218 GERMINATION OF SPORES 



tions in the absence of the enzyme carboxylase, and a second sys- 

 tem which is active in heated spores. Inactivity- of carboxylase 

 thus constitutes the respiratory block. 



The age of spores of Myxomycetes has been shown by Smith 

 (1929) to be of little significance in germination. Using herbarium 

 specimens, he secured germination in spores from 5 to 32 years 

 after collection, as is shown in Table 17. 



WATER RELATIONS AFFECTING GERMINATION 



Since water is profoundly important in all vital phenomena, it 

 may be anticipated that its presence is a primary requirement in 

 initiating spore germination. Spores, like seeds, do not all become 

 wet with equal facility, an observation that has been made by 

 everyone who has attempted to suspend spores in water for use as 

 inoculum. Ziegenspeck (1934) has clarified the physico-chemical 

 principles involved in the problem of wetting spores. Wetting 

 must be regarded as the displacement of a gas film at the surface 

 of a solid (spore) by a liquid (water). It implies an affinity of the 

 solid for the liquid and is governed by solid-liquid, solid-gas, and 

 liquid-gas tensions. The resultant forces are measurable in terms 

 of the angle of contact made by the solid with the liquid, as 

 Ziegenspeck shows. 



An examination of the literature on moisture requirements for 

 germinating spores shows conflicting results concerning whether 

 a film of water is necessary, since at high relative humidity a 

 slight decrease in temperature causes condensation. Experimenta- 

 tion becomes difficult if the effects of an aqueous film and of hu- 

 midity are to be distinguished. Far too little careful work has 

 been done on this problem. 



Doran (1922, pp. 334-335) recorded that Sclerotinia fructigena, 

 Peronospora pygmaea, Phyllosticta antirrhini, Cylindrocladium 

 scoparium, and urediniospores of Puccinia coronata germinate 

 only when in direct contact with water. On the other hand, his 

 observations show that aeciospores of Gy?tmosporangium clavipes 

 and conidia of Alternaria solani and Venturia inaequalis may germ- 

 inate in moist air. Stock (1931) failed to secure germination of 

 urediniospores of Puccinia graminis and P. coronata when the 

 spores were dusted on glass slides at relative humidities of 99% 

 or below. 





