Introduction to Fungi, Third Edition

240 ASCOMYCOTA (ASCOMYCETES)
Fig 8.12 Types of ascus dehiscence. (a) Prototunicate ascus; the wall dissolves to release the ascospores passively. (b,c) Operculate
asci before and after discharge; the ascus opens by means of a lid or operculum. (d) Discharged inoperculate ascus which has opened
through a pore. (e,f) Rostrate ascus as seen in Physcia. In (e) a thickened part of the upper wall of the ascus is being extruded and
is visible in a discharged ascus (f) as an extension of the inner part of the ascus wall, the rostrum. (g) Discharged bilabiate ascus
showing the longitudinal slit by which the ascus opens. (h,i) Bitunicate ascus before and after the first stage of spore release.
Rupture of the ectotunica has allowed the endotunica to expand. (e,f) after Honegger (1978).
ascus mother cell. During this stage the outer
layers of the wall making up the ectotunica
(¼ ectoascus) are deposited. In the second stage,
secondary wall layers making up the endotunica
(¼ endoascus) are laid down within the primary
wall. The development of bitunicate asci has
been studied by Reynolds (1971) and by Parguey-
Leduc and Janex-Favre (1982). At the beginning
of development, asci are surrounded by a single
homogeneous layer which is sometimes granular,
bearing externally a loose network (a fuzzy
coat) of interascal material. The ascus wall
becomes divided into a densely granular external
layer (the ectoascus) and a clearer, but equally
granular, inner layer (the endoascus). A clear
space then separates these two layers. The
granular material of the endoascus rearranges
itself into lines of fibrils at first following a wavy
pattern as seen in transverse sections of developing
asci. Later the fibrils become strongly
folded into pointed zigzag shapes, a development
which progresses from the inside towards the
outside of the endoascus. The folds of the zigzags
are closely pressed against the pointed teeth
which mark out the plasmalemma. The density
of the fibrils increases considerably throughout
the thickness of the endoascus. Finally the two
layers of the ascal wall, separated from each
other by a clear space, appear as a double-layered
ectoascus and a single-layered endoascus within
which the fibrils are strongly pleated into
accordion-like folds. The pointed crests of the
pleats lie parallel to each other and perpendicular
to the plasmalemma of the ascus. The
folding of the layers of the endoascus and
plasmalemma permit the rapid expansion of
the ascus prior to spore discharge, i.e. by
providing material which can unfold rapidly.
Towards the tip of the non-discharged ascus
the crests of the pleated folds of the endoascus