;o 



MORPHOLOGY 



sexual apparatus, from heterogamous alga-like forms to isogamous forms. 

 At the same time, although the sex organs lose their dissimilarity in 

 appearance, there is retained, at least in some forms, a physiological 

 differentiation which extends to individual mycelia. 



(2) ASCOMYCETES 



General character. — The sac fungi include the majority of fungi, and 

 their connection with the algae is very vague. In contrast with the 

 Phycomycetes, the mycelium is composed of septate filaments, and the 

 sex organs are much reduced and even suppressed. The common 

 character of this great assemblage of forms is the appearance of an 

 ascus (sac) in the life history, in which the ascospores are formed. The 

 ascus is a special cell, usually club-shaped or elongated, which at first 

 contains two nuclei. These nuclei fuse, and the fusion nucleus begins 

 a series of three successive divisions, resulting in eight nuclei. About 

 each nucleus a wall is formed, cutting out some cytoplasm, producing 

 eight ascospores (fig. 176). These definitely three successive nuclear 

 divisions, resulting always in eight ascospores, are found to be reduc- 

 tion divisions, resulting in reducing the number of chromosomes, and 

 therefore the ascus holds the same important place in the life history 

 of an ascomycete as does a spore mother cell in the higher plants (see 

 p. 95). In the majority of forms, a spore case is developed in con- 

 nection with the asci, more or less investing them with a protective 

 jacket. This investing structure is the 

 ascocarp, and it holds the same relation 

 to the asci as does the cystocarp of red 

 algae to the carpospores. 



The group is so extensive and varied 

 that no representative forms can be 

 selected. A few illustrations from the 

 eight usually recognized orders are as 

 follows: 



(a) Protoascales 



The yeasts (Saccharomycetes) are the 

 Figs. 168-173. — Yeast: 168, yeast most familiar forms, but their position 



cell; 169, 170. cells budding; i 7I( here ; g doubtful, for it is felt that 



172, sprout chains; 173, cell con- J . 



taining four spores.— 168-171, after their whole lite history IS probably not 



Coulter; 173, after Rees. known. They are solitary, oval cells, 



