SUGGESTED LABORATORY EXERCISES 423 



dishes in a paper towel or several in a paper bag before placing in the 

 autoclave. This wrapping should remain on the Petri dishes until they 

 are used, to prevent contamination. Do not remove wrapped glassware 

 from the autoclave until several minutes after the pressure is down. 

 Pipettes should be wrapped and placed in pipette cases. Sterilize in the 

 same way as Petri dishes. 



Use either a water bath or the autoclave for melting agar. Never melt 

 agar in or sterilize flasks which are more than half full. Test tubes should 

 not be more than one-fourth full. The reason for this lies in the fact 

 that the autoclave cools quicker than the medium. This leaves the 

 medium superheated, and under this condition it is likely to boil violently. 

 Never remove a flask of melted agar from the autoclave as soon as the 

 pressure is down. Agitation may cause violent boiling. Your instructor 

 will give you full instructions for operating the autoclave. Follow these 

 instructions carefully. 



On handling cultures. It will be necessary for each student to main- 

 tain the identity of his cultures. This may be done by name or by stock- 

 culture number. Each medium used in the course will receive a number. 

 If a medium is used more than once, it will be given another number. 

 The composition of each medium should be entered in the laboratory 

 notebook. The name of the fungus (or stock-culture number) and the 

 number of the medium should be also written on each culture vessel. 

 The date of inoculation and the kind of inoculum used should be entered 

 in the notebook. It is convenient to fasten together duplicate or tripli- 

 cate cultures in test tubes with a rubber band. 



Preservation of stock cultures. The maintenance of a stock culture 

 collection of filamentous fungi for class use is highly desirable. Such a 

 collection need not be extensive but should include a sufficient number of 

 selected species of known phj^siological reaction and any others which 

 may be desired for general use. The method of preserving cultures in 

 our laboratory has proved quite satisfactory when frequent transfers are 

 needed for research or class use. Test tubes with constricted tops and 

 plastic screw caps are used. Malt extract or any other suitable agar may 

 be used. After inoculation the tubes are allowed to remain at room tem- 

 perature for a few days until the inoculum starts to grow. Then the caps 

 are screwed down tightly and the cultures stored at 5 to 10°C. Most 

 species continue to grow slowly, and under these conditions the tube? 

 remain free from contamination and the agar dries out very slowly. Thi.= 

 method also excludes mites. Some vegetative cultures have remained 

 viable for a period of more than 2 years without being transferred. How- 

 ever, it is suggested that all cultures be transferred every year, and the 

 entire stock should be looked over carefully every few months, as some 

 species may require more frequent transfers. The first transfer from 



