126 



MYCOLOGY 



ascus now unite. The fusion or nucleus then divides to form those 

 of the eight ascospores, and the walls of the perithecium grow to inclose 

 the asci thus formed, including the paraphyses, which develop between 

 the asci. 



All of the typic ASCOMYCETALES have uninucleate hyphal cells, 

 while the ascogenous hyphse are binucleate, and in this case the nucleus 

 has a double chromosome number. Hence is suggested an alternation 

 of generations. 



The life cycle of Pyronema may be displayed in a graphic form 

 beginning with the ascospore and ending with its production again. 

 The diploid, or twenty-four chromosome condition, may be repre- 

 sented by the double lines. This life cycle is contrasted with the 

 well-known one of the fern where a well-marked alternation of genera- 

 tion is shown. 



Fern 

 Spore 



(After Claussen) 



Pyronema 

 Spore 



Prothalluim 



/ \ 



Antheridium Archegonium 



I. I 



Spermatozoid Egg cell 



I I 



Mycelium 

 Antheridium Ascogonium 



I 1 



T. i 



Antheridium Ascogonium 



Sperm nucleus ^Egg nucleus (Sperm) Nucleus (Egg) Nucleus 



\ 



/ 



Sporophyte 



il 



Spore mother cell 



4 Spores 



Ascogenous hyphge 



h 



Uninucleate ascus 



4 Nucleate ascus 



Brown, in his studies of Leotia, has shown that the asci are formed 

 at the tips of the ascogenous hyphae in several different ways (Fig. 

 41). In some cases, to quote him, "a hypha forms a typical hook, 



