VEGETABLE MEDIA 55 



4. Vegetable media. 



Potato. 



A. Petri dish method. 1. Select a number of perfectly sound potatoes, scrub 

 away the soil adhering to them in running water, then dry and peel them. 



2. Cut them into slices about 10-15 mm. thick parallel to their long axes, 

 and drop the slices into a dish of distilled water. 



The slices should not be touched with the fingers, and it is best to use a 

 silver blade as steel often turns potatoes black. 



3. Dry the pieces between folds of white filter 

 paper. 



4. Then lay them in Petri dishes (fig. 43) or other 

 suitable covered glass dish. 



5. Sterilize the potato in the dishes at 120 C. FIG 43 Petri dish 

 for 20 to 30 minutes. 



Potatoes must be sterilized at 120 C., because a highly resistant organism 

 (the potato bacillus), which is often present on the surface, may in slicing the 

 potatoes be carried by the knife on to the cut surface. 



B. Method recommended. 1. Wash and scrub the potatoes as above. 



2. Cut the potatoes, not into slices, but into elongated parallelepipeds or 

 semi-cylindrical pieces 4 to 5 cm. long, so that they can be put in special 



potato-tubes also known as Roux's tubes. 

 CQ^) These tubes (fig. 44) are rather wider than ordinary culture- tubes and 



the potato rests on a constriction situated about the lower one- fourth ; 



the bulb below collects the condensation water. 



A special cutter may conveniently be used for slicing the potatoes, 



but the only advantage to be gained is that the pieces are more neatly 



and regularly cut. The slices should not be too long, otherwise they 



will curl when boiled. 



3. Wash the pieces in distilled water : dry between blotting 

 paper. 



4. Put them into tubes. Plug with wool. 



5. Sterilize as above. 



Note. Potatoes, though generally neutral in reaction, are sometimes 

 strongly acid, in which case they are not suitable for the cultivation of 

 bacteria. If it be necessary to use these acid potatoes, they must be 

 soaked for some hours before being sterilized in a 0'5 per cent, solution 

 FIG. 44. of soda. 

 Potato-tube. 



[C. Glycerin-potato. 1. After cutting the potatoes into suitably 

 shaped pieces as above (B), soak them in a dilute (1-1000) solution of sodium 

 carbonate for 24 hours. 



[2. Transfer the pieces to a 5 per cent, solution of glycerin in water for a 

 further 24 hours. 



[3. Tube in ordinary test-tubes, which should have a pledget of wool at the 

 bottom. Fill up the tubes with the 5 per cent glycerin solution. 



[4. Sterilize at 100 C. on three successive days. 



[5. When required for use pour off nearly all the glycerin solution, and sow 

 the surface of the medium.] 



D. Potato mash. 1. Peel the potatoes, cut them into large pieces, and 

 boil them in water. 



2. Pass them through a sieve. 



3. Distribute the mash in layers 1-2 cm. thick in Petri, or other covered 

 glass, dishes. 



4. Sterilize at 120 C. for 20 minutes. 



