THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 289 



that they are formed by the branching and ramification of the ascogenous 

 hyphae over the floor of the perithecium. Meanwhile paraphyses develop 

 and separate the asci from one another. They are simple unbranched hyphae, 

 and it is suggested that they may arise not from the wall tissue of the 

 perithecium but as sterile branches from the ascogenous hyphae. 



When mature the asci are long and slender, but are considerably shorter 

 than the paraphyses, which partly fill the perithecium. Each ascus gives rise 

 to eight ascospores which are oval in shape and brown in colour (Fig. 283). 



Fig. 283. — Hypoxylon coccineum. Section through mature 

 perithecium showing asci with ascospores and ostiole 

 of perithecium. Note the basal layer in which the 

 ascogonia are embedded. 



Meanwhile the wall of the perithecium has continued to develop, forming 

 a thick plectenchyma of small cells containing, especially on the side nearest 

 the cavity, dense black material. The perithecium is finally extended through 

 the superficial zone as a short beak terminating in an ostiole. The inner 

 margin of the ostiole is beset by short thin-walled filaments of cells which 

 form a protective tissue between which the ascospores finally escape. They 

 germinate to produce a fresh mycelium. 



Relationships 



Hypoxylon coccineum is among the most highly specialized of the 

 Sphaeriales. It indicates the way in which, from a relatively simple type 

 where the perithecia are embedded in the host tissues, as are the apothecia 

 of Rhytisma acerinum, a complex external stroma is developed which protects 

 and nourishes the developing perithecia and enables a very much greater 

 spore output to be produced. It is worth noting that in certain allied genera 



