Bulletin of the British Museum (Natural History)

LICHEN GENUS MICAREA IN EUROPE 25
The species which at times grow on bryophytes usually have a superficial thallus, but even so,
the thallus is often, in part, endocuticular. When on bark Micarea thalli are generally
epiphloeodal, but at least partially endophloeodal thalli have been seen in M. cinerea and M.
peliocarpa.
Phycobionts
Among the 45 species of Micarea accepted in this revision three types (? genera) of 'grass-green'
phycobiont can be distinguished by LM observations on thallus squashes. Unfortunately the
identities of these algae are unknown and they await critical studies by an algologist. In addition,
two genera of blue-green alga may be involved in the formation of cephalodia (^.v.).
The commonest alga in Micarea is that found in the type species {M. prasina) and was
discussed at some length by Hedlund (1892, 1895). In the course of this study this alga is
hereafter referred to as 'micareoid' (Fig. 2). Its cells are fairly regular in appearance, ± globose,
thin-walled and c. 4-7 jxm diam. Reproduction within the thallus appears to be by a process of
cell division in which the protoplast divides in two, followed by the laying down of a dividing wall
which joins up with existing wall of the parent cell (Fig. 2A). No internal divisions into
aplanospores as found in, for example, Myrmecia and Trebouxia, have been observed. Contact
between the fungal hyphae and the algae cells is by intracellular haustoria (Peveling, 1974)
which are readily seen at x 1000 (mounted in 10% KOH, followed by ammoniacal erythrosin).
The haustoria may be peg-like (Fig. 2B, D-E), swollen to become clavate (Fig. 2G) or capitate
(Fig. 2C, H), or spreading into a foot (Fig. 2F). In collapsed algal cells (Fig. 2H) the haustoria
are intensively stained. A characteristic feature of the micareoid alga and its relationship with
the mycobiont is that a hypha frequently becomes closely aligned along the dividing line of two
separating algal daughter cells (Fig. 2D-E); the hypha concerned is often seen to penetrate one
or both of the cells by haustoria.
The second algal type is found in Micarea sylvicola and its presumed relatives M. bauschiana,
M. lutulata, and M. tuberculata. The cells are thin-walled and irregular in size and shape, varying
from 5-12 /xm diam when globose to up to c. 15 x 10 /xm when ± ellipsoid. I am undecided as to
their mode of reproduction within the thallus. The large number of cells at the lower end of the
size range is suggestive of aplanospore formation, but I have never observed a mother-cell
undergoing division. Haustorial penetrations by the mycobiont hyphae have been detected, but
they are never as distinct as those involving micareoid algae or the phycobiont of M. intrusa (see
below).
The third algal type is confined to M. intrusa, and has large globose cells, 7-21 /^m diam, with
thick hyaline walls c. l-lfxm thick. Cell division within the thallus seems to be by a process of the
division of a mother-cell into three or four daughter-cells (i.e. 'protococcoid' division). Many of
the cells are clearly seen to be deeply penetrated by a haustorium (Fig. 55). This phycobiont
looks very similar to that of Scoliciosporum umbrinum and may well be identical with it.
Cephalodia
For reviews of the morphological, taxonomic, and physiological aspects of cephalodia see Jahns
(1974), James & Henssen (1976), and Millbank (1976).
Cephalodia have been found in three species of Micarea: M. assimilata, M. incrassata and M.
subviolascens . The first two species have thalli composed of verrucose areolae with a micareoid
phycobiont. However, sections of their thalli reveal the presence of ± globose structures (c.
200-600 /xm diam) containing a blue-green alga of the genus Nostoc. In many cases these
structures are visible externally and closely resemble the areolae except that they are brown, and
usually darker, in colour. Internally they consist of numerous ramifying fungal hyphae (presumably
belonging to the Micarea) and dense masses of Nostoc cells which have lost their normal
(when free-living) filamentous arrangements; and I am in no doubt that these structures are
cephalodia. I am less certain of the status of the more loosely organized clusters of Stigonema
which are sometimes associated with the same two Micarea species, and also M. subviolascens.
However, the Stigonema filaments are, at least partially, disrupted and fungal hyphae are
present.