12 PHYSIOLOGY OF THE FUNGI 



chemical compound.s than synthetic media. They also contain com 

 pounds ordinarily not present in synthetic media. 



Specific metabolites. Only certain chemical compounds are utilized 

 by fungi, but not all fungi are able to utilize the same compounds. Any 

 compound utilized by a fungus is called a metabolite. Some fungi are 

 unable to synthesize certain essential metabolites and are said to be 

 "deficient" for the specific metabolites they are unable to synthesize. 

 In order to cultivate such deficient fungi, these metabolites must be pres- 

 ent in the medium. Natural media usually contain these metabolites. 

 If a fungus grows upon a natural medium and fails to grow upon a variety 

 of simple synthetic media, it may be suspected that specific metabolites 

 are involved in its nutrition. 



The following example will illustrate the role of specific metabolites in 

 fungus nutrition. Fellows (1936) investigated the ability of Ophiobolus 

 grayninis to utilize different nitrogen compounds for growth. A sucrose- 

 mineral salts solution was used as the basal medium to which various 

 nitrogen sources were added. Only complex nitrogen sources such as egg 

 albumen, peptone, casein, and nucleic acid allowed growth. Under the 

 conditions used no growth resulted when ammonium compounds, nitrates, 

 nitrites, and amino acids were tested. 0. graminis in the presence of egg 

 albumen utilized glucose, maltose, lactose, fructose, xylose, starch, and 

 dextrin, in addition to sucrose. From these experimental results it was 

 concluded that 0. graminis requires a complex source of nitrogen for 

 growth. 



Later, White (1941) found that this fungus requires two specific 

 metabolites, thiamine and biotin. When these vitamins were added to 

 synthetic media containing simple nitrogen sources (sodium nitrate, 

 ammonium nitrate, asparagine, or glycine), good growth was obtained. 

 Thus, it appears obvious that 0. graminis does not require a complex 

 nitrogen source, but that it is unable to synthesize two specific chemical 

 compounds. These papers illustrate the fact that fungus physiology is a 

 young and developing science. Much of the early work needs reevalua- 

 tion in the light of recent discoveries. A student should strive to develop 

 a critical attitude toward the work of others, but he should be no less 

 critical with regard to his own work. The evaluation of experimental 

 results depends upon the conditions under which the work was done, and 

 among these conditions the medium used is of first importance. 



Complexity of media. It is a common experience to find that a trace of 

 some crude natural product stimulates the rate of growth and sporulation 

 of a fungus. This stimulation frequently occurs with fungi which grow 

 well on synthetic media and which are not deficient for vitamins or amino 

 acids. It appears that the complexity of natural media offers a clue to 

 understanding this stimulatory effect. If a fungus is grown upon a simple 



