Introduction to Fungi, Third Edition

SORDARIALES
321
Fig12.3 Schizothecium tetrasporum. (a) Perithecium
showing asci through the semi-transparent wall.
(b) Ascus. (c) Projectile consisting of four spores
attached to the ascus cap and to each other by means of
mucilaginous appendages.
1996). It is similar to that described above for
Sordaria.
The development of the ascospores of
P. anserina has been studied by Beckett et al.
(1968). The ascospores (four in P. anserina, but see
below) are delimited by a double membrane
system as in other ascomycetes (Fig. 12.4). The
primary spore wall develops between the two
membranes and gradually pushes them apart.
The inner membrane continues to function as
the plasma membrane of the spore, whilst the
outer functions as the spore-investing membrane.
As the primary spore wall widens,
secondary wall material is laid down towards
the inside of the primary wall. These primary and
secondary walls enclose the whole of the spore,
including the spore head and the tail. A tertiary
wall representing the pigmented layer of the
spore head is laid down to the inside of the
secondary layer (Figs. 12.4e g). The elongated
tail of the spore is cut off from the spore head by
a septum. The tertiary wall layer does not extend
into the spore tail, which therefore remains colourless.
Its contents degenerate. This part of the
spore persists as the primary appendage, sometimes
termed the pedicel. Secondary appendages
develop at the apex of the spore head
and at the primary appendage. They arise
by localized evaginations of the spore-investing
membrane. A thinner area in the tertiary wall at