PREPARATION OF POTATOES. 



119 



FIG. 18. 



loosely into the test-tubes to be used. On each bit of 

 potato is then to be cut a slanting surface running from 

 about the junction of the first and second thirds of the 

 cylinder to the diagonally opposite end. These cylin- 

 ders of potato are to be left in running water over 

 night, otherwise they will be very much discolored by the 

 sterilization to which they are to be subjected. After 

 being thus washed they are placed in pre- 

 viously prepared test-tubes, one piece in each 

 tube, with the slanting surface up, the cot- 

 ton plugs of the tubes replaced, and they 

 are then to be sterilized in steam for fifteen 

 to twenty minutes on each of three successive 

 days. Or the entire sterilization may be ac- 

 complished in the autoclave, with the steam 

 under a pressure of one atmosphere, by a 

 single exposure of twenty to twenty-five 

 minutes. Potatoes thus prepared have the 

 appearance seen in Fig. 18, except that there 

 is no growth upon the surface as is shown 

 in the cut. 



For some purposes potatoes may be ad- 

 vantageously peeled, sliced into discs of 

 about 1 cm. in thickness, and placed in 

 small glass dishes provided with covers, 

 similar to the ordinary Petri dishes. The Potato in test- 



tube. 



dish and its contents are then sterilized 

 by steam in the usual way (method suggested by von 

 Esmarch). By this plan a relatively large area for 

 cultivation is obtained. 



Potatoes may also be boiled, or steamed, and mashed, 

 and the mass placed in covered dishes, test-tubes, or 

 flasks, and sterilized. By this method one obtains in 



