164 CULTURE MEDIA 



ning to the end except the cooUng down to below 60°, when 

 the egg white for clearing is added. Though filtration 

 through paper is possible as with gelatin, if the agar solution 

 is thoroughly boiled and filtered boiling hot, it is more satis- 

 factory for beginners to use absorbent cotton wet with boil- 

 ing water, and to pour the hot agar through the same filter 

 if not clear the first time. The solidified agar medium is 

 never perfectly clear, but always more or less opalescent. 

 The agar medium may be sterilized in the autoclave for 

 fifteen minutes at 15 pounds' pressure as the high temperature 

 does not injure the agar. 



Potato Media. — Potatoes furnish a natural culture medium 

 which is very useful for the study of many bacteria. The 

 simplest, and for most purposes the best, way to use potatoes 

 is in culture tubes as "potato tube cultures" (No. 8, Fig. 

 117). These are prepared as follows: Large tubes are used. 

 With a cork-borer of a size to fit the tubes used, cylinders 

 about one and one-half inches long are cut from fairly large, 

 healthy potatoes. The skin is cut off square from the ends 

 and each cylinder cut diagonally from base to base. This 

 furnishes two pieces each with a circular base and an oval, 

 sloping surface. The pieces are then washed clean and 

 propped for a minute into boiling water to destroy the oxi- 

 dizing enzyme on the surface which would otherwise cause 

 a darkening of the potato. (The darkening may also be 

 prevented by keeping the freshly cut potatoes covered with 

 clean water until ready to sterilize.) A bit of cotton one- 

 fourth to one-half inch in depth is put into each of the 

 test-tubes to retain moisture and a piece of potato dropped 

 in, circular base down. The tubes are then plugged with 

 cotton and sterilized in the autocla^'e at 15 pounds' pressiu-e 

 for not less than twenty-five minutes, since potatoes usually 

 harbor very resistant spores, and it is not unusual for a few 

 tubes to spoil even after this thorough heating. 



Potatoes are sometimes used in "potato plate cultures." 

 The term "plate culture" is a relic of the time when flat 

 glass plates were used for this and other "plate cultures." 

 Now glass dishes of the general form shown in Fig. 113, 

 called "Petri dishes," or plates are used for practically all 



