22 CULTIVATION AND GROWTH 



11). The range for spore germination is similarly more narrow (185, 

 291); this is to be expected, inasmuch as the design of the spore 

 germination experiment does not permit sufficient time for metabolic 

 activities of the organism to modify the pH of the medium. Soil-borne 

 plant diseases may or may not show the same pH curve as do the 

 fungi which cause them (96, 260); here, of course, the effect of pH 

 on the plant presumably exerts an important influence, and the com- 

 plex effect of pH on the soil itself may be even more decisive. 



In most physiological studies it is necessary that the pH be con- 

 trolled, at least within limits. Periodic addition of alkali (51) and 

 the use of solid calcium carbonate in the medium (99) are satisfactory 

 for certain problems, but in general it is necessary to employ a buffer 

 system. Of these, the most common and generally useful is the 

 phosphate buffer. The usefulness of phosphate is limited, however, by 

 the tolerance of the fungus to the ion; this may be as low as 0.003 M 

 (61) or as high as 0.20 M (66, 151), and must be determined for each 

 organism. Other buffer systems must be employed with caution: 

 acetate is often toxic (64, 183, 291), and citrate may be utilized by 

 the fungus as a source of carbon, with consequent displacement of 

 the apparent pH optimum (64). Phthalate, borate, and citrate buf- 

 fers are satisfactory for Neurospora crassa (253); a-hydroxybutyrate 

 has been proposed as a nonutilizable buffer for studies on fungi (303). 

 Equipment for automatic pH control has been described (50). 



Physiological buffer systems can be devised for particular problems, 

 e.g., acetate as carbon source and ammonium ion as nitrogen source 

 (188), or mixtures of ammonium and nitrate ions (141). In both these 

 systems, utilization of an anion is balanced by utilization of a cation 

 and the pH is thereby held relatively constant. 



In conclusion, the pH-growth curve has, to say the least, a limited 

 usefulness. In designing experiments of this type, the first question 

 to ask is whether the information to be gained is worth the effort 

 required. Once such an experiment is decided upon, two rather ele- 

 mentary precautions should be observed. First, the final pH of the 

 medium, after growth, must be reported. Second, the results will 

 be more meaningful if buffer systems are used to control pH; in 

 the alkaline range, addition of sodium hydroxide to control pH is 

 followed by absorption of carbon dioxide from the air, with a con- 

 sequent false impression of the upper pH limit of growth. 



The internal pH of the fungus cell — and the fungus cell is usually 

 large enough so that the concept of pH may legitimately be applied 

 — is not known with any precision. Indicator methods, which have 

 obvious uncertainties, yield in most hands a value of about 6.0 (4, 



