MAINTENANCE OF CULTURES 25 



medium, regardless of the nutritional defects of that substrate 

 and of whether the organism is pathogenic or saprogenic in its 

 natural state. First of all, a modicum of skill and good judgment 

 is required to keep a fungus alive by transfer at sufficiently fre- 

 quent intervals. Not only must it be kept alive, but also as a 

 result of manipulation and examination each fungus will come to 

 be recognized as having a distinctive appearance. If, then, numer- 

 ous isolates of that species are assembled and their appearance on 

 different substrates is compared, it will be evident that charac- 

 teristic differences between isolates exist. Some isolates may 

 reproduce more abundantly than others, their myceha may dif- 

 fer in color, the mycelial mass may vary in laxness or floccose- 

 ness and also in growth rate or in other features that do not lend 

 themselves well to description. Eventually the observer is forced 

 to conclude that each individual isolate must be regarded as a 

 specimen, and that the species consists of an assemblage of 

 closely related individuals. 



The frequency of subculturing will depend upon several fac- 

 tors, including (1) the peculiarities of the fungus, (2) the culture 

 medium, and (3) the temperature and humidity of the storage 

 cabinet. Fungi that produce sclerotia, such as Rhizoctonia solani 

 and Sclerotium rolfsiij will remain alive for several years even 

 though the medium has dried up and shrunken to a corneous 

 mass. 



The result of transfer of mycelium or spores from old cultures 

 that are thoroughly desiccated may indicate that the culture is 

 dead. It may be possible, however, to revive such an old culture 

 by flooding its surface with a liquid medium and then incubating 

 for a day or two before again attempting to subculture. 



Species that produce spores in such abundance that spore 

 clouds may be formed even when the culture is jarred only 

 slightly are difficult to maintain in a state of purity, and they may 

 become contaminants in the whole culture collection unless the 

 utmost care and faultless technique are employed during sub- 

 culturing. Monilia sitophila and members of the genera Asper- 

 gillus, Penicillium, Mucor, and Trichoderma are included in a 

 group having this peculiarity. 



If the temperature of the culture cabinet can be maintained at 

 about 20° C, most species of fungi need be transferred only 

 two or three times a year. Even when transfers are infrequently 



