SLIDE CULTURES 71 



Alcohol (50 per cent) 100 mL 



Formalin (40 per cent) 6.5 ml. 



Acetic acid (glacial) 2.5 ml. 



Immersion for a few minutes in this solution will suffice. From this 

 solution the cover slip is washed in several changes of water and 

 then stained. For simple staining a 1 per cent aqueous solution of 

 erythrosin or rose bengal is very satisfactory. These weakly acid 

 dyes may be used with agar preparations, since they do not stain 

 the agar so deeply as they do the mycelium. It may take up to 15 

 minutes to stain sufficiently. One may also obtain good preparations 

 with agar slide cultures by the use of the acid thionine solution in- 

 troduced by Frost for staining microcolonies of bacteria. The 



formula is 



Thionine 1.0 gram 



Phenol 2.5 grams 



Acetic acid (glacial) 20.0 ml. 



Water 400.0 ml. 



About 1 minute is sufficient for staining. If the staining is pro- 

 longed, the agar will stain too deeply. "With liquid media one may 

 use other stains. Heidenhain's iron hematoxylin gives very clear 

 preparations, but for routine use the simple aqueous erythrosin or 

 rose bengal solution is very satisfactory for molds. For actinomy- 

 cetes these stains are satisfactory but faint. Gram's stain gives a 

 clearer picture if cultures are not too old. 



One must not rely entirely upon slide cultures for the identification 

 of molds. In some cases growth in the small volume of medium on a 

 cover slip is not altogether typical. Spore heads may be small and 

 imperfectly developed. Some Penicillia, for instance, will form only 

 a single verticil of phialides in slide cultures, whereas they form 

 polyverticillate spore heads in Petri dish cultures. The slide culture 

 supplements but is not a substitute for the examination of the plate 

 culture. 



Another type of slide culture will prove of great value, particularly 

 in determining details of the arrangement of the aerial mycelium, for 

 example in untangling the branching of the sporangiophores of 

 Mucors. Rather large cover glasses (24 by 40 mm.) are cleaned. 

 With a small hot iron (for instance, the heated end of a small file) 

 a drop of sealing wax or, better, de Khotinsky cement is deposited on 

 each end. With the hot iron this is then spread out to form a layer 

 about 5 mm. wide and 1 mm. or less thick across the ends of the 

 cover glass. A clean slide is now heated in the Bunsen flame and the 

 cover glass is placed upon it with the cement side down. The slide 



