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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.


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Lichen thallus types

The ‘vegetative’ part of the lichen is the thallus, and this can develop into a morphologically diverse range of structures: multiply branched tufts, flat, leaf-like structures, filamentous structures, aggregations of tiny plates, thin crusts, layers of powdery granules, or hardly any visible thallus at all. Imposing a classification on this variation inevitably results in uncertain distinctions and borderline cases, but several general growth forms can be recognised. The treatment here matches that in Dobson (2011) and generally that in the multi-author accounts in Smith et al. (2009).


foliose, fruticose, squamulose, crustose, placodioid, leprose, filamentous, no visible thallus


 
1.   Foliose lichens

These are some of the largest and perhaps most complex lichens. The thallus generally forms flat, leaf-like lobes, with differentiated layers of tissue, the upper and lower cortices, forming the upper and lower surfaces. The lobes are commonly, but not always, appressed to the substrate surface, but can be lifted away. The lower cortex is often differently coloured, frequently brown or black and usually bears rhizines. In Peltigera the lower surface is ecorticate.

In foliose lichens with multiple branches of the thallus that may stand away from the substrate, the differentiated lower cortex distinguishes them from fruticose lichens, e.g. Evernia prunastri, in which the thallus lobes are white beneath, and Pseudevernia furfuracea, in which the undersides are black when mature.

 
 
Parmotrema perlatum, upper and lower surfaces.
(Saltash, Cornwall, 2008).

 
 
Typical arrangement of tissues in a foliose lichen
(from Smith, 1921).

As a foliose lichen, it has both an upper and a lower cortex. The photobiontic layer is in the upper part of the medulla, where the algal cells will receive enough light, yet be protected from the full strength of the sun's rays by the upper cortex.

[Physcia parietina is an old synonym for Xanthoria parietina.]

Systematics: lichenisation is a phenomenon that has developed many times within the fungi, and convergent evolution to similar thallus types is substantial and by no means surprising. The foliose morphology is, however, more restricted to groups of related species. The family Parmeliaceae (order Lecanorales) contains a substantial proportion of the foliose species, including such genera as Parmelia, Evernia, Hypotrachyna, Melanelixia, Parmotrema and numerous others mostly once placed in a larger concept of Parmelia. However the family Parmeliaceae also contains fruticose genera such as Bryoria and Usnea. The order Peltigerales contains several families of foliose lichens, including the Lobariaceae (Lobaria, the tree lungworts, and allied genera), the Pannariaceae (with, e.g., Degelia), and the Peltigeraceae (with Peltigera, the dog lichens, and Solorina). The family Umbilicariaceae (sole family of the order Umbilicariales) contains Umbilicaria and Lasallia. The order Teloschistales contains several important generally crustose genera, but also Physcia (family Physciaceae) and Xanthoria (family Teloschistaceae), with neither of these families being exclusively or even predominantly foliose. Here and below, systematic positions are taken from Smith et al. (2009), pp. 16–20.


Further examples of foliose lichens
Evernia prunastri
Parmeliaceae
Peltigera membranacea
Peltigeraceae
Xanthoria parietina
Teloschistaceae


 

 
2.   Fruticose lichens

The thallus is extended up into a tufted or pendant branched structure, the branches being covered by a single cortex. In fruticose lichens with flattened branches, e.g. Ramalina spp., the cortex extends round both surfaces of the branch. Consequently, they differ from foliose lichens with branched, aerial lobes such as Evernia.

 
 
Usnea subfloridana,
(Gleniffer Braes, Renfrewshire, 2009).

Here the thallus is tufted from a basal holdfast, which is blackened in this species. The branches are round in cross section, with a lax or compact medulla under the cortex and a solid, cartilaginous, central axis.

Systematics: fruticose morphologies appear to arise relatively easily from crustose or other thallus types and there may be variation even within genera. In Britian, Aspicilia is a predominantly crustose genus, but fruticose species are known elsewhere in the world. Stereocaulon contains richly branched, fruticose species, but a small number are best regarded as crustose. The family Cladoniaceae (order Lecanorales) contains predominantly fruticose lichens (or lichens commonly described as such), including Pycnothelia and Cladonia, but the primary thallus is crustose (if only briefly so) or squamulose. Generally, individual fruticose genera are most closely related to genera with other growth forms, e.g. Roccella is grouped with crustose genera such as Dirina, Peterjamesia, Schismatomma in the family Roccellaceae (order Arthoniales), Alectoria, Bryoria and Usnea are grouped with numerous foliose genera in the family Parmeliaceae (order Lecanorales), Stereocaulon is allied to the leprose/crustose genus Lepraria in the family Stereocaulaceae (order Lecanorales), Teloschistes is grouped with crustose, squamulose and foliose genera in the family Teloschistaceae (order Teloschistales). There is no generally fruticose line of evolution.


Further examples of fruticose lichens
Roccella phycopsis
Roccellaceae
Teloschistes flavicans
Teloschistaceae
Ramalina polymorpha
Ramalinaceae


 

 
3.   Squamulose lichens

In squamulose lichens, the thallus is composed of usually small, flat, usually massed, often overlapping scales – ‘squamules’. They differ anatomically from the smaller foliose lichens in that the squamules do not have a lower cortex, or at most it is weakly differentiated, though the underside may be differently coloured from the rest of the medulla and sometimes (as in Catapyrenium and Placidium), rhizoidal hyphae may be produced.

In some lichens, e.g. Cladonia species such as C. digitata or C. foliacea, the squamules are relatively large, several millimetres in length, and may be lobed and/or held away from the substrate; they nevertheless remain limited in size. Extension of the thallus is typically by growth of the underlying prothallus. In some genera, such as some Dermatocarpon species with massed, single lobes, the distinction between "foliose" and "squamulose" is arguable.

 
 
Squamarina cartilaginea, barren and fertile, squamulose thalli.
(Shap, Westmorland, 2012, and Giggleswick, Yorkshire, 2010).

Systematics: squamulose morphologies may variously be modifications of the crustose state or might be ancestral or reduced states of foliose morphologies, and so may be closely allied to lichens of different growth form. Most species of Cladonia subgenus Cladonia, family Cladoniaceae (order Lecanorales), have a squamulose primary thallus, though conspicuous podetia usually develop from this, but while podetia can be regarded as highly developed apothecial initials, the final appearance in many species is undeniably fruticose. Hypocenomyce, which may have a weakly differentiated lower cortex, is allied to the crustose genus Ophioparma in the family Ophioparmaceae (order uncertain, "incertae sedis"); Squamarina is doubtfully placed with both crustose and fruticose genera in the family Ramalinaceae (order Lecanorales); several squamulose genera such as Protopannaria and Psoroma are grouped with foliose genera in the family Pannariaceae (order Peltigerales); Catapyrenium, Dermatocarpon, Normandina and Placidium (all with perithecial fruiting bodies) are allied to the well known crustose genus Verrucaria in the family Verrucariaceae (order Verrucariales). The basidiomycete lichen, Lichenomphalia hudsoniana, a toadstool allied to the waxcaps (Hygrocybe) in the family Hygrophoraceae (order Agaricales) (FRDBI) also has a thallus composed of squamules.


Further examples of squamulose lichens
Vahliella leucophaea
Pannariaceae
Cladonia subcervicornis
Cladoniaceae
Lichenomphalia hudsoniana
Hygrophoraceae (Basidiomycota)


 

 
4a.   Crustose lichens

The thallus forms a crust over the substrate and is firmly attached to it. There is an upper cortex, at least in early development, but no lower cortex and the medulla is in direct contact with the substrate and commonly grows into it to some extent. Consequently, the lichen normally cannot be collected intact without collecting a portion of the substrate along with it.

There are various terms to describe the nature of the thallus surface (quite apart from sexual reproductive structures such as apothecia or asexual reproductive structures such as soralia or isidia). The surface may be smooth or lumpy ('warted') and frequently the surface is areolate, i.e. composed of small, separate islands of thallus seated on an underlying prothallus or hypothallus. These areoles may be clearly separate ('dispersed') or be closely contiguous and often represent break-up of the thallus surface ('cracked-areolate'). If the surface is cracked but not broken up into discrete areoles, it is said to be 'rimose'. Sometimes the surface is pruinose.

It follows that close examination of the surface, often with a hand-lens, is essential for identification. Any photographs must be in close view (macro mode or using a dedicated macro-lens) and avoiding any movement to ensure the result is pin-sharp. Snapshots of crustose lichens posted in Internet forums in the hope of identification are often completely useless for the purpose. In any case, only with considerable field experience (if then) can many crustose lichens be identified without microscopic and/or chemical confirmation. In American usage these are 'microlichens', liable to be excluded from identification guides.

The thallus is cracked-areolate, split into small islands of tissue, and the surface of each areole is minutely verrucose (warted). The thallus margin is not noticeably modified in this case, and there is no conspicuous prothallus.

 
Ophioparma ventosa, fertile, crustose thallus.
(Loch Eriboll, West Sutherland, 2012).

 
 
 
 
Lichen mosaic on iron-rich rock
(North Pennines, 2009).

Species of crustose genera, including Rhizocarpon, Porpidia, Lecanora and Lecidea, are growing together on the rock surface, with the original rock exposed only where there has been damage to a lichen thallus. Many thalli show a dark boundary; this is the prothallus – accentuated when adjacent colonies grow to meet.

Systematics: probably all major groups of lichenised fungi include simple, crustose forms, and their enumeration would be pretty much a listing of a substantial part of the British lichen mycota. Some of the better known genera include Arthonia, in the family Arthoniaceae (order Arthoniales), Verrucaria in the family Verrucariaceae (order Verrucariales), Acarospora in the family Acarosporaceae (order Acarosporales), Diploschistes, Graphis and other graphoid genera in the family Graphidaceae (order Ostropales), Aspicilia in the family Megasporaceae (order Pertusariales), Ochrolechia in the family Ochrolechiaceae (order Pertusariales), Pertusaria in the family Pertusariaceae (order Pertusariales), Lecanora and Lecidella in the family Lecanoraceae (order Lecanorales), Bacidia in the family Ramalinaceae (order Lecanorales), Rinodina in the family Physciaceae (order Teloschistales), and Caloplaca in the family Teloschistaceae (order Teloschistales), though many of the last would be better described as "crustose-placodioid".
In the past, a very large number of crustose lichens were placed in the genus Lecidea. The majority of these have been moved elsewhere, but the genus remains substantial, along with the allied genus Porpidia, both genera often being important on rock-faces and boulders. They are placed in the family Lecideaceae, but the precise taxonomic position of the family itself is uncertain. Of similar ecological importance but uncertain taxonomic position is Rhizocarpon, family Rhizocarpaceae.


Further examples of crustose lichens
Diploschistes scruposus
Graphidaceae
Caloplaca ochracea
Teloschistaceae
Graphis scripta
Graphidaceae


 

 
4b.   Placodioid lichens
(incl. 'crustose-placodioid', 'squamulose-placodioid')

The thallus is generally crustose, but the margin extends into distinct, radiating lobes, which still lack a lower cortex but which may not always be so firmly attached to the substrate. This is really a variant of "crustose" and many individual species may be simply crustose or may alternatively become placodioid. However, when well developed, the placodioid morphology may be striking – hence the separate treatment here.

Some placodioid species can be confused with foliose species, e.g. crustose-placodioid species of Caloplaca, especially C. flavescens, can can resemble the foliose Xanthoria elegans, but the latter has true foliose lobes with a lower cortex. There can also be intergradation between placodioid and squamulose morphologies. as in Fulgensia fulgens and Solenopsora holophaea.

This coastal species is noted for its characteristic, long, radiating lobes, giving the thallus a wheel-like appearance. However, on rough rock surfaces, the lobes may be poorly formed or sometimes even missing, making the thallus simply crustose with dispersed areoles.

 
Caloplaca thallincola,
(Great Cumbrae, Clyde Isles, 2009).

 
 
Fulgensia fulgens.
(Stackpole, Pembrokeshire, 2009).

A rare species of calcareous sand and soil, with a growth form that varies from placodioid to more or less squamulose.

Systematics: since 'placodioid' is generally an extension of 'crustose', and since the 'crustose' morphology is common throughout the fungal groups that form lichens, there is again no noteworthy evolutionary line of placodioid lichens. Numerous species of Caloplaca, in the family Teloschistaceae (order Teloschistales), have variable development of marginal lobes, and some, such as Caloplaca thallincola, described above, normally have these well developed. Many of these can be described as "crustose-placodioid". The large genus Lecanora, in the family Lecanoraceae (order Lecanorales), is predominantly crustose, but a very few species in Britain are placodioid. Fulgensia, shown above, is grouped with crustose, foliose and fruticose genera in the family Teloschistaceae (order Teloschistales).


Further examples of placodioid lichens
Diploicia canescens
Physciaceae
Placopsis lambii
Trapeliaceae
Lecanora muralis
Lecanoraceae


 

 
5.   Leprose lichens

In leprose lichens the thallus surface is composed of granules containing algal cells and fungal hyphae with no overlying cortex, even during early stages. There can be a thin, underlying medulla but even this may be missing, the lichen then being a film of granules with little, if any, further fungal matrix.

In the past, there has been a tendency to think that leprose lichens are reduced forms that really belong in other genera. Baron (1999), for example, actually defined leprose lichens as "those which have not, at least up to the present, been found to have fruiting bodies and so cannot, with certainty, be ascribed to a particular genus." However, fruiting bodies are not uncommon in some species, and while they remain unknown in the large, leprose genus, Lepraria, extensive DNA investigations have shown this to be a natural, independent genus (Ekman & Tønsberg, 2002). Nevertheless, the leprose morphology is not, in itself, now considered sufficient grounds for generic separation, e.g. the former genus Leproplaca, which is now subsumed within Caloplaca.

Note that in numerous crustose species, the upper cortex may disintegrate, with initially discrete soralia joining up to produce a continuously sorediate surface that then appears leprose. Common examples include Dirina massiliensis, Lecanora expallens and Porpidia tuberculosa. In such species, the thallus margins generally remain corticate (but sometimes scarcely so in L. expallens).

 
 
Lepraria incana.
(Paisley, Renfrewshire, 2008).

The thallus is diffuse and powdery; in this species there is no underlying medulla.

Systematics: probably the best known and certainly most frequent genus of leprose lichens is Lepraria, allied to the fruticose genus, Stereocaulon, and placed in the family Stereocaulaceae (order Lecanorales). Leprocaulon, with a leprose primary thallus resembling a Lepraria but then producing small, delicate branches (pseudopodetia), has also been placed in the Stereocaulaceae, but its correct taxonomic placement is uncertain. The family Pilocarpaceae (order Lecanorales) contains several lichen genera with reduced thalli that vary from crustose to granular in Micarea to granular to leprose and lacking a cortex in Psilolechia, Byssoloma and Fellhanera. The genus Chrysothrix is in its own family, the Chrysotrichaceae (order Arthoniales). As noted above, the species of the former leprose genus Leproplaca have now been included in the predominantly placodioid or crustose genus Caloplaca (family Teloschistaceae, order Teloschistales). The family Coniocybaceae, containing the 'pin-lichen' genera Chaenotheca and Sclerophora, constitutes another group of lichens of uncertain taxonomic relationships.


Further examples of leprose lichens
Chrysothrix candelaris
Chrysotrichaceae
Chaenotheca furfuracea
Coniocybaceae
Caloplaca chrysodeta
Teloschistaceae


 

 
6.   Filamentous lichens

In filamentous lichens, the fungal hyphae form sheaths around filaments of the alga (Trentepohlia or trichome-forming cyanobacteria). The lichen retains the morphology of the algal component, though will generally be a little more robust and generally darker in colour. In cyanolichens, e.g. Ephebe lanata, the lichen may actually differ little in appearance from the free-living cyanobacterium (Stigonema in the case of Ephebe). Fruiting bodies are unknown in some species (Cystocoleus ebeneus, Racodium rupestre), generally rare in others.

 
Cystocoleus ebeneus, macroscopic and microscopic views
(Ty Canol, Pembrokeshire, 2009).
 
The lichen looks like matted, black hair. The fungal hyphae (recognisable under the microscope by their contorted, nodulose cell walls) are tightly attached to and running the length of the filaments of the orange-pigmented alga, Trentepohlia, and although the fungal association macroscopically differs in colour from free-living Trentepohlia and has longer filaments, it retains the filamentous growth form of its photobiont.


Cystocoleus & Racodium, line illustrations
 
Cystocoleus ebeneus and Racodium rupestre, comparison of the two species, showing fungal hyphae enclosing the Trentepohlia filaments (images from Smith, 1911).
 

Systematics: relatively few lichens can be described as filamentous, and as with other thallus types, the fungi belong to several different orders and classes. Cystocoleus and Racodium are evidently related and belong to the order Capnodiales, but in Smith et al. (op. cit.) they are not assigned to any family within the order. Ephebe and Thermutis are both placed in the family Lichinaceae (order Lichinales), along with several other genera of cyanolichens with variably crustose to squamulose or fruticose thalli. Spilonema is in its own family, the Coccocarpiaceae (order Peltigerales); Polychidium is allied to the foliose genus, Massalonga, in the family Massalongaceae (also in the order Peltigerales).


Further examples of filamentous lichens
Ephebe lanata
Lichinaceae
Polychidium muscicola
Massalongaceae


 

 
7.   Lichens with no visible thallus

Sometimes, lichen fruiting bodies may be seen on a substrate and yet no thallus is visible. This may be because:
i)the thallus is evanescent (soon disappearing);
ii)the thallus is immersed (has developed within the surface of the substrate, e.g. is endolithic);
ii)   the lichen exists as hyphae (with minimal photobiont) within the thallus of another lichen, i.e. is lichenicolous.

In most such cases, the thallus is essentially crustose, with potentially an upper cortex, but if it is inside a rock surface or another lichen, or quickly disappears, this crustose nature is not fully developed or not apparent. Consequently this not really a different thallus type, but merely an acceptance that not everything fits into a morphological classification. Pyrenocarpous lichens (those with perithecia as fruiting bodies) commonly occur as black dots embedded in the substrate, with no visible thallus on the surface.

 
 
Acrocordia conoidea.
(Stackpole, Pembrokeshire, 2009).

An example of a pyrenocarpous lichen occurring embedded in tiny pits in the substrate surface, in this case limestone. In this species the perithecia are about 1mm in diameter — unusually large!
 
The thallus is usually immersed in the rock, as here, but can sometimes be visible and then crustose.

 
 
Collemopsidium foveolatum.
(Kennedy's Pass, Ayrshire, 2008).

A tiny pyrenocarpous species that dissolves pits in soft, maritime limestones and in the shells of limpets (Patella) and barnacles (usually Chthamalus montagui?). The thallus is immersed and minimal, with the algal cells in small groups or absent (Coppins & Orange, in Smith et al. (op. cit.)).

Systematics: numerous lichens in numerous families can have immersed thalli, often depending on the nature of the substrate. As noted above, pyrenocarpous lichens frequently have no visible thallus, so this phenomenon is more likely to be seen, e.g., in the order Pyrenulales or in genera of uncertain systematic position such as Collemopsidium in the family Xanthopyreniaceae, not assigned to any order in Smith et al. (op. cit.). The large genus Thelocarpon, consisting of very inconspicuous species with the thallus usually no more than an algal sheath around the perithecia, is placed in the family Thelocarpaceae, again not assigned to any order.


 

References
    •   
Baron, G. (1999). Understanding lichens, Richmond Publishing Co., Slough.
          
    •   
Dobson, F.S. (2011). Lichens. An illustrated guide to the British and Irish species, ed. 6, Richmond Publishing Co., Slough.
          
    •   
Ekman, S., & Tønsberg, T. (2002). Most species of Lepraria and Leproloma form a monophyletic group closely related to Stereocaulon. Mycological Research 106: 1262–1276.
          
    •   
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.
          
    •   
Smith, A.L. (1921). Lichens, Cambridge University Press, Cambridge.
          
    •   
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
October 2012, last updated April 2014
Line illustrations from out of copyright sources as noted, but scan manipulations © A.J. Silverside.