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).

See Pd.
As used in lichenology, 'Pd' is an abbreviation for para-phenylenediamene. A small drop of Pd is placed on the lichen and a positive result is a bright, yellow to orange reaction. It is valuable for identification in, e.g., Cladonia and Lecanora.
Although formerly used in hair dyes, Pd is toxic, mutagenic and probably carcinogenic. It must be used with great care. It is unsuitable for use in the field, except as Steiner's solution; see Dobson (2011).
A few crystals of Pd are best placed in a very small, screw-topped, glass bottle and a drop of strong alcohol is added. A clean, glass rod is used to transfer a drop to the specimen, which must be discarded safely afterwards. The mixture rapidly oxidises, turning brown, and lasts at most an hour or two, even with the screw-top replaced on the bottle. Another drop of alcohol can be added as it evaporates.
Fine particles of Pd leave brown spots on paper and work surfaces within a few days of use, indicating that the same may be happening in the user's lungs. Working surfaces must be cleaned after use and it is sensible to wear a breathing mask whenever handling the dry chemical. Once made up, Steiner's solution is easier to use and lasts a few weeks, but does not give such strong results.
This website does not encourage the use of Pd.

Positive (Pd+) reaction on Cladonia chlorophaea agg..
(plural: perithecia)  An ascomycete sexual fruitbody (ascocarp) which is subglobose to flask-shaped, enclosing the hymenium, with, at maturity, an apical pore (ostiole) through which the spores are released.
An illustrated account of perithecia is provided here.

perithecia of Pyrenula macrospora
The photosynthetic, i.e. algal, component of a lichen. Also called the "phycobiont" (q.v.).

photobiont in Caloplaca cerinella
photobiontic layer
Layer of tissue containing the cells of the photobiont.
The algal (i.e. photosynthetic) component of a lichen. Also called the "photobiont". Lichen photobionts are green algae (Chlorophyta) or blue-green algae (Cyanobacteria).
(plural: phyla)  The highest rank in the classification of organisms below that of kingdom. 'Phylum' was originally a purely zoological term, the equivalent rank under the botanical codes being 'division'. However, as part of moves towards harmonisation of the botanical (including fungal) and zoological codes, 'phylum' is adopted and preferred in botanical and fungal classification.
Almost all lichenised fungi are in the phylum Ascomycota, a few being in the Basidiomycota.
(plural: podetia)  Lichenised, stem-like portion of an apothecium, or, notably in Cladonia, the upright, often branched or cup-like structure arising from the primary thallus, generally bearing pycnidia and/or apothecia.

podetia of Cladonia uncialis subsp. uncialis
Possessing a perithecium as the fungal fruitbody.
Root-like projection from the underside of a foliose thallus, generally for attachment to the substrate.

tufted, brownish rhizines on the underside of thallus-lobes of Peltigera rufescens
Wrinkled. When the wrinkles are very small, the equivalent term is "rugulose".
Growing on stone.
(plural: scyphi)  Cup-like podetium, bearing pycnidia and/or apothecia on its rim. Scyphi characterise the "pixie-cup lichens" (Cladonia spp.).

scyphi of Cladonia diversa (pycnidial stage)
Lacking a stalk. When used of apothecia, the term refers to an apothecium seated on the thallus surface, i.e. superficial, neither stalked nor immersed.
(adjective: squamulose)  Small, often leaf-like scale, lacking a lower cortex. The primary thallus may be composed entirely of squamules, or, as in Cladonia, podetia may often bear squamules on the outer surface.
The squamulose thallus is more fully described here.

Cladonia squamosa:
basal squamules of primary thallus
Taxonomic rank below species but above variety (see classification). In the case of plants and fungi, as governed by the International Code of Nomenclature for algae, fungi, and plants (ICN), there are no trinomial names, so, for example, Aspicilia contorta subsp. hoffmanniana is written with its rank explicitely stated, and NOT as a trinomial ("Aspicilia contorta hoffmanniana"), which is strictly zoological usage, under the International Code of Zoological Nomenclature (ICZN).
This taxonomic rank is arguably under-used in lichenology and might be usefully employed for groups of semi-cryptic "species", such as those of the Cladonia chlorophaea complex.
Abbreviated "ssp." or "subsp." (plural "sspp." or "subspp.").
(plural: thalli)  As applies to lichens, the 'vegetative' body of a lichen, including both the fungus (mycobiont) and algal cells (photobiont), but excluding any sexual fruitbodies.
Lichen thalli vary much in morphology; a summary of thallus types is provided here.
Term used for a chlorococcoid, green-algal (photobiont) that is, or resembles, Trebouxia, and is presumably a member of the class Trebouxiophyceae (see 'classification').

trebouxioid photobiont in Caloplaca cerinella
Urn-shaped. When used of an apothecium, the term implies that the sides of the apothecium are extended up, becoming more like a perithecium in shape and with the hymenium partly enclosed.
Warted, covered with small, rounded lumps. When the warts/lumps are very small, the equivalent term is "verruculose".


Brodo, I.M., Sharnoff, S.D., & Shjarnoff, S., (2001). Lichens of North America, Yale University Press, New Haven.
Dobson, F.S. (2011). Lichens. An illustrated guide to the British and Irish species, ed. 6, Richmond Publishing Co., Slough.
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.