344 PHYSIOLOGY OF THE FUXGI 



pai't way through the pore to its broadest point and is then suddenly 

 squeezed out by the contraction of the ascus tip (Ingold, 1933). 



The increase in the osmotic pressure within the maturing ascus must 

 be preceded by an increase in the soluble materials in the cell sap. This 

 is believed to be accomplished by the digestion of glycogen, which is 

 known to occur in the young ascus. In the majority of t he Pyrenomycetes 

 the asci are produced within a spherical or flask-shaped perithecium. 



There are three general ways by which the ascospores are released 

 through the ostiole of the perithecium, two of which depend upon the 

 explosi^•e rupture of the ascus wall. In the first type, which is the most 

 common and believed to be the most primitive, the ascus wall remains 

 attached at its base, while the spores are discharged. This is accom- 

 plished by the elongation of the elastic ascus. until the tip reaches or 

 protrudes through the ostiole. The ascus then explodes, throwing the 

 ascospores into the air. The wall of the empty ascus contracts to the 

 base of the perithecium. and another ascus elongates. The process is 

 repeated successively as the asci mature. This type is illustrated by 

 Sordaria, Pleurage, and many other common fungi. An interesting 

 parallelism exists between the method of dissemination of Pilobolus and 

 that of Pleuragc, Sordaria, and other coprophilous Pyrenomycetes. The 

 short beaks of the perithecia are positi\ely phototropic and, as they 

 develop, are directed toward the source of light. In nature the ascospores 

 fall upon vegetation and are subsequently eaten and disseminated bj' 

 herbivorous animals. The vertical distances to which ascospores may 

 be shot have been reported as 6 cm. for S. fimicola and 45 cm. for P. 

 curvicolla (Weimer. 1920). 



A second general type of ascospore discharge occurs more commonly in 

 species with long perithecial beaks. The asci become detached from the 

 base of the perithecium and are pushed up through the beak to the ostiole, 

 where the spores are released simultaneously or successively. This is a 

 rapid method of spore discharge. Examples of this type are Etidothia 

 parasitica. Gnomonia rubi, Guignardia bidivcllii, and CcraiosfomclJa anipid- 

 lax:ea. !Most of these species are adapted to AA"ind dissemination of 

 ascospores. Some idea of the tremendous numbers of ascospores dis- 

 charged is given by Heald and Walton (1914), who reported that some 

 specimens of E. parasitica expelled ascospores every day for 168 days. 

 The rate of spore discharge from one perithecium was found to be as high 

 as one ascus explosion about every 2 sec. At this rate approximately 

 14,000 ascospores may be discharged per perithecium per hour. On the 

 basis of these figures, it is little wonder that the fungus spread so rapidly 

 among the American chestnuts. 



A third group includes the nonexplosive type of ascus, in which the 

 ascospores are released by the deliquescence of the ascus wall. Thej' are 



