Images of British Lichens

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Currently under development. This does not aim to be a comprehensive glossary, but rather an explanation of some common terms used in lichenology and those used on this website. For detailed, English-language glossaries, go (e.g.) to Brodo, Sharnoff, & Shjarnoff, S. (2001), Dobson (2011) or Smith et al. (2009).

An adjective is a "describing word" and has no separate meaning in lichenology. However, in scientific terminology, including lichen descriptions, the adjective often contains 'ul' as a diminishing modifier. For example, 'verrucose' means "warted"; 'verruculose' means "minutely warted", in this case a distinction between what may be seen with relative ease with unaided vision and what may need use of a hand-lens.
Fungus in the Basidiomycota that has a toadstool as its basidiocarp.
(plural: apothecia)  An ascomycete sexual fruitbody (ascocarp) which is usually cup- or saucer-shaped, sometimes convex, and with an exposed hymenium, at least when mature. Fungi with apothecia as their fruiting bodies are termed "discomycetes".
An illustrated account of apothecia and their variation is provided here.

apothecia of Pannaria rubiginosa
(adjective: areolate)  An island of thallus tissue, seated on the hypothallus. The effect is that the crustose thallus has the appearance of crazy paving, with the areoles sometimes well separated (dispersed), sometimes closely set or even overlapping to become squamulose. A continuous crust may partly or entirely crack during growth to give a 'cracked-areolate' thallus.

areolate thallus in Acarospora sinopica
Sexual fruiting body in the Ascomycota. It produces ascospores in asci. Lichen ascocarps are apothecia or perithecia.
(plural: ascomata)  Alternative term for ascocarp.
The largest phylum in the kingdom Fungi (see 'classification') and that to which the great majority of lichenised fungi belong. The fruiting body produces spores (ascospores) in an ascus.
Spore formed in sexual reproduction, by a fungus in the Ascomycota. Ascospores are produced in asci.

1-septate ascospores of Lecania cyrtella
(plural: asci)  The structure that contains and releases the sexual spores in fungi of the Ascomycota. The shape of the ascus varies but commonly it is cylindrical and contains most often 8 ascospores, sometimes a subdivision or power of 8, the effect analagous to a cannon with 8 cannon-balls similtaneously loaded for firing. Asci are produced (in lichens) in basically two types of fruitbody, an apothecium or a perithecium, in the tissue layer termed the 'hymenium'.

ascus & ascospores
Sexual fruiting body in the Basidiomycota. It produces basidiospores on basidia. Lichen ascocarps are commonly toadstools (genus Lichenomphalia. An illustrated account of lichen basidiocarps is provided here.
Lichenised member of the Basidiomycota.
The phylum in the kingdom Fungi (see 'classification') that contains the majority of the larger fungi, including toadstools and bracket fungi. Few are lichenised, but the phylum includes, in Britain, the lichenised agaric genus Lichenomphalia (e.g. L. umbellifera), one lichenised fairy-club, Multiclavula vernalis, and what might be best described as a highly reduced, lichenised bracket fungus, Dictyonema interruptum (Smith et al., 2009). The fruiting body (basidiocarp) produces spores from a basidium.
Spore formed in sexual reproduction, by a fungus in the Basidiomycota. Basidiospores are produced externally on basidia.
(plural: basidia)  The structure that releases the sexual spores in fungi of the Basidiomycota. Typically it is narrowly balloon-shaped and produces usually 2 or 4 basidiospores externally on sterigmata. Basidia develop in the hymenium. Basidia often have "clamp connections" adjacent to basal septa, but these are not found in Lichenomphalia, the main genus of lichenised basidiomycetes, and so are not depicted here.
Image modified from Berkeley (1860),
[a: basidium; b: sterigma; c: basidiospore].

basidia with basidiospores
(plural: blastidia)  A small, asexual propagule, containing both fungus and alga, produced by budding from the tips of other blastidia and from the thallus. They are typically partly corticate.
Blastidia are further described here.
As used in lichenology, 'C' is short-hand for an approx. 5% solution of calcium or sodium hypochlorite, i.e. undiluted household bleach. A small drop of bleach is placed on the lichen and any colour change noted. Commonly there is no reaction (C-) or alternatively there may be production of a bright colour (C+), often red, sometimes bright green, specific for the species concerned. It is important to note the tissue that must be tested for a given species, usually the cortex or the medulla, sometimes the apothecia. The reaction may be fleeting, sometimes lasting only a second or so, and must be carefully observed.
Bleach for field tests must be carried safely, usually in a small plastic dropping bottle within a rigid container, and the test must be applied with due care, bearing in mind it is a corrosive irritant.
The solution remains active for only a few weeks and must be replaced frequently. Unfortunately, commercial bleaches now commonly contain additives, e.g. sodium or potassium hydroxide, which cause false results, and regrettably such additives are usually not specifically named on the bottle. Dobson (2011) gives more details of 'C' testng and recommends testing a new bleach on the medulla of Parmelia saxatilis or P. sulcata, a postive yellow or orange result showing that additives render the bleach unsuitable for lichenological work.

Positive (C+ orange-red) reaction on the thallus of Pertusaria lactea.
Rich in calcium carbonate, as used of rocks, sand or soil (compare siliceous). Chalk and limestone are calcareous rocks in Britain that are largely composed of calcium carbonate and carry distinctive lichen communities, but some sandstones are also strongly calcareous.
Celtic Rainforest
A term used for the high-rainfall Sessile Oak (Quercus petraea), Downy Birch (Betula pubescens) and Hazel (Corylus avellana) forests of western Wales, western Scotland and Ireland. On the fringe of the Atlantic Ocean, the high humidity, high total rainfall, high number of rain-days, frequent mists and equable temperatures combine to provide rich, internationally important habitat for mosses, liverworts and lichens, with many rare species. These forests include the Atlantic Hazelwoods (Coppins & Coppins, 2012). For an outline of the Celtic Rainforest and its lichens, go here.

Atlantic Hazelwood, Jura, May 2011
(plural: cephalodia) Variably-shaped and often gall-like structure, containing a cyanobacterial photobiont, on or in the thallus of a lichen that has a green alga (Chlorophyta) as its main photobiont. The assumption is that cephalodia aid with nitrogen metabolism. Rarely, a cephalodial state can persist independently of the normal thallus.

Lobaria amplissima with black, shrubby cephalodia
A general adjective used for green algae (Chlorophyta) that consist of globose, single cells, as in the case of many lichen photobionts. In the past, many of these were classified in the order Chlorococcales, but DNA studies have confirmed that this formerly large taxonomic group includes quite independent lines of evolution. Recent treatment (e.g. John et al., 2011) has the Chlorococcales as a relatively small order (probably not including any lichen photobionts?). Many, including the important lichen photobiont, Trebouxia, are placed in the quite separate class Trebouxiophyceae (see 'classification') and may be more precisely described as 'trebouxioid'. In many lichens, the exact identity of the photobiont is unknown and "chlorococcoid" remains as a convenient term that no longer implies a defined position in green-algal classification.

chlorococcoid (trebouxioid) photobiont in
Caloplaca cerinella
The phylum in the kingdom Plantae (see 'classification') that contains the green algae, some of which are lichen photobionts.
Molecular data show that some green algal groups should be excluded from the Chlorophyta and grouped more closely with the land plants (see John et al. (2011), pg.364), but the green-algal lichen photobionts remain in the Chlorophyta as defined more narrowly.
As applied to lichens, lichen species are classified and named according to the internationally accepted International Code of Nomenclature for algae, fungi, and plants (ICN), which replaces the various versions of the International Code of Botanical Nomenclature (ICBN). The ICN was adopted at the International Botanical Congress in Melbourne in July 2011, but, despite the significant changes in this Code, it has not, at the time of writing, yet been published. Information here follows recent versions of the ICBN and is assumed not to to have been changed. The ICN is entirely separate from the International Code of Zoological Nomenclature (ICZN), which covers organisms traditionally treated as animals, though there are moves towards harmonisation. A problem at present is that current knowledge of evolution, from molecular studies, is resulting in some authors failing to follow the codes as regards classification at the higher levels, though fortunately the fungi (including lichens) are unaffected.
Lichens are true fungi and thus classified in the kingdom 'Fungi', the highest classificatory level sanctioned by the ICBN/ICN, and the vast majority of lichenised fungi belong to the Ascomycota. Some belong to the Basidiomycota.

primary ranks in classification of lichens
(plural: cortexes, cortices)  The layer of tissue that forms the outer 'skin' of the thallus, overlying the medulla, i.e. the equivalent of the epidermis of leaves and stems in higher plants (rather than of the cortex in plant stems). Fruticose lichens have a single cortex that encircles the branches, even when the branches are flattened; foliose lichens have separate upper and lower cortices, crustose, placodioid and most squamulose lichens lack a lower cortex; true leprose lichens lack any cortex.
Possessing a cortex. The term may refer to a thallus or structures such as isidia or podetia, and may be used in a context where part of the structure is covered by a cortex and yet other parts have the underlying medulla exposed, and are ecorticate.
In Cladonia the distribution of corticate areas on the podetial surfaces is of identification importance; squamules are produced only from the cortex.

Partly corticate podetia of Cladonia cornuta. The lower parts of the podetia are corticate (with a few squamules), the upper parts are ecorticate and sorediate.
Growing on bark. (Compare lignicolous.)
See areole.
Having an edge toothed, with small, rounded teeth. When the teeth are very small, as often applies to the margin of an apothecium, the equivalent term is "crenulate".
Thallus type in which the thallus forms a crust over the substrate, to which it is firmly bonded. The thallus has an upper cortex, at least when young, but lacks a lower cortex.
The crustose thallus is more fully described here.

mosaic of crustose lichens on iron-rich rock
The formal name of a phylum of bacteria, characterised by relatively large size, an oxygenic photosynthetic process, presence of phycocyanin as (usually) the predominant accessory pigment and, often, the ability to fix (chemically incorporate) atmospheric nitrogen. See also "cyanobacterium".
(plural: cyanobacteria)  Member of the bacterial phylum Cyanobacteria, popularly known as the "blue-green algae". Unlike other photosynthetic bacterial groups, the cyanobacteria possess an oxygen-evolving photosynthetic process similar to that of other algae and higher plants, and some also fix atmospheric nitrogen. Many lichens contain cyanobacteria as their photobiont, and are known as "cyanolichens" (q.v.).

cyanobacterial photobiont of Lichina confinis
Lichen in which the primary photobiont is a member of the Cyanobacteria. Cyanolichens include many small, black species, often on limestone, but also jelly lichens (Collema, Leptogium, etc.), and large, foliose species of Peltigera, Lobaria, Degelia. etc. Often these latter species are distinctly grey-blue, at least when damp, and many are characteristic of the Lobarion communities of higher rainfall areas (the Celtic Rainforest) in western Britain. An important identification source for cyanolichens is that by the Nordic Lichen Society (2007).

Degelia atlantica, a characteristic cyanolichen of Lobarion communities in western Scotland
The often concave, flat or sometimes convex upper (or inner) surface of an apothecium, as bounded by the apothecial margins and occupied by the hymenium.
Fungus, member of the Ascomycota, in which the sexual fruitbody is an apothecium. The term is applicable to a large number of lichens but is more regularly applied to non-lichenised apothecial fungi.
Lacking a cortex, the opposite of corticate.
Term used for the margin of an apothecium (proper or thalline), where continued growth and development of the apothecium has resulted in the margin being turned under and out of sight.
The image shows apothecia of Lecidella elaeochroma, convex when mature and with excluded margins, the young apothecium to the right still with a visible proper margin.

apothecia of Lecidella elaeochroma
Thallus type in which the thallus is composed of leaf-like lobes, not firmly bonded to the substrate, and with distinct upper and lower surfaces (separate upper and lower cortices). The under-surface commonly bears rhizines.
The foliose thallus is more fully described here.

young, foliose thallus of Parmelina pastillifera


Berkeley, M.J., (1860). Outlines of British fungology, Lovell Reeve, London.
Brodo, I.M., Sharnoff, S.D., & Shjarnoff, S., (2001). Lichens of North America, Yale University Press, New Haven.
Coppins, A., & Coppins, B., (2001). Atlanic hazel. Scotland's special woodlands, Atlantic Hazel Action Group, Kilmartin.
Dobson, F.S. (2011). Lichens. An illustrated guide to the British and Irish species, ed. 6, Richmond Publishing Co., Slough.
John, D.M., Whitton, B.A., & Brook, A.J. (eds.) (2011). The freshwater algal flora of the British Isles. An identification guide to freshwater and terrestrial algae, ed 2, , British Phycological Society and The Natural History Museum, London.
Nordic Lichen Society (2007). Nordic lichen flora, vol. 3, Museum of Evolution, Uppsala University, and Nordic Lichen Society, Uddevalla.
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, unattributed line illustrations, scan manipulations of out of copyright sources, and text © A.J. Silverside.
Uploaded January 2013, last modified or updated April 2014.