ASCOMYCETES 



135 



each other by a gelatinous appendage. The rest of the ascus collapses, 

 withdraws and makes place for the next. The height of projection of 

 the ascospores occasionally attains a relatively enormous value if one 

 considers that the perithecia are only about half a millimeter high in the 

 middle. Thus in Podospora fimiseda, it reaches 15 cm. and in P. curvi- 

 colla, as high as 45 cm. (Weimer, 1920). 

 In other forms, as Leptosphaeria 

 acuta (Hodgetts, 1917), Pleospora her- 

 barum (Atanasoff, 1919) and P. scirpi- 

 cola (Pringsheim, 1858), this discharge 

 is favored by an anatomical differentia- 

 tion of the ascus wall. This consists of 

 two layers which are only recognizable 

 at the moment of spore liberation, a 

 rigid inelastic cuticular outer layer 

 which does not swell in water, and a 

 thicker gelatinous inner which absorbs 

 water and swells. The paraphyses also 

 swell in damp weather. By the pres- 

 sure of the swollen inner layer, the outer 

 ruptures at the tip, the inner together 

 with the ascospores pushes out, tears 

 open laterally with a jerk at the point 

 of emergence or at the top and dis- 

 charges the ascospores. 



In still other forms, as in Endothia 

 parasitica and in many Gnomoniaceae, 

 the length of the perithecial neck does 

 not permit the asci, as in Podospora, 

 to reach the opening and there to eject 

 their spores. Hence they break loose 

 from their point of formation and closely 

 press together, more or less parallel, with 

 the tip, directed toward the entrance 

 canal and fill up the interior of the peri- 

 thecium. Hereupon they are gradually 

 pressed out through the canal by the 

 periphyses. On the drying of the peri- 

 thecial neck, the asci are pressed together until they rupture under the 

 lateral pressure and discharge their spores. Thus the mechanism of 

 discharge here rests in the perithecial opening; if this is cut off, at least 

 in Endothia parasitica, spore discharge ceases (Heald and Walton, 1914). 

 The apothecium, the second basic form mentioned above, differs 

 from the perithecium mainly in the greater development of the fertile 



Fig. 83. — Humaria convexula. 

 Above, cross section of apothecium. 

 ( X 20.) Below, section of hymenium 

 showing asci and paraphyses. (X 550; 

 after Sachs.) 



