Chapter 7 

 EFFECTS OF REACTION OF SUBSTRATE ON FUNGI 



Amono- the chemical environmental influences to which fungn 

 are generally known to respond is the reaction of the substrate. 

 Long ago the theory was advanced that the chemical activities of 

 acids, bases, and salts may be attributed chiefly to the ionized 

 portions. Abundant experience has shown that fungi are more 

 tolerant of acid ions [H + ] than of basic ions [OH - ]. If, for 

 example, it becomes necessary to separate mixed cultures of fungi 

 and bacteria, the growth of bacteria may be inhibited by the 

 addition of lactic acid in the proportion of 1 drop of 50% lactic 

 acid to 10 ml of agar in making the poured plates that are to be 

 planted with the mixed cultures. 



Many fundamental facts regarding the effects of the ionized 

 portions were established before the differences between total 

 acidity (titratable acidity) and active acidity (hydrogen-ion-con- 

 centration) were appreciated. The work of Clark (1899) is not- 

 able in this connection. He studied the effects of the concentra- 

 tion of a variety of mineral and organic acids upon the germina- 

 tion of spores and mycelial development of Sterigmatocystis 

 nigra, Oedocephahim albidzmi, Pemcillium glaiicwn, and Botrytis 

 cinerea. As a result he found that the OH~ group is rather 

 more toxic to all species than the H+ ions and that molds differ 

 specifically in tolerance. Furthermore, to inhibit the germina- 

 tion of these molds, a concentration of the mineral acids 200 to 

 400 times that fatal to higher plants is required. 



Subsequently the classical studies of Michaelis and S0rensen on 

 the theory of the hydrogen ion and its measurement laid the 

 foundations of present-day knowledge. These matters, an under- 

 standing of which is essential to all biologists regardless of their 

 special field of interests, are summarized and elucidated in a vol- 

 ume by Clark (1928). With the help of this book the student can 

 learn the fundamentals of ionization, conductivity, and use of in- 

 dicators, at least to a sufficient degree to be able to measure hvdro- 



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