Bulletin of the British Museum (Natural History)

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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.
26 BRIAN JOHN COPPINS A B Fig. 2 'Micareoid' phycobiont of Micarea alabastrites and its relationship with mycobiont hyphae ; see text for further details. Scale = 10 /xm. The three above-mentioned Micarea species are morphologically closely related and occur in exposed arctic-alpine situations. In such habitats the formation of cephalodia with a blue-green alga(e) which has the ability to fix atmospheric nitrogen, is of undoubted nutritional benefit to the mycobionts. Most morphological and physiological studies concerning cephalodia have involved macrolichens. Reports of cephalodia in crustose lichen are relatively few, although some are Hsted by James & Henssen (1976). Additional examples in the Lecideaceae include Huilia aeolotera, H. elegantior, H. panaeola (Hertel, 1977), and Lecidea pallida (Fries, 1874). Apothecia: external features As a rule the apothecia in Micarea are small to medium sized, convex to ± globose or tuberculate, and usually immarginate. However, this generalization encompasses considerable variation even within a single given species. With regard to shape, some species (e.g. M. denigrata and M. prasina) are very variable with apothecia that may be shallow-convex to convex-hemispherical (Figs 3A-B, 4A-B) and finally ± globose (Fig. 3D) or tuberculate (Fig. 3C). Apothecia of other species may be less variable: those of M. alabastrites and M. cinerea are broadly convex to hemispherical, sometimes tuberculate, but never ± globose; at the H
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LICHEN GENUS MICAREA IN EUROPE 95 T
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Infrageneric considerations LICHEN
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5. Micarea bauschiana (Korber) V. W
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32. Micarea osloensis (Th. Fr.) Hed
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34. Micarea prasina Fr. LICHEN GENU
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conferanda (Lecidea) 205 confusula
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populina (Lecidea, Micarea) 31,32 p
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