CULTIVATION 21 



drops. Each has its advantages and disadvantages. With the 

 first method the cultures can be kept indefinitely, and subcul- 

 tures can be made at such intervals as maintenance of viability 

 demands. Observations are necessarily intermittent. With the 

 second method cultivation in van Tieghem cells makes continu- 

 ous observations possible as growth proceeds. Although usually 

 the period of observation is limited to a few hours, by governing 

 atmospheric humidity and availability of oxygen in van Tieghem 

 cultures the period may be extended, and much information re- 

 garding the developmental history of the fungus may be gained. 



CULTIVATION 



The substrata upon which fungi are cultivated in the labora- 

 tory are called media. Many different kinds of media have been 

 compounded or synthesized. All are made up according to 

 standard specifications, which are usually included in laboratory 

 manuals. Although these media may contain all the essential food 

 substances, namely, carbohydrates, proteins, fats, amino acids, vita- 

 mins, minerals, air, and water, none of them constitutes an ideal 

 substratum. Their deficiency is evidenced by the fact that the 

 mycologist has thus far been unable by use of artificial media to 

 cultivate certain fungi, notably the rusts, downy mildews, and 

 powdery mildews. Furthermore, numerous fungi remain sterile 

 in culture, others cannot be induced to complete their normal 

 developmental cycle in the test tube, and some are manifestly 

 teratological. For these reasons many different kinds of media 

 not Hsted as standard are continually being devised and tested. 

 The mycologist realizes all too keenly that there is no one best 

 medium. The ideal medium is, perforce, most nearly like the 

 substratum on which the specific fungus occurs in nature. Until 

 more fundamental knowledge has been acquired regarding the 

 nutrition of fungi, the investigator will continue to depend upon 

 empirical methods for their artificial cultivation. 



On the basis of their composition, media may be regarded as of 

 two kinds: synthetic (mineral) and non-synthetic (organic), and 

 they may be either Hquid or semisohd. Agar in the proportion 

 of about 2% is used almost exclusively in making the media semi- 

 solid. 



