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What are lichens?

Belorussian translation Belorussian translation

Lichens are 'dual' organisms. They consist of two (or more) different life-forms living together symbiotically in a more-or-less well defined body or thallus.

The main partner is the fungus or mycobiont. The other partner is a green alga or a cyanobacterium (the photobiont). Many cyanobacteria are able to fix nitrogen which is an essential nutrient. In 10% of lichens (e.g. Collema and Leptogium) the photobiont is a cyanobacterium. The nutritional benefits of having a nitrogen fixing capability is why some lichens, where the primary photobiont is a green alga, have evolved separate structures called cephalodia which contain cyanobacteria (the secondary photobiont). So for these lichens there are three partners in the symbiosis.
Lichens are named after the fungal partner.

Placopsis gelida

The fungal partner provides a cosy, sheltered environment for the alga, protecting it to some extent from the elements and providing mineral nutrients. The actual benefits to the alga or cyanobacterium are debatable but the obvious advantage for the fungus is the in-house food factory provided by the photobiont. The photobiont partner is more than simply an imprisoned food-slave, however. It is such a closely evolved alliance that the fungus is dependant on the presence of the photobiont for its shape and structure. If the fungal partner is isolated and grown on an agar plate it forms a shapeless, infertile blob.

Lichen structures.

Below is a leafy lichen (tree lungwort Lobaria pulmonaria) cut open to show what is inside. The green algal layer can be clearly seen at the top of the section with a white algae-free zone below consisting of just fungal hyphae. The middle layer of the thallus is the medulla. There is also a very thin upper and lower cortex and a layer of rhizines on the lower surface which attaches the lichen to the substrate.

lichen cross-section

Growth forms

There are four basic lichen growth forms:-

  • crustose
  • squamulose
  • foliose
  • fruticose

  • Each have various subcategories:-

    Crustose lichens

    Crustose lichens grow completely attached to the substrate. The main body of the lichen is called the thallus and there is usually a distinct layer of prothallus between the thallus and the substrate (a bit like a carpet underlay).

    The lichen thallus is initially
    made up of separate lumps
    growing on the prothallus.
    These lumps often grow
    together to produce a
    continuous cracked surface.
    The prothallus is often
    visible between the cracks.
    areolate lichen
    Protoparmelia ochrococca
    The thallus is initially
    continuous but becomes
    cracked into irregularly-
    shaped and sized pieces.

    rimose lichens
    Lecanora rupicola
    The margin of the
    thallus is formed into
    radiating lobes which
    are attached directly
    to the substrate.

    placodioid lichen
    Caloplaca flavescens
    The lichen thallus
    does not have a cortex
    even when young
    but consists entirely of
    tiny, fluffy granules.
    made up of fungus
    and algae.

    leprose lichen
    Lepraria nivalis

    Squamulose lichens

    Squamules are scale-like outgrowths. The lower surface doesn't have a cortex. They can be very small and sometimes difficult to tell apart from a crustose growth form.

    squamulose lichen
    Normandina pulchella
    squamulose lichen
    Squamarina cartilaginea
    squamulose lichen
    Lopadium disciforme
    squamulose lichen
    Catapyrenium cinereum

    Foliose lichens

    Leaf-like lichens where the upper and lower surfaces are distinctly different. Both surfaces are corticate (except in Peltigera). Usually attached to the substrate by outgrowths of the lower surface such as rhizines or hapters.

    The lobes have multiple attachment points such as hundreds of rhizines.
    The lobes are attached by a single stalk or umbilicus.
    foliose lichen
    Flavoparmelia caperata
    Peltigera hymenina
    Umbilicaria torrefacta
    Umbilicaria torrefacta
    Lasallia pustulata
    Lasallia pustulata
    Foliose:- jelly lichens
    The photobiont is a cyanobacterium. Lobes often swell markedly when wet.

    jelly lichen Collema auriforme jelly lichen Leptogium gelatinosum

    Fruticose lichens

    Shrubby, branched, beard-like or strap-shaped lichens. The branches are rounded or flattened but both sides are similar (not different upper and lower surfaces).

    Cup lichens
    Stick or goblet-like
    podetia growing out of
    a crustose or squamulose
    basal thallus. Often with
    red or brown reproductive
    structures on the end
    of the podetia.

    Cladonia lichen
    Cladonia pocillum
    Shruby lichens
    A branched and bushy
    lichen where the basal
    thallus disappears very early.

    Cladina lichen
    Cladonia portentosa
    Beard lichens
    Erect or pendent, branched
    lichens with a tough
    central core.

    beard lichen
    Usnea subfloridana
    Hair lichens
    Pendant, thin, branched
    lichens without a
    central core.

    hair lichen
    Bryoria fuscescens

    Reproductive structures

    Most lichens reproduce sexually by the production of spores but vegetative reproduction is also very common.
    Many species use both methods.

    Sexual reproduction involves the process of genetic recombination which enables genetic stability to be maintained within the population whilst allowing variation. But there is a slight problem with spores:- A spore represents just a fungal propagule and to become a lichen the germinating spore has to re-lichenise. The spore must germinate in a suitable location and find an algal cell of an appropriate species in order to start a new lichen. Whereas a vegetative propagule already consists of both fungal and algal partners and only has to land and stay put in a favourable location in order to start growing.

    Sexual reproduction

    Most lichens belong to the Phylum Ascomycota in which the spores are made inside a sac-like structure or ascus (plural asci).
    The asci are contained within a "fruiting body" or ascoma (plural acomata). These ascomata occur in a wide variety of different shapes
    and colours but there are two distinct types:-

  • Perithecia. Where the asci are contained within a protective flask-shaped or pimple-shaped structure.
  • Apothecia. Where the ascus layer is generally open and exposed. As shown below.

    There are also a few lichens which belong to the Phylum Basidiomycota in which the spores are produced on the outside of a cell called
    a basidium (pl. basidia). For these lichens the spore-bearing body is called a basidioma (pl. basidiomata).

    Although technically incorrect, many of us simply refer to them all as "fruits"!

    Some lichen genera such as Lepraria and Thamnolia have never been found fertile.

    Types of spore-producing bodies:-

    Tiny flask-shaped structures containing asci. When mature the spores are extruded through the central pore or ostiole.

    Thelidium papulare
    Thelocarpon olivaceum
    The layer of spore-containing asci is usually uncovered and visible.

    With a thalline margin
    which contains algae and
    is the same colour as the
    lichen thallus.

    lecanorine lichen
    Lecanora pulicaris
    Without a thalline margin
    but with a 'proper'
    margin which is usually
    the same colour as
    the central disc.
    lecideine lichen
    Lecidella elaeochroma
    The apothecia are
    elongated or star-shaped,
    immersed and flush with
    the lichen surface.

    arthonioid lichen
    Arthonia radiata
    The apothecia appear twisted, ropey or ridged.

    gyrose lichen
    Umbilicaria cylindrica

    Elongated and/or branched.

    lirellate lichen
    Graphina anguina
    Pin lichens

    With a powdery mass of spores on top
    pin lichen
    Chaenotheca bruneola

    Twigs with apothecia on top.

    cladonia lichen cladonia lichen
    Cladonia polydactyla             Cladonia gracilis
    Mushroom-fruited lichens.

    mushroom lichen mushroom lichen
    Lichenomphalia umbellifera             Thallus
    mushroom lichen mushroom lichen
    Lichenomphalia hudsoniana           Thallus

    Stuff what ain't lichens

    Because lichens are so varied in appearance it is easy to mistake them for other life forms.
    So here are some non-lichen examples.

    Trentepohlia algae
    Free-living Trentepohlia algae.
    Klebsormidium algae
    Algal "gunge" that covers
    twigs and rocks in areas
    with high Nitrogen deposition.
    Bark fungus
    Bark fungus. Easily mistaken
    for lirellate lichens
    but have no lichenized thallus.
    Glue fungus
    Glue fungus on hazel
    Hymenochaete corrugata.
    Nostoc algae
    Cyanobacteria Nostoc commune.
    Merveille de Jour
    Moth with lichen camouflage
    Merveille de jour
    (Unmistakable but interesting).

    Vegetative reproduction

    Types of vegetative reproductive structures:-

    Vegetative propagules get dispersed by wind and rain and creatures great, small, slimey, feathered or furry. If one ends up somewhere good - that's the next generation started.

    Soredia are tiny fluffy parcels of fungal hyphae and algal cells without a cortex (think of peas mixed with spaghetti, only smaller - around 1/20th mm).

    soredia soredia soredia soredia
    Isidia are outgrowths of the lichen body containing fungal hyphae and algal cells with a smooth cortical surface (like a pea and spaghetti pasty?).

    isidia isidia isidia isidia

    Why lichens are special.

    Adding interest to the landscape.

    Lichens add another layer of diversity and beauty to our environment with their different colours and shapes. A landscape without lichens on the rocks, walls, fences and trees would be so much less interesting. Photographers, artists (and poets) have been inspired by the beautiful patterns that lichens make - the Lichenscapes page has some great examples.

    Contribution to a rich ecosystem.

    Lichens provide shelter and/or food for a very wide variety of invertebrates, such as mites, molluscs, spiders and moths which are near the bottom of the food chain. A woodland rich in lichens has been shown to have a far greater diversity of other wildlife than a woodland where the trunks and branches are relatively bare. Small birds (chaffinch, hawfinch, goldcrest and long-tailed tit) use lichen fragments to camouflage their nests.

    Ecological indicators

    They are excellent barometers of air quality, environmental management and ecological value.

    Many lichens are sensitive to airborne pollutants such as sulphur dioxide, oxides of nitrogen, NPK fertilizer and ammonia etc. Lichen distribution data can be used to indicate pollution problems.

    Some species are highly adapted to a particular ecological niche such as a dry underhang on an old oak trunk or a dried up wound track from a broken branch. Also these species are often poor colonisers. This means that their existence at a particular location is dependent on the continuous local availability of that particular niche. Lichens can therefore be used to infer continuity of management and as part of the evidence to determine the age of a woodland. This has led to the development of Indices of Ecological Continuity for various woodland types where the total number of certain indicator species is used as a measure of the conservation importance of that woodland.

    A good site for rare lichens is also likely to be rich in other things such as invertebrates, bryophytes and higher plants. In contrast, higher plants are not such good indicators; if you clear-fell an ancient woodland, almost all the herbs and trees will still survive. But the rare lichens, bryophytes and invertebrates will almost all disappear, leaving only common, weedy species, and it may take many centuries for them to return (if ever), depending on how far it is to the next population.


    To start out all you really need is a hand lens. For a modest investment it is amazing what facinating and beautiful detail can be revealed with a simple x10 hand lens.

    The next step usually involves getting hold of a dissecting microscope, to see fine details more easily. You will also need a variety of chemicals to carry out 'spot tests'. Spot tests are where a tiny drop of chemical is applied to parts of the lichen. The most common chemicals are bleach (Calcium or Sodium hypochlorite or 'C' for short - cheap, thin bleach is best), Potasium hydroxide (K), Paraphenylene diamine (Pd) and Iodine (I). These chemicals produce a variety of different colours (or no reaction) depending on the presence of various lichen compounds which are characteristic to different species. You do have to be careful when using and handling these chemicals for obvious reasons. Pd is considered to be (probably mildly) carcinogenic so I always open the jar outside to let the built up fumes escape before taking a very small amount to make up a fresh solution.

    To get fully into lichens you do need a compound microscope in order to examine tiny details such as the size and shape of spores etc. Obviously, how much you want to get involved with lichenology is completely up to you. But you don't need to be an expert to join the British Lichen Society. The BLS is made up of a wide range of abilities and degrees of involvement. An interest in the subject is all you need. But there is no better way to learn than to spend time with fellow enthusiasts.

    Facts and figures

    Number of species

    There are 1873 species listed in The Lichens of Great Britain and Ireland. However, some of these are considered to be extinct in the British Isles so the actual figure is around 1800. But more are still being discovered each year, either new to the region or new to science. This compares with about 1760 species of native vascular plant.

    The world lichen list probably stands at around 17,000 although it is difficult to be precise since many regions have not been thoroughly searched.

    Fossil record

    Since lichens are primarily composed of soft tissues they are very rarely found as fossils. The earliest evidence comes from a 600 million year old lichen-like fossil from China. The next record comes from our own Rhynie chert (Aberdeenshire) at around 410 MY. The Rhynie chert was formed by a hot spring system and is of world importance for its geology and the exceptionally well preserved animals and plants it contains.

    Then there is a long gap where at some stage a recognisable lichen thallus evolved as revealed by leafy or bushy lichen fossils aged around 65 MY (Tertiary period). There is a modern-looking Lobaria fossil dating back 12 - 24 MY from California and many examples of excellently preserved specimens in amber from around the world dating from 55 to 15 MY.


    The above is just a very brief selection of information. Much more can be obtained from:-

  • The Lichens of Great Britain and Ireland
  • . Ed. by Smith, Aptroot, Coppins, Fletcher, Gilbert, James and Wolseley (2009). British Lichen Society.
  • Lichens. Gilbert, O.L. (2000). The New Naturalist, HarperCollins.
  • Lichen Biology (2nd Edition), Edited by Nash, T.H. (2008). Cambridge University Press.
  • Indices of Ecological Continuity for Woodland Epiphytic Lichen Habitats in the British Isles. Coppins, A.M. & B.J. (2002). British Lichen Society.
  • Rhynie chert website links:- Scottish Geology Website