Images of British Lichens
Frequently Asked Questions (if anyone actually asked any)
These pages will be used to provide various notes and explanations.
I intend that the FAQ will expand into an illustrated glossary, but this will take time.
Lichen sexual fruiting bodies (ascocarps, basidiocarps)
The great majority of lichens belong to the Ascomycota, in which the principal types of sexual fruiting body (ascocarp) are the perithecium and the apothecium. Perithecia are typcally flask-like; apothecia are variable in shape but commonly disc- or saucer-shaped, and then often likened to "jam tarts". Both are described in detail below.
A very small number of lichens belong to the Basidiomycota, with fruiting bodies (basidiocarps) club-like, bracket-like or sheet-like, or a toadstool.
perithecium, apothecium, basidiocarp
1. Perithecium (plural: perithecia)
Perithecia are usually flask-shaped fruiting bodies containing the asci. At maturity, an opening at the top, the ostiole, allows release of the ascospores. Perithecia are usually at least partially immersed in the thallus or in the substrate and are relatively inconspicuous, rarely more than 1mm diameter and commonly much less. They may be scattered or grouped, sometimes clustered on or in discrete areas of blackened tissue, a stroma. The walls of the perithecium, commonly black or darkened, constitute the exciple (or excipulum), and there may be a partly covering, shield-like structure, the involucrellum. Lichens in which the fruiting body is a perithecium are termed 'pyrenocarpous'. Their identification commonly requires a vertical section of the perithecium to show the structure of the exciple, the involucrellum (if any), and the arrangement of the tissues within, as well as details of the asci and ascospores. Orange (2013, especially section 1.2.1) provides numerous useful diagrams.
Pyrenula nitida, appearance on branch and cross-section of immature perithecium showing asci, but not yet the ostiole, also showing immersed position in the bark tissues.
(Adapted from Smith, 1911, original figures out of copyright).
The hymenium, the tissue containing the asci, is seated at the base of the perithecial cavity. The arrangement of tissues within the hymenium is important in differentiation of genera, though this can be difficult to judge without careful sections with a microtome. Tissues penetrating between the asci, collectively the hamathecium, may consist of:
It will be difficult to resolve these structures in a squash preparation. See Orange (2013, section 1.2.2) for a fuller, illustrated explanation of hamathecial structure.
- paraphyses, septate, usually unbranched filaments, developing and extending upwards from the hypothecium, the tissue below the hymenium;
- pseudoparaphyses, septate filaments, often branched and anastomosed, initiating from the roof of the perithecial cavity and extending downwards into the hymenium;
- paraphysoids, other tissue within the perithecium, becoming stretched into branched and anastomosed, slender filaments as the perithecium develops and generally with septa remote and inconspicuous.
Pyrenula macrospora, (Ballygrant, Islay, 2011).
The perithecia in this species are relatively large and project above the thallus, which is partly immersed in the bark. Note the ostioles, which are relatively conspicuous here.
The formation of an ostiole at maturity, with release of the spores through this, is fundamental to the definition of a perithecium. Another type of ascocarp in the Ascomycota is the 'cleistothecium', in which no ostiole is formed and the spores are released by splitting of the ascocarp wall. As yet, there appear to be no cleistothecial lichens known. (?)
2. Apothecium (plural: apothecia)
Apothecia typically are more open structures, cup or saucer-shaped, often with a distinct margin and popularly likened to jam tarts. The hymenium, the tissue containing the asci, forms the disc, the often concave, flat or sometimes convex upper surface of the apothecium, and it is typically exposed from an early stage in development. Apothecia that are much more deeply cup-shaped, with incurved margins hiding and protecting the disc, are said to be 'urceolate'.
Xanthoria parietina, (Wicken Fen, Cambridgeshire, 2007).
The apothecia are commonly abundant in this species. The margins are thalline (see below), and match the colour of the rest of the thallus, while the discs are rather darker orange.
The hymenium, composed of asci and paraphyses, occupies the entire expanse of the disc surface.
In this case there is no visible 'proper margin' (see below) inside the thalline margin.
Two basic types of apothecium are recognised in lichens, differing in their margins and underside (together the exciple). Their distinction is important in classification and identification and, while often easily recognised, may require microscopc examination of the apothecium in section for certainty. (See below for more specialised apothecial structures.)
i) lecideine apothecium
Lecideine apothecia match the structure of those in non-lichenised discomycetes. The exciple forms the underside and outer layer of the apothecium, extending up to the rim, where it forms the "proper margin". This excipular tissue is part of the true apothecial tissue and does not contain algal cells. The proper margin is sometimes conspicuous and a different colour from the disc, but more often is narrow, the same colour as the disc, inconspicuous, and is often excluded (turned under) as the apothecium grows.
vertical section showing exciple and proper margin (from Smith, 1921).
("Lecidia parasema" is an old name that was used for what is now Lecidella elaeochroma.)
Porpidia platycarpoides, (Isle of Muck, 2012).
The lecideine apothecia have rather thick proper margins in this species, and these margins are somewhat lighter in colour than the disc.
(N.B., the discs are frequently grey-pruinose in this species.)
ii) lecanorine apothecium
In lecanorine (or "thalline") apothecia, the thallus tissue extends up the outside of the apothecium to form the exciple and the rim. In theory there may still be the true exciple tissues of the apothecium itself inside this and a narrow proper margin may still be visible, but commonly only a thalline margin can be seen. This thalline margin generally retains the colour of the thallus and so is commonly a different colour from the disc. It normally contains algal cells and sometimes also crystals. It may show other properties of the thallus, including formation of soredia.
top view and vertical section showing exciple and thalline margins
(adapted from Smith, 1921).
No proper margin or non-thalline excipular tissue is depicted.
("Lecanora subfusca" is an old name for what is now known to be an aggregate of species. Many of these have large crystals in the thalline margins, but these are not depicted here.)
Haematomma ochroleucum var. porphyrium, (Isle of Muck, 2012).
The greyish white, powdery thallus extends up to form the sides and rims of the apothecia.
Further types of apothecia are recognised, based on markedly different morphology:
iii) lirella (plural: lirellae)
Lirellae are apothecia that are long or at least elongated, narrow, often branched, often with hard, black outer margins, and which sometimes show a superficial resemblence to ancient or Arabic script and may then be termed 'graphoid'. Often they are slit-like and initially immersed in the thallus, or in the substrate tissue in those species that occur on bark. Important genera with graphoid lirellae are Graphis, Phaeographis, and Graphina, which resemble each other closely and make microscopic confirmation important; Opegrapha species can also be graphoid in appearance though tend to have smaller lirellae, and genera with smaller and often less distinctive lirellae include Arthonia, Lithographa and Xylographa. Depending on genus, margins may be fundamentally lecanorine, lecideine, or lack diferentiated marginal tisseue
Graphis scripta, (Glen Tilt, Perthshire, 2008).
The apothecia (lirellae) are long, very narrow, generally curved, and often forked, with raised, black, unfurrowed margins, and opening to expose the grey hymenium.
iv) mazaedium (plural: mazaedia)
In mazaedia, the asci break down, leaving the ascospores as a powdery mass. Gently touching mature mazaedia can leave sooty marks on the fingers. Mazaedia are frequently on long stalks, characterising 'pin-lichens' such as Calicium, Chaenotheca and Sclerophora, but they are sessile in Cyphelium.
Calicium viride, (Kindrogan, Perthshire, 2008)).
A widespread pin-lichen, with mazaedia for fruiting bodies (1–2mm in height).
Tissues and zones in apothecia
|Vertical section of thalline apothecium showing arrangement of tissues (adapted from Smith, 1921)|
A basidiocarp is a sexual fruiting body produced by a member of the Basidiomycota, and could take any of a diversity of shapes. The hymenium is usually external, but not so in the puffballs and other gasteromycetes, and not infreqently the hymenium has increased surface area from production on sides of gills, tubes or spines, and increased protection by being on the underside of a cap or bracket. The hymenium generally consists of basidia, which produce the basidiospores externally on short projections, sterigmata, most often two or four to a basidium. Packing and maybe protection is provided by basidioles, undeveloped basidia.
Only a small part of this basidiocarp diversity is to be seen in the few lichenised basidiomycetes, but some major basidocarp types are represented. A short illustrated sumary of basidiolichens is provided by Purvis (2000, pp. 34–35).
One lichenised fairy-club is known, Multiclavula vernalis, which grows on algal mats in heathland and moorland areas, most of the few British records being from the islands of north and west Scotland, but recently recorded also from Surrey (BLS record, National Biodiversity Network (2012). Illustrations of this simple basidiolichen are provided by Ryman & Holmåson (1984) and by Purvis (op. cit.).
i) club-shaped basidiocarp
Regarded as a relatively primitive structure, club-shaped fruiting bodies are seen in the "fairy-clubs" such as Clavaria (non-lichenised fungi especially of grasslands). The club (or 'clavulum') is generally stalked, smooth or somewhat furrowed above, and the hymenium coats the entire surface of the upper, fertile portion. The hymenium is unprotected and unless furrowing is pronounced, there is no significant increase in surface area.
ii) sheet-like or resupinate basidiocarp
Dictyonema is a genus of mainly tropical and subtropical basidiomycete cyanolichens. They are variable in form and evidently can form delicate and attractive brackets (see, e.g., photographs of D. glabratum in Purvis (op. cit.) and in Lücking et al. (2009)). One rarely seen, British species, D. interruptum, is known, but from available descriptions (Coppins & James, 1979; Woods & Watling in Smith et al., 2009), the thallus form is encrusting or forming sheets (resupinate), not extending into brackets, on wet. mossy rocks or underneath sheets of liverworts, the hymenium forming on the (under-)surface of the thallus. [The correct name for the British species is uncertain (Lücking et al., op. cit.).]
ii) toadstool (agaric)
One genus of toadstools (agarics) in Britain is lichenised: Lichenomphalia. The basidiocarp is of the typical toadstool shape, stalked, with a cap (pileus) and gills formed on the underside of the cap. The hymenium occupies the faces of the gills.
Lichenomphalia umbellifera, (Hare Cleuch, East Lothian, 2010).
The basidiocarps are omphalioid with well-formed gills underneath the cap. While this species is easily confused with non-lichenised agarics such as Omphalina pyxidata, the algal mat on the peat surface here can be seen to be granular, and microscopic examination confirms the presence of a binding, fungal matrix.
Species of Lichenomphalia are delicate, and have gills that extend a short distance down the stem ('decurrent') and in this respect can closely resemble non-lichenised agarics in such genera as Omphalia and Arrhenia, some of which are associated with or fruit through algal mats. The basidiocarps of Lichenomphalia species are associated with granular algal mats that are at least set in a matrix of the fungal hyphae, and in one species, L. hudsoniana, the thallus is squamulose. All four currently accepted British Lichenomphalia species are depicted, with extended notes, on this site — see index (L–M).
Coppins, B.J., & James, P. (1979). A British species of Dictyonema. Lichenologist 11: 103–108.
Lücking, R., Lawrey, J.D., Sikaroodi, M., Gillevet, P.M., Chaves, J.L., Sipman, H.J.M., & Bungartz, F. (2009). Do lichens domesticate photobionts like farmers domesticate crops? Evidence from a previously unrecognized lineage of filamentous cyanobacteria. American Journal of Botany 96: 1409–1418.
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Purvis, W. (2000). Lichens, Natural History Museum, London.
Ryman, S., & Holmåson, I. (1984). Svampar. En fälthandbok, Interpublishing AB, Stockholm.
Smith, A.L. (1911). A monograph of the British lichens. A descriptive catalogue of the species in the Department of Botany, British Museum, part II, British Museum, London.
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Smith, A.L. (1921). Lichens, Cambridge University Press, Cambridge.
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Smith, C.W., Aptroot, A., Coppins, B.J., Fletcher, A., Gilbert, O.L., James, P.W., & Wolseley, P.A. (eds.) (2009). The lichens of Great Britain and Ireland, British Lichen Society, London.
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Photographs and text © A.J. Silverside
March 2013, last modified or updated April 2014
Line illustrations from out of copyright sources as noted, but scan manipulations © A.J. Silverside.