POTATOES AS CULTURE MATERIAL 



45 



with 1-1000 corrosive sublimate, and a piece of circular filter 

 paper, moistened with the same, is laid in its bottom. On this 

 latter are placed four sterile watch 

 glasses. Two firm, healthy, small, 

 round potatoes, as free from eyes 

 as possible, and with the skin whole, 

 are scrubbed well with a brush 

 under the tap and steeped for two 

 or three hours in 1-1000 corrosive 

 sublimate. They are steamed in 

 the Koch's steriliser for thirty 

 minutes or longer, or in the auto- 

 clave for a quarter of an hour. When 

 cold, each is grasped between the 



FIG. 9. Potato jar. 



FIG. 10. Cylinder of potato 

 cut obliquely. 



left thumb and forefinger 

 (which have been sterilised with sub- 

 limate) and cut through the middle with 

 a sterile knife. It is best to have the 

 cover of the jar raised by an assistant, 

 and to perform the cutting beneath it. 

 Each half is put in one of the watch 

 glasses, the cut surfaces, 

 which are then ready for /#>-,_ 

 inoculation with a bacterial growth, being upper- m* 

 most. Smaller jars, each of which holds half of a 

 potato, are also used in the same way and are very 

 convenient. 



(6) By Slices in Tubes. -This method, intro- 

 duced by Ehrlich, is the best means of utilising 

 potatoes as a medium. A large, long potato is well 

 washed and scrubbed, and peeled with a clean knife. 

 A cylinder is then bored from its interior with an 

 apple corer or a large cork borer, and is cut obliquely, 

 as in Fig. 10. Two wedges are thus obtained, each 

 of which is placed broad end downward in a test-tube 

 of special form (see Fig. 11). In the wide part at 

 the bottom of this tube is placed a piece of cotton 

 wool, which catches any condensation water which 

 may form. The wedge rests on the constriction 

 above this bulbous portion. The tubes, washed, 

 dried, and with cotton wool in the bottom and in the 

 mouth, are sterilised before the slices of potato are potato. 

 introduced. After the latter are inserted, the tubes 

 are sterilised in the Koch steam steriliser for one hour, or in 

 the autoclave for fifteen minutes, at 115 C. An ordinary test- 



FIG. 11. 

 Ehrlich's 

 tube contain- 

 ing piece of 



