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: 
172 
(a) Protoascales 
The yeasts (Saccharomycetes) are the 
FIGS. 168-173. Yeast: 168, yeast most familiar forms, but their position 
cell; 169, 170, cells budding; 171, here j s doubtful, for it is felt that 
172, sprout chains; 173, cell con- . ,,,.,.,. 
taining four spores. -168-171, after their whole life history is probably not 
COULTER; 173, after REES. known. They are solitary, oval cells, 
