366 REPRODUCTION 



Plant pathogenic fungi usually, and with an obvious ecological ad- 

 vantage, form their reproductive structures on the surface of the host. 

 It is reasonable to speculate that either the higher oxygen requirement 

 or the lower carbon dioxide tolerance of sporulation is the mechanism 

 that ensures extramatrical sporulation; it is significant that active 

 anaerobic respiratory mechanisms are common in the lower phycomy- 

 cetes and the genus Fusarium, forms in which resting spores or conidia 

 often occur within host tissues (93). 



Several Penicillium spp. sporulate in shaken cultures, but the de- 

 terminants are not known (102, 122). In our laboratory we have ob- 

 served that Fusarium solani forms abundant macrospores in shake 

 culture, particularly in certain media. Conidia of Streptomyces griseus 

 appear in aerated cultures at the time of autolysis (67). 



In a very brief note, Richards (242) reported that ozone appears to 

 stimulate sporulation in Alternaria spp.; Mycosphaerella citrullina 

 forms more pyenidia if exposed to ozone, but the spores are non-viable. 



The striking effect of carbon dioxide and bicarbonate on sporangial 

 development in Blastoclaclia and Blastocladiella spp. is considered later 

 in this chapter. 



5. ACIDITY 



Although pH has definite effects on reproduction, no unitary hy- 

 pothesis or generalization is possible; as mentioned elsewhere (Chapter 

 1), pH effects are exerted in a variety of ways. Furthermore, the com- 

 mon failure to determine the final pH casts doubt on much of the 

 available data. In general, it appears that the pH range for sporula- 

 tion is somewhat narrower than that for growth (5, 20, 23, 192, 193, 

 276). Within the physiological range, an acid reaction is often less 

 favorable than a neutral or mildly alkaline reaction (170, 245, 276), 

 but exceptions to this rule have been reported (5, 156). 



Lockwood (192) showed that in several fungi perithecia form over 

 a wide range of pH values, but asci mature only at neutral or slightly 

 alkaline reaction. This finding is of particular importance because 

 often the only data submitted in pH studies concern macroscopically 

 visible fruit bodies. 



In Mycosphaerella pinocles pyenidia, pseudothecia, and chlamydo- 

 spores are reported to have different pH optima (266), but other data 

 on the same species are not easily reconciled with this claim (23). The 

 pH range for sporangium formation by Physarum polycephalum is 

 narrowed by high temperature cultivation (116). 



