46 



ASCOMYCETES 



[CH. 



received a considerable impetus in 1907 and 1912, from the work of Claussen 

 on Pyronema confliiens. According to tlie latter author the male nuclei enter 

 the oogonium and pair with the female nuclei, but do not fuse with them. 

 The sexual nuclei pass in pairs into the ascogenous hyphae (fig. 15), and 



eventually they or their descendants 

 fuse in the ascus. For Claussen, there- 

 fore, there is a single fusion (that in 

 the ascus) and a single reduction (the 

 meiotic) in the life-history of Pyrow^wa. 

 His interpretation has been followed 

 by Schikorra on Monas.cus, by Faull on 

 Laboidbenia, and by Ramlow on Asco- 

 bolusiminersuszwAAscophanuscarneus. 

 The last-named author figures certain 

 "fusions" in the oogonial region of his 

 fungi, but regards them as pathological. 

 Claussen's hypothesisdemandsthat 

 the attraction between the sexual 

 nuclei, though not sufficient to cause 

 fusion, yet holds them together in the 

 multinucleate ascogenous hyphae and 

 is transmitted to their descendants 

 when nuclear division has occurred'. 

 It is based on three chief grounds : 



(i) the failure to observe fusions in 

 the oogonium or their interpretation, 

 if found, as pathological phenomena ; 



(2) the recognition of as many 

 chromosomes in the third division in 

 the ascus as in the iirst; 



(3) the observation of paired nuclei 

 in the ascogenous hyphae. 



The first of these grounds has a mainly negative value ; in regard to 

 the second, further investigation is very much to be desired ; the statement 

 of the chromosome number without figures is of little value, nor is any 

 figure of the third division significant except that of the late anaphase; in the 

 earlier stages chromosomes are scattered about the spindle, so that there is 

 no criterion by which a metaphase showing efight undivided chromosomes 

 can be distinguished with surety from an early anaphase showing two sets of 



' A comparison is sometimes instituted between the sporophyte of the Ascomycetes and that of 

 the rusts. In the rusts, however, each pair of nuclei is enclos ed in a separate cell and the hypothesis 

 of nuclear attraction throughout the vegetative phase is accordingly not required. 



Fig' 15- Pyronema confuens; sexual apparatus 

 and paired nuclei in the ascogenous hyphae; 

 a. antheridium ; b. trichogyne ; c. oogunivim ; 

 d. ascogenous hyphae; x 1040 ; after Claussen. 



