The main characteristic which distinguishes the Ascomycetes is the
_ascus_; a name given to a sporangium of a definite shape and size,
and containing a definite number of spores. The shape is generally
club-like or spherical, the number of spores 8 (in some 2, 4, 16 or
more), see Figs. 103, 105, 108, 110, 113, 116, 120, 121, 123, 129.
In the lowest forms, the EXOASCI, the ascus springs directly from the
mycelium without the formation of a fruit-body (_i.e._ ascocarp).
In the higher forms, which contain many species, the CARPOASCI, the
asci are united and form ascocarps which may be more or less enclosed
(angiocarpic, hemiangiocarpic, and probably gymnocarpic).
[Illustration: FIG. 103.--Endogenous formation of spores in
_Peziza confluens_. In the youngest asci there is only one
nucleus (_b_, _e_); this divides into two (_f_); and the division
is repeated so that there are 4 nuclei in _c_ and 8 in _g_. These
surround themselves with protoplasm and a cell-wall (_h_, _i_).
The protoplasm of the mother-cell is not entirely used up.]
The hyphæ of the _Mycelium_ in some remain free, in others they are
felted together and form thick strands or flat, cushion-like bodies
(compare in particular the stromata of the Pyrenomycetes). Some species
form _sclerotia_ (Figs. 116, 128).
Asexual reproduction by means of _conidia_ is known in many species
as the principal means of reproduction, and the one which affords
the most rapid means of distribution. The conidia may be produced on
conidiophores (Fig. 109), in conidial-layers (Fig. 122), and often in
conidiocarps (pycnidia, Figs. 120 _d_, _e_; 123 _a_; 124 _b._). These
last occur partly as the so-called “spermogonia” (that is, pycnidia
with microconidia). The conidiophores never approach the basidia.
In many species the ascospores germinate and form conidia
immediately (_Nectria cinnabarina_, _Sclerotinia_, _Taphrina_,
etc.), sometimes while they are still in the ascus and before
their ejection (_Taphrina_, Fig. 105 _a_). In many instances
the conidia by means of continued budding can, for a longer
or shorter time, produce yeast-conidia, _e.g. Taphrina_. In
many other cases the conidia arise from the germ-tubes of the
ascospores, or at any part of the mycelium. The unripe asci
of _Taphrina_, when placed in water, develop conidia at their
apices. The _Sclerotinia_-species produce numerous conidia
whose germination has never been observed. The formation of
conidia and asci sometimes takes place on the same fruit-body.
In _Heterosphæria patella_ the conidia and asci are developed
successively in the same fruit-body; in the ascocarps of
_Dermatea frangula_ and _Sclerotinia sclerotiorum_ the formation
of conidia may take place. The ascocarps frequently arise
from the conidial-layers (_Nectria cinnabarina_, etc.). This
relationship of the two forms of reproduction to each other
may be explained by considering that both have descended
phylogenetically from sporangia.
Sometimes _chlamydospores_ and _oidia_ also appear in the Ascomycetes;
on germination, however, they do not, as in _Protomyces_, form
sporangia, and on this account cannot be distinctly distinguished from
conidia.
The asci are morphologically the highest form of reproduction and
are always found at the close of the development of these Fungi; the
accessory forms of reproduction are first developed, but a well-defined
alternation of generations does not occur.
In the Ascomycetes there are more than 11,000 described species,
which can be classed as follows:--
Series 1. EXOASCI. Only one order.
„ 2. CARPOASCI.
Family 1. _Gymnoascales_, }
„ 2. _Perisporiales_, } Angiocarpic Carpoasci.
„ 3. _Pyrenomycetes_, }
„ 4. _Hysteriales_, } Hemiangiocarpic Carpoasci.
„ 5. _Discomycetes_,}
„ 6. _Helvellales_, Gymnocarpic (?) Carpoasci.
Additional _Ascolichenes_: Lichen-forming Ascomycetes.