90 CARBON NUTRITION 



is strongly influenced by substrate (205), metal deficiencies (29, 158, 

 167, 182), and nitrogen nutrition (157, 211). The influence of age is 

 variable (28, 155). As one would expect, increasing the glucose sup- 

 plied decreases the economic coefficient (119, 159, 195). 



All of the factors just listed as influencing the economic coefficient 

 almost certainly act by changing the amount of soluble metabolic 

 products formed; it is a possibility not so far explored that the loss 

 of carbon as C0 2 is affected, but this seems doubtful. A low economic 

 coefficient should suggest in the first instance a search for soluble 

 carbon compounds in the medium. 



11. CONCLUSION 



The most important single facet of carbon nutrition is the high 

 degree of specificity; differences between species and strains are the 

 rule in the fungi. Nevertheless, a search for some general principle has 

 been and should be pursued. One such is to relate carbon nutrition 

 to phylogeny. 



It has often been pointed out, e.g., by Garrett (69), that the major 

 groups of fungi differ with respect to cellulose and lignin utilization. 

 The Zygomycetes do not use either of these materials; in the ascomy- 

 cetes and the Fungi Imperfecti, cellulose utilization is common; the 

 basidiomycetes contain most of the known lignin-utilizing fungi in 

 addition to many species able to grow on cellulose. This progression 

 toward utilization of more complex materials would be more impres- 

 sive if the capacity to break down cellulose were not widespread in the 

 Chytridiales and perhaps in the Oomycetes. 



The phycomycetes, usually conceded to be the most primitive major 

 group of fungi, have certain nutritional and metabolic characteristics 

 which, taken together, separate them rather clearly from the higher 

 fungi : 



1. The fungi which produce large amounts of lactic acid from 

 carbohydrate are all phycomycetes (Chapter 6). 



2. The fungi able to grow at low oxygen tensions are usually phy- 

 comycetes (Chapter 1). This ability may, of course, be related to the 

 ability to form lactic acid. 



3. Requirements for organic sulfur among fungi occurring in nature 

 are known only in the phycomycetes (Chapter 9). 



4. The ability to use the lower fatty acids appears to be more com- 

 mon in the phycomycetes than in other groups (p. 81), although the 

 data are still rather limited. 



