36 THE COMPOSITION OF FUNGUS CELLS 



1. THE MAJOR CONSTITUENTS OF THE CELL 



Growing fungus cells are, of course, largely water; early measure- 

 ments indicate that 85-90 per cent of the fresh weight of mycelium 

 and fleshy sporophores is water. Presumably the more leathery sporo- 

 phores and sclerotia have less water (25, 197). The conidia of most 

 fungi appear to have a relatively low water content, e.g., 17.4 per cent 

 in Aspergillus oryzae (171), 25 per cent in Monilinia jructicola, and 6 

 per cent in Penicillium digitatum (195). It is probable, as suggested 

 by Yarwood (195), that most of the water in these spores is hygroscopic. 

 Their water content is largely, therefore, a function of the environ- 

 mental humidity; this may explain some of the rather high values re- 

 ported in the early literature. 



Species of Erysiphe are unusual in that the conidia have a high water 

 content (195, 196). With this is associated the ability to germinate at 

 low relative humidity (Chapter 12). 



The bound water of Aspergillus niger mycelium, defined as water 

 which is removed by heat but not by in vacuo drying over a desiccant 

 at moderate temperatures, amounts to 7.1-8.4 per cent of the dry 

 weight when the fungus is cultivated on 5 per cent glucose; at higher 

 glucose concentrations the bound water rises to as much as 30 per cent 

 of the dry matter (182). 



The elementary composition of fungi has been studied chiefly with 

 respect to carbon and nitrogen. The carbon content of a number of 

 fungi studied by Heck (78) was 40-44 per cent of the dry weight; 

 Pinck and Allison (128) report a somewhat wider range — 43 to 53 

 per cent. Aspergillus oryzae mycelium is about 49 per cent carbon 

 (173), and A. niger mycelium about 46 per cent (129), but other species 

 of Aspergillus may be as high as 55 per cent (194). Most fungi are 

 40-50 per cent carbon, and the value is relatively little affected by 

 cultural conditions (173, 194) or age (126). 



Bacteria and viruses in general have carbon contents of the same 

 order as fungi (130). The mycelium of Streptomyces griseus has 

 slightly less carbon — about 38 per cent of the dry weight (58). 



In contrast to the relative uniformity of carbon content, the total 

 nitrogen of fungal mycelium is quite variable; both the kind and the 

 concentration of the nitrogen source influence the composition. 



The major constituents of the mycelium of several fungi are listed 

 in Table 1. These data cannot be considered typical of all fungi, 

 since it is known that at least four of the fractions — carbohydrate, 

 protein, lipid, and ash — are strongly affected by cultural conditions 



