THE PREPARATION OF CULTURE MEDIA 21 



be streaked on to the surface of a potato, the red colour 

 again reappears. There are many ways of j^reparing 

 potatoes for cultivation purposes, but in all the difficult}^ 

 of their effectual sterilisation is experienced. The two 

 methods which are mostly used are those in which small 

 circular dishes and test-tubes are respectively employed. 

 The dishes are shallow, being about -^ inch deep, and 

 from 1^ to 4 inches diameter ; they are fitted with an 

 overlapping glass cover (see fig. 7). After thoroughly 

 washing and finally rinsing with distilled water, these 

 dishes are placed either in the steam steriliser or in the 

 hot-air oven for a couple of hours. Meanwhile the 

 potato is carefully washed and scrubbed with a nail- 

 brush, and after peeling it is cut into slices which will 

 fit easil}^ into the dish. On replacing the lid the dish 

 with its contents is immediately placed in the steam 

 steriliser, and allowed to remain there for an hour or 

 more ; as an additional precaution it can be again 

 sterilised on the following day. 



Another method consists in cutting a cylindrical 

 piece out of the peeled potato by means of an ordinary 

 laboratory cork-borer, and placing this in a test-tube 

 with a cotton-wool stopper. One end of the cylinder 

 should be sliced off obliquely, so as to permit of a larger 

 ■surface being used, as in the case of sloped agar tubes. 

 A drop of water is put at the bottom of the test-tube, to 

 prevent the potato drying up, and the tubes with their 

 contents are then sterilised in the steamer in the usual 

 manner. This is a particularly convenient form of 

 potato culture, as it can be preserved ready for use for 

 a long period of time. 



Eoux has modified this method, originally devised 

 by Meade Bolton, by constricting the test-tube towards 

 the bottom, as shown in the accompanying figure, so 

 that the condensation water which forms on the walls 



