406 



SPORE GERMINATION 



been shown that spore volume decreases during germination in dry air. 

 Although somewhat conflicting data and a theory based on the hypo- 

 thetical conversion of bound to free water have been reported (33, 34), 

 it seems that the high water content of conidia of the powdery mildew 

 fungi explains their ability to germinate at low humidity. The occur- 

 rence of plasmoptysis in pure water argues for a rapid entrance into the 

 spore; this is still to be reconciled with the observation that the loss of 

 water from ungerminated spores is very slow — approximately 0.08 per 

 cent of the loss from an equivalent free water surface (312). 



These studies on the powdery mildews have been generalized by 

 Yarwood (310) in the proposition that fungus spores of the usual type, 

 i.e., those with a low water content, must absorb water to about the 

 level of that in the powdery mildew conidia (70 per cent) before they 

 can germinate. This hypothesis explains the swelling of these spores 

 so frequently reported (p. 391), and in fact demands that all spores of 

 low water content swell on germination. This appears to be the most 

 promising general explanation of humidity relations. 



As is to be expected, fungi tolerant of low humidity are also in gen- 

 eral tolerant of high osmotic pressure in the germination medium. 

 This relation is brought out in Figure 6. 



Conidia of Erysiphe graminis germinate in concentrations of deu- 

 terium oxide up to 100 per cent, but the heavy water is markedly in- 

 hibitory to germ tube elongation (220); Macrosporium sarcinaeforme 

 is unable to germinate in 60 per cent deuterium oxide (213). 



Figure 6. The relation of the 

 minimum relative humidity to 

 the maximum tolerated concen- 

 tration of sodium chloride for 

 spore germination in several 

 fungi (Aspergillus spp., Penicil- 

 lium spp., Fusarium monili- 

 forme). Each plotted point 

 refers to one species. Drawn 

 from data of Armolik and Dick- 

 son (4). 



15 20 25 30 



Maximum sodium chloride, per cent 



