Bulletin of the British Museum (Natural History)

84
BRIAN JOHN COPPINS
In the case of M. denigrata it is a general rule (but with exceptions) that when on worked wood
(fence-posts, garden furniture, window frames, etc.) its thallus has numerous pycnidia (with
mesoconidia) and often few (if any) mature apothecia. On natural substrata (e.g. fallen tree
trunks in old woodlands) pycnidia with microconidia or macroconidia are more prevalent,
although they are usually relatively fewer in number and associated with numerous apothecia. It
seems highly likely that the success of M. denigrata as a primary coloniser of newly available
substrata is largely due to the successful role of its mesoconidia as asexual propagules. Its
microconidia probably function as spermatia, but the function of the macroconidia is less
obvious, although I suspect that they, like the mesoconidia, act as asexual diaspores (perhaps in
a different way).
Another observation pertinent to this discussion is that I have frequently observed the mesoand
macroconidia of Micarea species being extruded through the ostioles of the pycnidia as
white mucilaginous blobs; such phenomena are usually seen after periods of wet weather. It is
tempting to suggest that these extrusions facilitate dispersal beyond the limits of the parent
thallus by the action of rain or passing arthropods and mollusca. Dispersal by invertebrates may
be of especial importance for species such as M. botryoides, which occur in sheltered situations
rarely subjected to rain-wash. I have only rarely observed microconidia being extruded as white
blobs, thus suggesting that there is no need for them to be dispersed beyond the limits of the
parent thallus. Indeed, if microconidia do function as spermatia, and if the mycobiont of the
Micarea is homothallic, there would be no requirement for them to be dispersed more than a few
millimetres.
Conidiogenous cells
The conidiogenous cells of Micarea species are always phialidic, although in many cases the
phialides are seen to undergo 'percurrent proliferation' (Figs 42B, 44B). The conidiogenous
cells belong to Types I or II of Vobis & Hawksworth (1981) and arise from the inner wall of the
pycnidium (or on the outer surface of the sporodochium in M. adnata). Their subtending cells
are never sufficiently regular in shape for them to be termed 'conidiophores'. In several species
whose pycnidial walls are intensely pigmented the bases of the conidiogenous cells are often
similarly pigmented (Figs 47B, 51B). The nearest approach to the differentiation of wall-tissue
from a conidiogenous layer is found in the thick-walled pycnidia of M. elachista.
There is much variety in the shape of conidigenous cells (e.g. ampuUiform, doliiform,
lageniform, cyUndrical) and there is sometimes much variation within a single pycnidium
(Figs 42B, 44A). Conidiogenous cells with long cyhndrical necks often have swollen bases (Figs
38A, 47B, 5 IB). Although critical observations and measurements have not been made for all
species, the size and shape of conidiogenous cells have rarely been found to be useful characters
for the separation of closely similar species. However, one exceptional case is that of saxicolous
forms of M. olivacea (Fig. 47A) versus M. tuberculata (Fig. 5 IB), in which the conidiogenous
cells of the latter are much longer.
Chemistry
The discussions below are presented under two sub-headings. The first deals with 'lichen
substances', which are readily extractable in acetone and identifiable by thin-layer chromatography
(t.l.c; see 'Methods'). The second part deals with pigments that cannot be analysed in this
way; their chemical nature is at present unknown and they can only be characterised by their
colour in water and subsequent reactions with reagents such as K and HNO3.
Lichen substances
Prior to the present studies there is little evidence in the literature to suggest that species of
Micarea contain any lichen substances. However, there are a few hints given in some early
descriptions: for example, Leighton (1879: 362) gives 'K+ yellow, C+ orange-red' reactions for
Lecidea milliaria [Micarea lignaria], which probably relate to his specimens of the var.
endoleuca. The only reported t.l.c. analysis of a Micarea appears to be that of Huneck &