Introduction to Fungi, Third Edition

456 LICHENIZED FUNGI (CHIEFLY HYMENOASCOMYCETES: LECANORALES)
Trebouxia, which forms single globose cells in a
defined layer beneath the upper cortex of the
stratified thallus (Fig. 16.6c). The apothecia are
saucer-shaped and about 2 3 mm in diameter.
They are located on the upper surface of the
thallus, and the algal zone extends into the
apothecial margin (Fig. 16.6a). The ascospores are
at first one-celled, but ingrowth from the wall of
the ascospore eventually divides the contents
of the spore into two. The yellow colour of the
thallus is due to the presence of the anthraquinone
parietin (Fig. 16.5) in the upper cortex.
Unusual carotenoids are also produced by
Xanthoria spp. (Czeczuga, 1983). Xanthoria parietina
does not produce soredia, but it is very
abundant none the less. One reason for this may
be that germinating ascospores display a tendency
towards cleptobiosis, i.e. the theft of
photobiont cells from soredia or mature thalli
of other lichens (Ott, 1987). Another means of
dispersal may be by mite browsing and feeding
as mentioned earlier.
16.3.3 Peltigera
About 45 60 species are known, and the genus
has been thoroughly examined by Miadlikowska
and Lutzoni (2004). They and many other workers
now consider it to be part of a separate order,
Peltigerales, which is closely related to Lecanorales.
Species of Peltigera form large lobed leaf-like
thalli attached to the ground or to rocks by
groups of white rhizinae. The thallus is rather
fleshy and is highly stratified (Fig. 16.7a). The
commonest species are P. polydactyla and P. canina
(Plate 8d), both of which have now been split up
into several species. They grow among grass on
heaths, on sand dunes and on rocks amongst
moss. The usual photobiont is the cyanobacterium
Nostoc. In some species, however, the
primary photobiont can be either a Nostoc,
giving rise to the usual greyish-black thallus, or
a green alga (Coccomyxa), in which case the
thallus is vividly green, with Nostoc sometimes
present as a secondary photobiont in cephalodia
(Brodo & Richardson, 1978). The two different
Fig16.6 Xanthoria parietina. (a) V.S. thallus and apothecium showing the extension of the algal zone into the apothecium. (b) Asci,
paraphyses and two germinating ascospores. (c) V.S. thallus.