Bulletin of the British Museum (Natural History)

LICHEN GENUS MICAREA IN EUROPE 31
This difference can be due to the much branched and entangled apices of the paraphyses, or
(and) deposits of pigment in the gel-matrix, or on (or in) the walls of the paraphyses. The upper
layer of the hymenium differentiated in such ways is perhaps more correctly termed the
'epihymenium' (Poelt, 1974a) or 'pseudoepithecium' (Korf, 1973). In its strict (original) sense
an 'epithecium' refers to a layer of branches of paraphyses that overtop the asci. Such a layer is
approached in some species of Micarea and is best exemplified by M. contexta and M.
melanobola. In Micarea the difference between an 'epihymenium' and an 'epithecium' is not
clear-cut, and so I have chosen to employ the latter term which has been long and widely
employed in lichenology.
The height of the hymenium in Micarea species is sometimes difficult to measure accurately
because the hymenium often merges rather imperceptibly into the hypothecium. Measurements
are most accurately made by mounting thin sections in Lugol's iodine, in which the strongly
amyloid (dark blue) hymenium contrasts strongly with the non-amyloid (or ± so) hypothecium.
Even when accurately determined the height of the hymenium is rarely a useful character in
distinguishing similar species. The overall range in height is about 23-90 /am, and in most cases is
within 35-50 /xm. Shallow hymenia (rarely exceeding 35 /am) are characteristic of many of the
species with small spores, viz.: M. hedlundii, M. melanobola, M. misella, M. myriocarpa, M.
nigella, M. olivacea, M. osloensis, M. rhabdogena, and M. tuberculata. Likewise, tall hymenia
are characteristic of species with large spores, and the tallest hymenium (65-90 /u,m) belongs to
the species with the largest spores, namely M. subleprosula. The height of the hymenium
sometimes increases with the age of the apothecium, and this is particularly true in M.
bauschiana and M. sylvicola: small, yet mature, apothecia usually have a hymenium c. 40 /zm
tall, but as the apothecium enlarges and increases in convexity, the hymenium often increases in
height up to about 60 /xm; at the same time the paraphyses appear to 'stretch' and become
thinner (especially in the lower part of the hymenium).
The hymenium in Micarea never contains hyaline, crystalline inclusions that adhere to
paraphyses (as found in some species of Lecidella, and many Graphidaceae) or numerous oil
droplets (as found in several species of Buellia and Caloplaca). Epithecial granules that dissolve
in K and often give the apothecia a pruinose appearance are also never found in Micarea. Minute
granules of blue-violet (K+aeruginose) pigment, like those found in Lecidea hypnorum, are
occasionally observed in the hymenium of M. contexta and M. lignaria.
The colour of the hymenium and epithecium is an important character in the identification of
Micarea species. A discussion of the various pigments involved is given in the sections on
'chemistry'. The hymenium may be dilutely and ± evenly coloured throughout, or the
colouration may be more intense in the upper part. In many species the hymenium is often
intersected by dark vertical streaks, a feature usually due to dense pigment surrounding
individual, or small fascicles of stout paraphyses (see p. 61).
Asci
The asci of Micarea species are clavate or cylindrical-clavate in shape and belong to the
Lecanora-type of Honegger (1978). When mounted in Lugol's iodine (following treatment in
10% KOH) the ascus is seen to have a non-amyloid wall surrounded by an amyloid outer-layer
('fuzzy coat') and an internal staining amyloid apical dome (Fig. 6). Micarea has sometimes
been placed in the Arthoniaceae (e.g. Lamb, 1953) but the ascus of members of that family does
not have an amyloid outer-layer and the apical dome is not deeply amyloid. In a few species
of Arthonia and Arthothelium I have observed a faint bluing in the part of the apical dome
immediately adjacent to the ocular chamber. In some Arthonia species a tiny amyloid ring is
apparent above the apex of the ocular chamber, and, despite statements to the contrary by
Eriksson (1981), such a ring is found in the asci of ^. radiata (type species oi Arthonia Ach.) and
A. fuscopurpurea. The same type of ring was also described for Bryostigma leucodontis by Poelt
& Dobbeler (1979). Such aspects of ascus structure in the Arthoniaceae clearly merit more
detailed investigation. The ascus structure in Micarea supports my opinion that the genus should
be placed in the Lecideaceaes. str., at least for the time being.
The asci of all European Micarea species are 8-spored. The North American Lecidea populina