

 to the ordinary crystallizing dishes. -The dish and its con- 

 tents are then sterilized by steam in the usual way. By 

 this plan a relatively large area for cultivation is obtained. 



