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Hypoxylon cohaerens (Pers.: Fr.) Fr.
Stromata pulvinate, constricted at base, often containing few
perithecia and then shortly stipitate, becoming coalescent, with
inconspicuous perithecial mounds, 1.5-5 mm diam x 1.5-4 mm thick; surface
dark brick (60) to chestnut (40), eventually black, orange brown granules just beneath
surface with KOH-extractable pigmentsdark vinaceous (82) when young, greyish
olivaceous (90) to olivaceous (48) when mature; the tissue below the perithecia
brown to blackish brown, up to 3.5 mm thick.
Perithecia spherical to ovoid, 450-520 µm diam x 500-680 µm
high.
Ostioles papillate, black, frequently, on mature stromata, at
the centre of a discoid depression 120-150 µm diam.
Asci 118-180 µm total length, the spore bearing-parts 56-88 x
6-6.8 µm, the stipes 60-95 µm long, with apical ring amyloid or inamyloid,
1-1.2 µm high x 2.2-2.5 µm broad.
Ascospores brown to dark brown, ellipsoid-inequilateral,
8.8-12.2 x 3.4-5.5 µm ( M = 9.9 x 4.3 µm), with straight germ slit
spore-length; perispore dehiscent in 10% KOH, smooth, with a thickening on
the more convex side.
Anamorph in nature: fawn (87) to greyish sepia (106), velvety, covering
the young stromata.Conidiogenous cells 15-30 x 2-2.8 µm; conidia ellipsoid, 5-6 x 3-4 µm.
The conidiogenous structure is Virgariella-like.
Habitat: primary saprophyte on bark or wood of Fagus
sp.
Known distribution: Europe, North America.
Specimens examined: FRANCE, Ariège (09): Rimont,
Las Muros, 11 Sept. 1996, JF-96091, on Fagus sylvatica;
Rimont, Grand Bois, 26 Apr. 1999, JF-99080, on Fagus
sylvatica; Rimont, Grand Bois, 23 Sept. 1999, JF-99236, on
Fagus sylvatica; Rimont, Las Muros, 03 Aug. 2000, JF-00141,
on Fagus sylvatica; Rimont, Las Muros, 17 Jan. 2001, JF-01005, on
Fagus sylvatica; Ustou, Cirque de Cagateille, 26 Aug. 2001, JF-
01175, on Fagus sylvatica.
Notes: Hypoxylon cohaerens is a common saprophyte of
beech forests, characterized by black pulvinate stromata constricted at
base with papillate ostioles. Two other taxa,
H. cohaerens var. microsporum and
H.
multiforme, share similar stromatal features, and field
identification relies mainly on host identification: H. cohaerens
is restricted to Fagus while
H.
cohaerens var. microsporum is restricted to Quercus and
Castanea and
H. multiforme
grows mostly on Betula, Alnus and
Corylus. However, one collection made on Ligustrum vulgare,
a member of Oleaceae, [Hautes Pyrénées
(65), Bagnères de Bigorre, L'Arbizon, 04 Sept. 2002, JF-02154,
leg. FC] proves that host specificity is rarely absolute.
Stromatal KOH-extractable
pigments vary as stromata become mature, they are dark vinaceous when
recorded from anamorphic state, while black mature
stromata yield typical olivaceous pigments. Along with BNT, unknown
compounds were already reported from
H. cohaerens by
Mühlbauer et al. (2002),
which were identified by
Quang et al. (2005)
as two new azaphilones named cohaerins
A and B, which lack in related species
H. cohaerens var. microsporum
and
H. multiforme
and other members of section Annulata submitted
to HPLC analyses. These cohaerins are only present in mature stromata, while
yet unknown metabolites occur in younger stromata
(Quang et al., 2005), which
corroborates coloured reactions
in KOH we observed. This peculiarity makes this last feature somewhat difficult
to use in distinguishing H. cohaerens from its allies, while HPLC profiling
allows for an unambiguous identification, even from very old material like the
Persoon's type collection dating back prior to 1797
(Quang et al., 2005).
When HPLC is not available, the separation of H. cohaerens from its
allies relies on microscopic observation of
ascospores germ slit: it is spore-length in H.
cohaerens but shorter in
H. cohaerens var. microsporum
and H. multiforme. The both latter differ in ascospore size.
It is noteworthy that, beside its variability in KOH-extractable
pigments, H. cohaerens shows variability in the reaction of ascal
apical ring to Melzer's reagent and a tendency to have stromata containing
very few perithecia, some of them being even uniperitheciate.
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