POTATOES AS CULTURE MATERIAL 



45 



FIG. 9. Potato jar. 



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 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. 



FIG. 10. Cylinder of potato Each nalf is P ut in one of the watch 

 cut obliquely. glasses, the cut surfaces, 



which are then ready for ^,=,_ 



inoculation with a bacterial growth, being upper- 

 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, Ehrlich's 



-, . i i ., , . ., , . i ji tube contain- 



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

 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. 



