88 PHYSIOLOGY OF THE FUNGI 



associate to form polymers. At room temperature water consists mostly 

 of (H20)3, which is sometimes called trihydroL For a further discussion 

 of water see Barnes (1937). 



The chemistry of life processes is largely confined to reactions which 

 take place in the presence of water or in solution. In addition to being a 

 solvent of remarkable powers, water is associated with the colloids which 

 comprise protoplasm. Gortner (1949) has distinguished between "free" 

 and "bound" water. Free water is mobile within the cell and serves as a 

 solvent and for the purpose of translocation of the various products of 

 metabolism. Bound water is firmly adsorbed by protoplasm, and in this 

 form water does not freeze. This property of bound water enables cells 

 to withstand low temperatures. The ability of fungus spores to with- 

 stand low temperatures may well be due to their having most of their 

 water content in the bound form. 



Water ionizes to form hydrogen (H+) and hydroxyl (0H~) ions. 

 The effects of these ions on biological processes are so important that they 

 will be discussed in detail in Chap. 8. 



OXYGEN 



Apparently none of the fungi are obligate anaerobes. Many are 

 strictly aerobic, and some are facultatively anaerobic. An aerobic 

 organism requires uncombined oxygen, while a facultative anaerobe may 

 use combined oxygen in addition to free oxygen. The amount of oxygen 

 required for optimum growth varies with the species. It is common to 

 express the amount of oxygen available in terms of millimeters of mercury. 

 Approximately 21 per cent of air is oxygen. The amount of oxygen may 

 be regulated by controlling the air pressure within the culture vessel. If 

 the barometric pressure is 740 mm. Hg, the partial pressure due to oxygen 

 is ^Hoo X 740, or 155.4 mm. Hg. If the pressure within a culture vessel 

 is reduced to 100 mm. Hg, the partial pressure of oxygen amounts to 21 

 mm. Hg. Tamiya (1942) has reported that Aspergillus onjzae has a 

 maximum rate of respiration when the partial pressure of oxygen is 500 

 to 630 mm. Hg. Such partial pressures of oxygen are readily obtained by 

 using oxygen-nitrogen mixtures. Ternetz (1900) reported the following 

 effects of reduced oxygen supply on Ascophanus carneus: at 10 mm. Hg the 

 mycelium grew with difficulty; at 20 mm. Hg growth was good, but no 

 spores formed; at 40 mm. Hg some fructification occurred; at 120 to 140 

 mm. Hg growth was somewhat better than at atmospheric pressure. 



The ability of certain soil fungi to exist under conditions of low oxygen 

 supply is important for survival. The amount of oxygen in soil depends 

 upon the soil type and the amount of water present. Soil saturated with 

 water contains but a trace of free oxygen. Hollis (1948) found Fusarium 

 oxysporum to survive under essentially anaerobic conditions for 13 weeks, 



