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2. Taxonomic part 22
some species, a variably thick true cortex of highly conglutinated and ±densely arranged to
cartilaginous irregular or periclinal hyphae is present. Many species also show intermediates
of these cortex types, sometimes even within a single specimen. In contrast, Frisch (2006),
who follows the terminology of Poelt (1989) and Büdel & Scheidegger (1996) summarizes all
cortex structures under the term ‘phenocortex’ and lists four different types (see there).
The species treated here all have a trentepohlioid photobiont. Abundance of algal cells
differs widely between specimens. In some species the algal layer is well-developed and
continuous but can be also discontinuous or almost absent. In this case photobiont cells are
scattered throughout the entire thallus. In predominantly hypophloedal specimens, algal cells
are also found amongst the hyphae that penetrate the substrate layer. The algal layer is often
interrupted by large oxalate crystals, in some species these crystals are organized in columns.
The presence of oxalate crystals is also variable. Species that produce secondary metabolites
may contain additional thalline crystals; the most prominent example is Leptotrema wightii,
which contains bright red anthraquinone crystals. A distinct medulla layer is rather unusual
and occurs in epiphloedal taxa with thicker thalli. In Reimnitzia santensis sometimes lower
cortex-like structures are found in the basal thallus regions.
Vegetative propagules
Several forms of vegetative reproduction are known for thelotrematoid lichens, isidia occur
in two Australian species of Myriotrema and in Reimnitzia (in Pseudoramonia isidia-like
structures are present, which probably represent immature ascomata). Soralia are known from
one member of Leucodecton, otherwise soralia as wells as schizodiscs are predominantly
known for columellate species (not treated here).
2. 5. 2. Ascomata
Morphology
Ascomata in thelotremataceaen Graphidaceae are predominantly hemiangiocarpous, perito
apothecioid or Geaster-like and open by a single pore, or mazaedious. Sizes of mature
ascomata range from c. 100 µm to c. 5 mm in diameter. They are predominantly roundish to
slightly irregular, occur solitary to distinctly fused and their position relative to the thallus
reaches from deeply immersed to distinctly emergent or stipitate. In some genera, ascomata
develop in succession (Pseudoramonia, Melanotopelia) or are regenerative (Chapsa,
Leptrotrema schizoloma-group). A well developed, incurved to recurved, entire to lacerate
thalline rim is mostly present, in some cases it can become strongly eroded with age. The
thalline rim is usually concolorous with the thallus or brighter, rarely whitish or
conspicuously stained. Ascomata discs are flat to concave and often visible from the surface.
They are predominantly whitish, grayish, brownish, flesh-colored or rarely distinctly reddish,
and are epruinose to distinctly pruinose.
The ascoma morphology is an important character in thelotrematoid lichens that delimits
several genera. The following morphological ascomata types can be distinguished:
– Geaster-like (chroodiscoid): predominantly large, conspicuous, immersed-erumpent,
gaping, with distinctly exposed disc and fissured to lacerate or eroded, often layered
margin; margins erect to recurved or exfoliating and usually proper exciple fused and not
visible; ascomata regeneration common: Acanthotrema (not treated), Chapsa, Chroodiscus
and Reimnitzia.
– mazaedious: large, conspicuous, distinctly emergent with visible mazaedium; nonregenerating:
Nadvornikia.
– stipitate: medium-sized, conspicuous, peri- to apothecioid with distinctly stiped base
and successive growth: Pseudoramonia.