4 CULTIVATION AND GROWTH 



interest; these include devices in which only the solution is renewed 

 (325) and those in which both nutrients and cell population are con- 

 trolled (138). 



Culture media for fungi are described by Fred and Waksman (101), 

 Levine (176), and Lilly and Barnett (180). In general, it has been too 

 readily assumed that fungi need and tolerate high nutrient levels and 

 high acidity; this is true of many fungi but by no means of all. For 

 certain purposes, concentrated media are of course useful, but mor- 

 phology, physiology, and reproduction are best studied in more dilute 

 media than those often used. 



2. PRESERVATION 



Interest in the preservation of fungi springs from the desire to avoid 

 excessive labor in maintaining stock cultures and, more important, 

 the wish to maintain cultures with a minimum of genetic change. The 

 problem is to maintain a viable inoculum in a non-growing condition. 



Cultures and herbarium specimens often survive for surprisingly 

 long periods after drying, probably by virtue of the formation of long- 

 lived spores. Zobl (344) reviews early records of up to 21 -year survival; 

 in his own work he found that survival of dried cultures is common in 

 ascomycetes and basidiomycetes, but the Mucorales are relatively short- 

 lived. The sclerotia of several fungi, which may live for as long as 13 

 years (336), are less viable if stored dry (294). A few of the more 

 leathery basidiomycete sporophores survive dry for a period of years; 

 more succulent types are less durable (41, 43). 



In the soil tube method (5, 121), a fungus is transferred to moist 

 sterile soil, preferably brought to pH 6-7 with calcium carbonate, and 

 grows and sporulates until the soil dries out. The limited data avail- 

 able indicate that although some fungi survive at least 5 years in soil 

 tubes, too many do not live that long, and the method therefore is to 

 be regarded as a special one not adapted for general use. 



Preservation of cultures under sterile mineral oil is relatively sim- 

 ple and has the advantage that sporulation is not essential (39, 267). 

 Limited tests indicate, however, that although many fungi survive at 

 least 2 years, others die out earlier (146, 331, 333). 



Lyophilization of spores suspended in serum, gelatin, dextran, or 

 other colloidal materials appears to be a usable method, although again 

 not successful with some fungi, e.g. the Entomophthorales and the 

 dermatophytes (199, 239, 265, 337). Simple vacuum drying without 

 freezing is suitable for many fungi (244). The structure of the fungus 

 is important; thus, oogonia but not sporangia or mycelium, of Pythium 



