Introduction to Fungi, Third Edition

332 HYMENOASCOMYCETES: PYRENOMYCETES
Fig12.9 Chaetomium globosum. (a) Perithecium showing
cirrhus of ascospores. (b) Asci. (c) Ascospores.
other arthropods (Wicklow, 1979). When the
perithecia are ripe, a column-like mass (cirrhus)
of dark ascospores is extruded through the
ostiole, supported by the perithecial hairs
which surround it (Fig. 12.9a). The spore
column results from the breakdown of the asci
within the body of the perithecium, i.e. the asci
do not discharge their spores violently. The
young asci are cylindrical to club-shaped, but
this stage is very evanescent and is only found
in young perithecia (Fig. 12.9b), often before
the spores have become pigmented and sometimes
before the perithecium has developed an
ostiole. Spores of most species are pale brown to
grey and lemon-shaped, with a single terminal
germ pore.
The development of perithecia in
Chaetomium shows some variation between
species (Whiteside, 1957, 1961). The ascogonia
are coiled and lack antheridia. Investing hyphae
arise from the ascogonial stalk or from vegetative
cells surrounding the ascogonium.
Perithecial hairs develop early from the external
cell layer. The centrum is at first filled with
hyaline pseudoparenchyma. At the apex of the
perithecium, certain of these cells become
meristematic and give rise to the elongate
periphyses which line the inside of the ostiole.
Ascogenous hyphae develop from the ascogonium
at the base of the centrum and, at about
this time, the surrounding pseudoparenchyma
cells of the centrum deliquesce to form a cavity.
In some species croziers develop, but in others
they are absent. Paraphyses of two types have
been described. In C. brasiliense, lateral paraphyses
arise from the pseudoparenchyma cells
outside the hymenium, whilst hymenial paraphyses
have been illustrated in C. globosum
(Whiteside, 1961). The paraphyses are evanescent
and disappear as the asci mature.
Most species of Chaetomium are homothallic,
but a few, e.g. C. cochliodes, are heterothallic.
Both homo- and heterothallic strains of C. elatum
have been reported. Conidial states are rare in
Chaetomium, but simple phialides and phialospores
occur in C. elatum and C. globosum, whilst
C. piluliferum forms both phialospores and
globose thalloconidia of the Botryotrichum type
(Daniels, 1961).
12.3 Xylariales
The order Xylariales is probably polyphyletic and
comprises about 800 species in 8 families (Kirk
et al., 2001). Members of this group produce dark
perithecial stromata with asci which have an
iodine-positive apical apparatus. Ascospores are
typically melanized. Xylariales are saprotrophs
or plant pathogens and are associated especially
with the bark and wood of trees.
We shall only consider one family, the
Xylariaceae, which is by far the most important.
The dark-coloured ascospores are generally
smooth-walled and inaequilateral (one face
being more strongly curved than the other),
with a hyaline germ slit. Anamorphs are sometimes
also stromatic, producing hyaline or
lightly pigmented conidia holoblastically from
a sympodially proliferating conidiogenous
region, the sympodula (Fig. 12.13). They have