GROWTH 



37 



of ions and molecules of solute contained in a unit \'olume of solution. A 

 mole of a non-ionized compound in 1,000 g. of water at 0°C. has an 

 osmotic pressure of 22.4 atm. if separated from pure water by a semiper- 

 meable membrane. For a fuller discussion of osmosis and osmotic pres- 

 sure the student is referred to Gortner (1949), Seifriz (1936), and Meyer 

 and Anderson (1948). 



If concentration were the sole factor which determines whether growth 

 is possible, all solutions having the same osmotic pressure would be 

 equally inhibitory. Table 7 indicates that this is not true. 



Table 7. Highest Osmotic Pressures (Atmospheres) op Solutions of Four 

 Compounds in Which Various Fungi Grew 

 (Hawkins, Jour. Agr. Research 7, 1916.) 



* Limiting concentrations not used. 



These data and others show that the limiting osmotic pressure depends 

 upon the fungus and the compounds used. It is difficult to evaluate the 

 effects of osmotic pressure upon the fungi, for the cell membrane is per- 

 meable to other compounds in addition to water. Calculations of osmotic 

 pressure are made by assuming that an indifferent semipermeable mem- 

 brane separates solutions of different concentrations. The effect of 

 osmotic pressure upon the fungi cannot be considered as a simple physio- 

 chemical process. However, the ability of many fungi to grow in solu- 

 tions having high osmotic pressures is advantageous. Parasitic fungi 

 characteristically have a higher osmotic pressure than the cell sap of the 

 plants they parasitize (Thatcher, 1939). For further references to the 

 effect of osmotic pressure on fungi, see Kroemer and Krumbholz (1931). 



Another process involved in the entrance of water into fungus cells is 

 imbibition. Gortner (1949) has defined imbibition as the process whereby 

 colloidal substances such as protoplasm take up water, and imbibition 

 pressure as the pressure against which a colloid will imbibe liquid. 

 Raciborski (1905) grew a species of Torula in saturated lithium chloride 

 (1,000 atm.) and Aspergillus glaucus in a saturated sodium chloride 

 solution. 



Aside from osmotic effects, the concentration of the medium has a great 

 effect on the rate and amount of growth of fungi. The concentration of 



