CH. Il] 



ASCOMYCETES 



35 



The Ascus. The ascus or mother-cell of the spores is a spherical, oval, 

 club-shaped, or almost cylindrical organ with a narrow, more or less elongated 



Pleospora sp.; germinating spores, x looo. 



base. When moderately young it contains a single nucleus which undergoes 

 three karyokinetic divisions giving rise to 

 eight daughter nuclei (fig. 3). Asci of the 

 short, stout type are full of dense cyto- 

 plasm ; in the relatively cylindrical forms 

 the ends are usually vacuolate, but a broad, 

 granular belt fills the middle region and 

 contains the nuclei. The spores are cut 

 out by free cell-formation so that a certain 

 amount of cytoplasm remains outside them, 

 constituting the epiplasm. It becomes 

 charged with glycogen and other food 

 substances, and is a source of supply to 

 the developing spores. 



In the vast majority of cases the mature 

 ascus contains eight spores, but in a certain 

 number of species, though eight nuclei are 

 produced, only one or a few are utilized as centres of spore-formation. 

 Thus a single spore is sometimes developed in Tuber, two is the regular 

 number in Phyllactinia, and four in many of the Laboulbeniales ; in 

 Bulgaria pofymorpha eight spores are initiated but only four reach maturity. 

 In Endomyces it is probable that the ascus nucleus divides only twice, 

 and the four spores utilize all the available nuclei. On the other hand ad- 

 ditional nuclear divisions sometimes precede spore-formation so that the 



Fig. L . Spores of a. Geoglossiim diffbrme 

 Fr.; b. Delitschia furfuracea Niessl; 

 c. Rhytisnia acerinuin (Pers.) Fr. ; d. 

 Chaetomiuni Kuntzeanutn Zopf ; e. 

 Podospora minuta (Fuck.) Wint. ; 

 X500. 



3—2 



