[Illustration: FIG. 13.--_Ascobolus furfuraceus._ Diagrammatic section
of the fructification. (After Janczewski.)
m, Mycelium.
c, Archicarp.
l, Pollinodium.
s, Ascogenous filaments.
a, Asri.
r, p, The sterile tissue from which the paraphyses h spring.]
_Discomycetes._--Used in its widest sense this includes the
Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is
characterized in general by the possession of an ascocarp which,
though usually a completely closed structure during the earlier stages
of development, at maturity opens out to form a bowl or saucer-shaped
organ, thus completely exposing the layer of asci which forms the
hymenium. Such an ascocarp goes by the name of _apothecium_. Owing to
the shape of the fruit-body many of these forms are known as
cup-fungi, the cup or apothecium often attaining a large size,
sometimes several inches across (fig. 12). Functional male and female
organs have been shown to exist in _Pyronema_ and _Boudiera_; in
_Lachnea stercorea_ both ascogonia and antheridia are present, but the
antheridium is non-functional, the ascogonial (female) nuclei fusing
in pairs; this is also the case in _Humaria granulata_ and _Ascobolus
furfuraceus_, where the antheridium is entirely absent. In _H.
rutilans_, however, both sexual organs are absent and the ascogenous
hyphae arise apogamously from the ordinary hyphae of the mycelim. In
all these cases the ascogonium and antheridium contain numerous
nuclei; they are to be looked upon as gametangia in which there is no
differentiation of gametes, and since they act as single gametes they
are termed coenogametes. In some forms as in _Ascobolus_ the
ascogonium is multicellular, the various cells communicating by pores
in the transverse walls (fig. 13).
In the Helvellaceae there is no apothecium but a large irregular fruit
body which at maturity bears the asci on its surface. The development
is only slightly known, but there is some evidence for believing that
the fruit-body is closed in its very early stages.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.
FIG. 14.--Perithecium of Podospora fimiseda in longitudinal section
After v. Tavel.
s, Asci.
a, Paraphyses.
e, Periphyses.
m, Mycelial hyphae.]
The genus _Peziza_ (in its widest sense) may be taken as the type of
the group. Most of them grow on living plants or on dead vegetable
remains, very often on fallen wood; a number, however, are found
growing on earth which is rich in humus. The genus _Sclerotinia_ may
be mentioned here; a number of forms have been investigated by
Woronin. The conidia are fragrant and are carried by bees to the
stigma of the bilberry; here they germinate with the pollen and the
hyphae pass with the pollen tubes down the style; the former infect
the ovules and produce sclerotia, therein reducing the fruits to a
mummified condition. From the sclerotia later the apothecium develops.
One species, _S. heteroica_, is _heteroecious_; the ascospores
infecting the leaves of _Vaccinium uliginosum_, while the conidia
which then arise infect only _Ledum palustre_. This is the only case
of heteroecism known in the vegetable kingdom outside the Uredineae.
_Pyrenomycetes._--This is an extraordinarily large and varied group of
forms which mostly live parasitically or saprophytically on vegetable
tissue, but a few are parasitic on insect-larvae. The group is
characterized by a special type of ascocarp, the _perithecium_. This
is typically of a flask-shaped form opening with a small pore at the
top. The asci live at the bottom often mixed with paraphyses, while
the upper "neck" of the flask is lined with special hyphae, the
periphyses, which aid in the ejection of the spores (fig. 14). The
simpler forms bear the perithecia directly on the mycelium, but the
more highly developed forms often bear them on a special mycelial
development--the stroma, which is often of large size and special
shape and colour, and of dense consistence. The cytological details of
development of the perithecia are not well known; most of them appear
to develop their ascogenous hyphae in an apogamous way without any
connexion with an ascogonium. Besides the special ascocarps, accessory
reproductive organs are known in the majority of cases in the form of
conidia.
_Tuberineae._--These are a small group of fungi including the
well-known truffles. They are found living saprophytically (in part
parasitically) underground in forests. The asci are developed in the
large dense fruit bodies (cleistothecia) and the spores escape by the
decay of the wall. The fruit-body is of complicated structure, but its
early stages of development are not known. Many of the fruit-bodies
have a pleasant flavour and are eaten under the name of truffles
(_Tuber brumale_ and other species). The exact life-history of the
truffle is not known.
_Laboulbeniineae_ are a group of about 150 species of fungi found on
insects, especially beetles, and principally known from the researches
of Thaxter in America. The plant is a small, dark brown, erect
structure (receptacle) of a few cells, and 1-10 mm. high, attached to
the insect by the lowermost end (foot), and easily mistaken for a hair
or similar appendage of the insect. The receptacle ends above in
appendages, each consisting of one or a few cells, some of which are
the male organs, others the female organs, and others again may be
barren hairs. The male organ (antheridium) consists of a few cells,
the terminal one of which either abstricts from its end, or emits from
its interior the non-motile spermatia, reminding us of those of the
Florideae. The female organ is essentially a flask-shaped structure;
the neck of the flask growing out as the trichogyne, and the belly
composed of an axial carpogenic cell surrounded by investing cells,
and with one cell (trichophoric) between it and the trichogyne. These
three elements--trichogyne, trichophoric cell, and carpogenic
cell--are regarded as the procarp. The spermatia have been shown by
Thaxter to fuse with the trichogyne, after which the axial cell below
(carpogenic cell) undergoes divisions, and ultimately forms asci
containing ascospores, while cells investing this form a perithecium,
the whole structure reminding us essentially of the fructification of
a Pyrenomycete. Many modifications in details occur, and the plants
may be dioecious. No injury is done to the infested insects. It has
lately been shown that there is a fusion of nuclei in connexion with
ascus formation, so that there can be no doubt of the position of this
extraordinary group of plants among the Ascomycetes. The various cells
of these organisms are connected by large pits which are traversed by
thick protoplasmic threads connecting one cell with the next. In this
point and in their method of fertilization the Laboulbeniineae suggest
a possible relationship of Ascomycetes and the Red Algae.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.