pH 403 



no evidence of a nutritional mechanism in this as yet unexplained phe- 

 nomenon. 



Conidia of Erysiphe graminis respond slightly to nutrients, but the 

 response is strongly conditioned by temperature and by the time of 

 collection of the spores, i.e., by their previous history (316, 317). 



Ammonium salts, often a utilizable nutrient, are toxic to some spe- 

 cies of Mycena, but not to all; mycelial growth is much less sensitive, or 

 possibly insensitive, to concentrations of ammonium tartrate which 

 completely inhibit basidiospore germination (97). Chlamydospore 

 germination in Tilletia spp. has repeatedly been found to be inhibited 

 by complex biological products which are nutritive for other organisms 

 (171). 



Ecologically, a nutritional requirement for spore germination may 

 be an advantage, helping to insure that germination will not occur in 

 environments devoid of the materials needed for continued growth. 



7. pH AND GERMINATION 



It has been mentioned earlier, in Chapter 1, that spore germination 

 usually has a narrower pH range than growth, presumably because 

 there is less time for metabolic products to modify an unfavorable 

 medium. In the same chapter, some of the ambiguities of acidity data 

 were considered; the same ambiguities attach to germination experi- 

 ments, in particular to the finding that several fungi show a double pH 

 optimum under some conditions (90, 283, 295). 



Analysis of spore germination data reveals several complicating fac- 

 tors: (1) strains of a given species differ in response (43), (2) the type 

 of buffer — and a buffer is essential in pH studies — affects the shape and 

 limits of the curve (274), (3) nutrient materials often modify the pH 

 response profoundly (43, 295), and (4) the previous history of the spore 

 population influences its behavior (2). These considerations suggest 

 a certain caution in the interpretation of the rather extensive literature 

 on pH-response curves, to which we now turn. 



Spores of most fungi germinate best at pH 4.5-6.5, with limits at 

 about pH 3 and pH 8. These rough figures apply to, for example, 

 uredospores of several rust fungi (2, 90, 266), ascospores (14, 43, 301), 

 chlamydospores of Urocystis tritici (206), and spores of myxomycetes 



(257). 



Variations from this general pattern are several: a few fungi have 

 more acid optima (283, 295), some have optimum zones extending to 

 about pH 8 (60, 146, 160, 161, 295), and many germinate over a some- 

 what wider range than the typical (53, 198, 257, 295, 301). 



