398 SPORE GERMINATION 



long obligatory rest periods. If we define dormancy as a rest period 

 after the spore has completed both its morphological and its cytological 

 development, and if we assume that at least part of the rest period in the 

 phycomycete and basidiomycete spores just considered is such true dor- 

 mancy, then the most promising hypothesis is that some factor associ- 

 ated with the heavy spore wall — possibly impermeability, possibly sim- 

 ple mechanical strength — is determinative. It is not likely that the 

 same explanation will fit ascospore dormancy in Neurospora, in which 

 an internal, biochemical mechanism seems more probable. However, 

 ascospore dormancy in Onygena equina develops only after the heavy 

 spore wall is laid down — the immature spore germinates without a 

 dormant period (30). 



5. SPORE LONGEVITY 



Several fungi form asexual spores whose survival under the most 

 favorable conditions is measured in days only; examples include the 

 conidia of the powdery mildew fungi (317), the sporangium of Perono- 

 spora tabacina (302), and the sporidia (basidiospores) of some rust 

 fungi (178, 222, 262). The short life of the powdery mildew conidia 

 is probably related to the high water content of the spores (317). 



Aeciospores and uredospores of the rust fungi are intermediate in 

 longevity. Although in a few species these spores may under excep- 

 tionally favorable conditions survive for as long as a year (57, 77, 89, 

 214), their lives under most conditions of storage are weeks or months 

 only (6, 20, 72, 73, 74, 221) and are even less under uncontrolled nat- 

 ural conditions (57, 237). Early work on spore survival in the rust 

 fungi is reviewed by Maneval (193) and by Zimmerman (322). 



Types of spore characterized by a long or very long life include the 

 chlamydospores of the Ustilaginales (88, 171, 278), basidiospores of 

 Hymenomycetes (95, 127), and at least some ascospores (46, 127). 

 Spores with long periods of inability to germinate — resting spores of 

 phycomycetes, teliospores of the rust fungi, and certain ascospores — are 

 ipso facto long-lived. 



Although the rule in the fungi is that asexual spores are relatively 

 short-lived, the conidia of certain imperfect fungi and of actinomycetes 

 may survive very long periods (15, 175, 212, 234, 297). Ecologically, it 

 appears that the survival function typical of sexual spores is taken over 

 in these forms, which lack a sexual stage, by the conidia. Dried swarm- 

 spores of the Myxomycetes similarly are reported to survive many years 

 (78, 258). 



