CULTURE OF FUNGI 213 



best arranged for in this manner. Smaller moist chambers may be made by- 

 placing filter paper above wet Sphagnum on the bottom of shallow glass 

 dishes, such as crystallizing dishes, three or more inches high, covered by a 

 piece of glass. The substance to be used as the substratum is placed on the 

 filter paper, which is kept moist by the Sphagnum. Such chambers are well 

 suited to cultures on small pieces of fruit or other vegetable matter, and on 

 the dung of various animals. Cultures on horse dung are best made in larger 

 dishes or under bell jars. 



203. Pure cultures on potato agar. Most saprophytic fungi may be cul- 

 tivated on potato agar, which is one of the simplest and most satisfactory 

 of the culture media. 



To make potato agar, pare two or three medium-sized potatoes, cut into 

 thin slices, place in a stewpan, and cover with tap water. Allow the water to 

 simmer gently for one half hour, or until the potatoes are soft but not dis- 

 organized. Do not let it boil. Strain the liquid, which should be as clear as 

 possible. Add enough tap water to make one half liter, and place in a fiask 

 with 10 grams of agar-agar cut up in small pieces. Heat the flask in a steam 

 sterilizer until the agar has melted and mixed with the culture fluid. 



Clean about thirty test tubes (six inches long and three fourths of an inch 

 across), rinse, drain, and dry. Fit cotton plugs of a good quality into the 

 dry test tubes. They should enter the tubes at least an inch and project 

 somewhat beyond. Place the tubes fitted with cotton plugs in a dry-air 

 sterilizer and expose to a temperature of 140° C. for an hour ; this will kill 

 all spores of fungi, including bacteria, in the tubes or on the cotton. The 

 tubes are best handled in a receptacle made of heavy wire netting, and of a 

 size which will slip into the steam sterilizer. 



Pour the melted potato agar into the test tubes by means of a funnel, 

 removing the cotton plug carefully and holding between the fingers while fill- 

 ing each. The tubes should be filled up about one and one-half inches from the 

 bottom. It is very important that no agar become smeared around the top 

 of the test tube where the plug is inserted. The filled tubes, carefully plugged, 

 are now sterilized twice a day (morning and night) on three successive days 

 in the steam sterilizer for an hour each time. This is necessary to render the 

 tubes free from bacteria, for the spores of bacteria are not generally killed 

 by the temperature of 100° C, but they germinate quickly in the potato 

 agar, and the vegetative bacteria produced by them are then killed by that 

 temperature. 



After the sterilization of the third day the tubes are taken out and laid on 

 a table inclined against a board so that the surface of the hot fiuid agar runs 

 three or four inches up the sides of the tubes. As they cool, the agar stiffens 

 in this position, forming a long, slanting surface in each tube, wliich is now 

 ready for inoculation. 



