14 PHYSIOLOGY OF THE FUNGI 



Solid versus liquid media. Both solid and liquid media are used in 

 cultivating fungi. Media solidified with agar, or semisolid substrates 

 such as corn meal, offer many advantages in that the culture vessels can 

 be freely handled without disturbing the fungus. This feature is particu- 

 larly valuable when one wishes to follow the development of a fungus. 

 Microscopic examination is facilitated, and contaminants are more easily 

 detected. Single-spore isolations can be made more easily from solid 

 media. Agar media are used to maintain stock cultures and are recom- 

 mended for many preliminary experiments. 



Frau Hesse (Kitchens and Leikind, 1939) introduced the use of agar 

 into microbiological procedures in 1881. Agar, which is obtained from 

 various marine red algae, is a complex polysaccharide sulfate ester 

 (Pigman and Goepp, 1948). It forms colloidal solutions at elevated 

 temperatures and sets to a gel at temperatures around 45°C. On acid 

 hydrolysis both D-galactose and its enantiomorph, L-galactose, as well 

 as sulfuric acid, are formed. Agar must exist in the form of a salt 

 (Ca, Mg, Na, K, etc.) to form a gel. Agar introduces physiologically 

 active elements into media. It may contain significant amounts of 

 zinc (Leonian and Lilly, 1940) and other micro essential elements. 

 Mulder (1940) found that magnesium could be efficiently removed from 

 agar by repeated soakings in 10 per cent sodium chloride solution, followed 

 by washing with distilled w^ater until the filtrate was free from chloride 

 ion. Agar also contains growth factors such as thiamine (Day, 1942) 

 (see Fig. 1). Many fungi make some growth on water agar, which indi- 

 cates that agar or the ''impurities" contained in it are utilized by fungi. 

 Robbins (1939) found that leaching agar with 5 per cent aqueous pyridine 

 removed many of the physiologically active compounds. 



Liquid media should be used for precise investigations where it is 

 desired to control as many variables as possible. The composition of the 

 medium may be controlled and the amounts used measured accurately. 

 Cultures may be aerated by shaking or by blowing sterile air through the 

 media. Weighing the mycelium is facilitated. When it is desired to 

 study the metabolic by-products of fungus metabolism (except gaseous 

 products), it is almost necessary to use liquid media. Isolation of 

 by-products is less complicated when liquid media are used. Studies of 

 various metabolite deficiencies and many microbiological assays (Chap. 

 10) almost always require the use of liquid media. The choice between 

 the use of solid or liquid media should be made on the basis of the known 

 advantages and disadvantages of both and with regard to the purpose of 

 the problem under investigation. 



Designating media. It is common to find references to a medium by 

 the name of the investigator who first used it. These names have served 

 as convenient abbreviations and commemorate the pioneers in the art of 



