400 SPORE GERMINATION 



est at intermediate relative humidity and is less at either extreme (6, 

 57, 129, 214, 215, 221, 237). Chlamydospores of Urocystis tritici respond 

 in the same way (206). Damage at low moisture levels is probably sim- 

 ple water loss; consideration of the possible effects of high humidity 

 leads us to the third and final category of humidity response. 



This last type of response is exemplified by both ascospores and en- 

 doconidia of Endoconidiophora fagacearum (201) and by conidia of 

 Helminthosporium oryzae (211); these remain viable longest at low 

 humidity, especially if the temperature is unfavorably high for survival. 

 It has been suggested that at high humidity the metabolism of this type 

 of spore is accelerated, with a consequent reduction in longevity (201); 

 the mechanism of such an effect is obscure, but this is at present the 

 most attractive hypothesis and may be extended to the problem of 

 uredospore survival at high humidity. 



Spores in nature are often protected from rapid loss of water by a 

 gelatinous matrix, by the structure in which they are formed, or by 

 host plant tissues; several such phenomena are reviewed by Gottlieb 

 (113). 



Spores of Myrothecium verrucaria are strongly affected by the history 

 of the culture, in particular by the composition of the medium (64). 

 Spores from a mycelium grown on filter paper survive more than twice 

 as long as those from any of three agar media; this greater viability 

 is not, however, correlated with the rate of loss of dry weight. The 

 growth medium also exerts a significant effect on the dry weight, nitro- 

 gen content, and respiratory behavior of spores of M. verrucaria. Co- 

 nidia of nutritionally deficient mutants of Ophiostoma multiannulatum 

 die less rapidly in a starvation medium than do conidia of the parent 

 non-deficient stock; it has been suggested that the deficient spores 

 metabolize less and hence survive longer (96). 



Physiologically, senescent spores are characterized by poor germina- 

 bility, by a slower rate of germination (36, 57, 194), and by lower re- 

 spiratory activity (64). 



In Streptomyces sp., ageing of the spore population at low tempera- 

 ture increases the recovery of mutant types; the mechanism is obscure 

 (294). 



6. NUTRITION AND GERMINATION 



Germination may be considered as a growth process in which some 

 or all the necessary nutrients are present in the spore ab initio. We 

 may therefore expect two extreme types of relation to external nutri- 

 ents — complete independence in some fungi and complete dependence 



