ORGANIC NUTRIENTS OF FUNGI 11 



criterion. These studies, as might be expected, have determined 

 that cosmopolitan species of Aspergillus and Penicillium thrive 

 on a wide variety of substrata. Specialized pathogens, on the 

 other hand, either grow poorly in artificial culture on organic sub- 

 strata or may even fail to grow at all. A survey of this situation 

 clearly indicates that the underlying reasons for these differences 

 in organic food requirements of fungi should be sought by inten- 

 sive studies. 



Carbon requirements. Those carbon compounds that can be 

 oxidized with the least expenditure of the energy stored in the 

 compound or can be assimilated most readily appear to constitute 

 the food of first choice for fungi. Evidence indicates that, in 

 general, fungi, like bacteria, prefer carbohydrates as food sources, 

 with proteins as second choice, and that few species thrive well 

 on fats. 



Fungi grow more rapidly in proportion to their body weight 

 than do green plants, and consequently expend relatively more 

 energy in converting their food into an assimilable form. With 

 molds an increase in body weight amounting to a thousandfold 

 within a 10-day period, such as occurs in Phy corny ces nitens and 

 Aspergillus niger, is not uncommon. 



The method that has been generally employed to determine the 

 food value of carbon compounds is to grow the fungus in a basal 

 mineral-nutrient solution and to vary the carbon or the nitrogen 

 added. By preliminary trials the time required to attain maximum 

 growth can be determined. The mycelial mat, if removed at the 

 end of this period, can be desiccated and weighed. In comparison, 

 another figure, which is the result of an analysis to determine the 

 amount of compound that has been used by the mold, can be con- 

 sidered. Unfortunately, as investigators have indicated, inaccu- 

 racies appear as a consequence of the formation and accumulation 

 of by-products, such as acids, alkalis, staling products, and toxins, 

 and of the autolysis and utilization of dying and dead parts of the 

 mycelium. 



Another method that has been employed to only a limited extent 

 makes use of microrespirometers. This method, considered in 

 Chapter 3, is adapted for use in determining whether the given 

 carbon compound is acceptable and also the rate at which it is 

 consumed. 



