Ill] 



EXOASCALES 



93 



mother-cell during development. Two nuclei can frequently be recognized 

 in the cells of the fertile mycelium, and the young ascus, in all investigated 

 cases, is binucleate. The two nuclei fuse, the fusion nucleus undergoes three 

 successive divisions and eight spores are formed (fig. 48). In many species 

 budding of the ascospore takes place, so that the mature ascus contains 

 numerous minute conidia (fig. 50) by means of which the fungus is distributed. 

 The Exoascales include the single family Exoascaceae ; with this is 

 sometimes associated the Ascocorticiaceae containing the saprophytic 



Fig. 50. Taphrina aurea (Pers.) Fr. ; mature asci, x 500. 



genus Ascocorticium with five known species. The asci of Ascocorticiitm, 

 like those of the Exoascaceae, are cylindrical in form, parallel in arrange- 

 ment and quite unprotected. It is open to question whether the parallel 

 arrangement of the asci in the Exoascaceae has any phylogenetic significance 

 or is not rather the result of their development on the surface of the carpel 

 or leaf. The cylindrical form of the ascus, however, does not suggest a 

 primitive group. It may perhaps be inferred that the Exoascaceae are 

 reduced forms derived from one of the phyla with protected asci ; there 

 does not appear at present to be any clue to their probable ancestry. The 

 same is true of the Ascocorticiaceae, of the development of which even less 

 is known. 



Exoascaceae 



The classification of the Exoascaceae, and especially the separation of 

 the two principal genera, Exoascus and Taphrina, has been based on a 

 variety of different characters including the form of the ascus, the develop- 

 ment or not of conidia from the ascospores, the annual or perennial character 

 of the mycelium and the presence or absence of a stalk cell. Boudier 

 defines Exoascus as having asci usually octosporous and usually provided 

 with a basal cell; Taphrina as having asci usually polysporous, sometimes 

 provided with a basal cell. 



