This is a “fake” taxon meant to group all lichenized fungi. Most belong in Ascomycota, but there are a few in Basidiomycota, and there are several distinct lineages even within Ascomycota due to the lichenized condition apparently evolving independently several times. (There is some debate on this matter still, as one leading lichenologist claims that all existing ascomycetes share a lichenized ancestor! (Does anyone have a citation for this?))

In general, a lichen is any fungus that grows with a green alga and/or cyanobacteria, where the relationship is obligate (at least for the fungus), and the resulting symbiosis results in a specialized thallus which neither the fungus nor the alga/cyanobacteria would produce on their own.

See the section on Terminology below for an overview of lichen morphology.

World wide.

Rock, bark, wood, soil, sand dunes, moss, plant detritis, leaves, even aquatic.

Some liverworts can look surprisingly like lichens, but microscopically there can be no question, as they have much larger cells and spores. Another tell-tale clue is that no internal structure will be visible if you cut a liverwort thallus in half, while almost all lichens will have a display cortex, algal layer, and medulla, easily visible with a handlens.

Some lichens are primary food source for animals (e.g. reindeer moss in the boreal zone). Some have been used to make dyes (the cudbear used in some scottish tartans, for example, is derived from a lichen). Lichens are also an excellent indicator of pollution and climate change, both on a global and local scale. (See the section on Ecology below.)

Terminology

Coming from a mycological background it won’t take long at all to get the hang of the new terminology.

Lichens are fungi (the mycobiont) and either green algae and/or cyanobacteria (the photobiont) growing together to form a single distinct organism. In most cases the photobiont grows in a distinct thin layer just under the surface (e.g. Parmeliaceae), but in other cases the fungus merely forms an invisible sheath around algal filaments (e.g. Coenogonium), and in yet others the photobiont is distributed uniformly throughout the lichen (e.g. Collema).

The basic lichen forms are:
foliose – flattened with distinct (and different) upper and lower surface
fruticose – non-foliose lichen that is either 1) bushy and well-branched, 2) beard-like, or 3) having unflattened erect stalks
crustose – attached so tightly to a surface that you can’t remove it without removing part of the substrate
umbilicate – foliose lichen that is attached in a single spot underneath
squamulose – tiny foliose lichens, typically made of tiny separate, often overlapping scales
jelly – foliose, fruticose or crustose lichens that turn gelatinous or at least rubbery when wet, typically without recognizable layers — i.e. appears to be uniformly black/brown throughout in section
calicioid – crustose lichens with minutely-stalked fruiting bodies

The entire structure of the lichen is called the thallus, and is typically layered with a tough, usually shiny cortex outside, and fluffly, usually white medulla inside, generally with the algae sandwiched in a thin layer just below the cortex.

Fruiting bodies are nearly always apothecia, typically disks with a rim (lecanorine) or without a rim (lecideine or biatorine), or cups. Rarely perithecia or mazaedia (powdery masses of spores caused by the disintegration of the hymenium), and even more rarely true basidiate mushrooms (e.g. Lichenomphale). See the section on Microscopy below for more info on the specialized terminology of microscopic structures.

Asexual propagules are very common and very important to identification:
soredia – powder or granules, containing just algae and medulla
isidia – minute protuberances, shiny from presence of cortex (and containing algae and medulla as well)
lobules – just normal bits of thallus that break off easily for whatever reason (also called blastidia, schizidia, among other things)

Another odd asexual reproductive structure is the pycnidium, which is essentially a primitive perithecium with conidia instead of ascospores. These are present as dots scattered within or protruding from the surface, often in addition to apothecia.

Root-like structures are called rhizines. They can either be simple (e.g. Physcia), branched or squarrose (e.g. Physconia), or they can grow from the margins, where they are called cilia (e.g. Heterodermia).

Stalks (e.g. in Cladonia) are podetia. They generally emerge from a mat of tiny scales called squamules.

Two other important terms, maculae and pseudocyphellae, both refer to light-colored markings on the surface. In the former the cortex is still present, and the color is due to discontinuities in the algal layer beneath the cortex; in the latter the marks are full-on cracks that reach all the way into the medulla (to facilitate gas exchange). (See Parmelia and Punctelia, for example.)

Occasionally, in addition to green algae, a lichen will contain cyanobacteria in cephalodia, little dark bumps or blobs attached to the surface (several Peltigera most notably).

Glossary

Glossary of terms used for lichens, from the Flora of Australia Glossary by D. J. Galloway

Chemistry

Two chemical reagents are critical to getting anywhere with lichens, both easy to acquire:
C – plain old undiluted household bleach
K – lye (Red Devil drano is a good source for NaOH, about 1/4tsp in 1/4cup water, as much as you can fit in there and still have it dissolve completely!; KOH might be marginally better but is harder to find)

Other reagents of various importance:
P or PPD – paraphenylenediamine; this is carcinogenic and very difficult to find (ask Jason if you need help finding it)
K/I – iodine, either lugol’s solution or meltzer’s reagent, typically a stain used to observe microscopic structures of the apothecium and asci: apply KOH first, rinse with a drop of water, then apply a drop or two of Lugol’s or Melzer’s
stains – such as Congo red, something-or-other blue, etc.; used to improve contrast of hyphal tissue where otherwise difficult to see under the microscope

(Sulphuric and nitric acid are also rarely used to determine if hymenial granules dissolve.)

Lichens in general have among the most complex chemistry among all other comparable groups of organisms (a widely used vague figure that I have been completely unable to make more scientific!) There are good introductions to this in various guide books (e.g. Irwin Brodo’s Lichens of North America). Aside from spot tests mentioned above, there are other techniques, most notably TLC — thin layer chromatography — that enable lichenologists with properly-equipped labs to distinguish hundreds of different lichen compounds. And in some cases such subtle chemistry is the primary basis for distinguishing spcecies. But this is entirely outside the reach of the amateur, and indeed many professionals I’ve met.

Spot Tests

The two most common spot tests are on the cortex and the medulla. For the cortex, just place a drop of lye or bleach on the surface of the lichen. For the medulla, scratch the surface off with the edge of a razor or the point of a knife (for tiny lichens this might have to be done under the dissecting scope). This should expose the cottony, usually white interior. This is often highly hydrophobic, making it difficult to get a drop of liquid to actually contact the material. The easiest thing I’ve seen is to use a sharp toothpick: just dip the tip in a drop of lye or bleach, then jam it into the medulla, trying to avoid touching the cortex.

The K test is a drop of lye, C is bleach, KC is a drop of lye followed by a drop of bleach (take your time, they only need to be done within a few minutes of each other).

Here are some common spot test results with K and C: (there are many many more)

Note that C reactions can tend to be fleeting, while K reactions are always persistent.

The KC+ gold spot test is very useful for separating out several genera of Parmeliaceae, e.g., Flavoparmelia from Parmelia, and for distinguishing several species of Cladonia (the ones typically with red apothecia or pycnidia).

The K+ yellow spot test is critical for distinguishing the genera of Physciaceae. Although with experience you will quickly learn to recognize “usnic yellow” and “atranorin gray” with the naked eye.

The K+ wine-red reaction on bright orange lichens is among the most satisfying. It is also useful, since there are other yellow or even somewhat orangish pigments that are not K+ red (e.g. vulpinic acid in Letharia, or calycin in Candelariella).

Microscopy

The basis for higher taxonomy of lichens is in microscopic details of the mycobiont’s reproductive structures. (Indeed, presently and for the forseeable future, lichen taxonomy is based entirely on the fungal taxonomy.)

The vast majority of lichens are ascomycetes, and thus spores are borne in specialized hyphae called asci.

The fruiting bodies of most lichens are apothecia. These structures are typically discs, although they are occasionally elongate, angular or even branched and effigurate (in which case they are called lirellae, such as in Graphis scripta). In section, the disk is composed of a hymenium, typically hyaline, containing the asci and sterile hyphae called paraphyses. The top layer of the hymenium is called the epihymenium, composed of the tips of the paraphyses, and often various pigments, typically dark. Beneath the hymenium is the subhymenium, then the hypothecium, however these layers are not necessarily differentiated, and there is some disagreement in the literature as to their exact definition.

The rim or margin of the disc is composed of one or more of various layers. The entire rim can be called the exciple, however the proper exciple refers only to the layer immediately surrounding the hymenium. The rim can have a cortex, algae, and medulla just like the rest of the thallus, in which case the apothecium is called lecanorine. If cortex, algae and medulla are absent, and the rim contains only the proper exciple, the apothecium is called either lecideine or biatorine. In the former the outside of the exciple is darkly pigmented, sometimes carbonized and brittle; in the latter it is hyaline to lightly pigmented.

Sometimes the thalline part of the rim and the layer beneath the hypothecium are collectively called the amphitheicium, while the layers immediately inside of that are called the parathecium, but I’ve never found a satsifying definition of these terms. They have something to do with ontogeny, so potentially they may vary from genus to genus.

The ascus itself has internal structure that is critical to higher lichen taxonomy. Most lichen asci and hymenia stain strongly blue in K/I (see section on chemistry above). The structure tip of asci, called the thollus is important. Parts of the thollus will stain differently, often a thick dark blue mass with lighter canal of variable shape penetrating partway or entirely through the tip. This is extremely difficult to see without high-quality micrscope and a great deal of patience and experience. Irwin Brodo (Lichens of North America) and Nash et al. (Lichen Flora of the Greater Sonoran Desert Region, vol. I) both have excellent overviews of this subject.

(Lecanora-type ascus of Lecidella carpathica)

Lichen spores are often more interesting than the average mushroom spore. A fair percentage of lichens have multicellular spores, some with interesting (and taxonomically important) internal structure (see, for example, Physciaceae). Basic properties of spores are critical even to determine genus of most crustose lichens, and are usually visible even at 400x:
color: generally either hyaline or brown;
number of cells: simple (1), septate (2-more, all cell walls parallel, cells forming a chain, e.g. Graphis), or muriform (brick-like, both horizontal and vertical septae, e.g. some Buellia);
number per ascus: typically 8, but as few as one (e.g. some Pertusaria), as many as hundreds (e.g. Acarospora)
cell wall shape: typically thin throughout, but sometimes they are thick and multilayered throughout (e.g. Ochroechia), or the septum is thickened pushing the cells to either end of the spore (polarilocular, e.g. Caloplaca), or the walls are weirdly thickened, creating angular locules (e.g. Rinodina).
texture: typically smooth, but sometimes textures (e.g. cracked into plates or spirals in Calicium)

Ecology

Lichens are especially sensitive to environmental and microclimatic conditions. This is partially due to being so exposed to the elements, partially due to their ability (like amphibians) to absorb gasses and liquids through their cortex, and partially due to complex and finicky metabolic requirements resulting from the mycobiont and photobiont responding quite differently to temperature, moisture and chemical conditions.

As a result lichens are excellent indicators of climate change, pollution, and natural conditions, such as soil pH, moisture, light, etc. Lichen morphology is consequently also more variable than in most other life forms, changing color and/or form from time to time through the year, from place to place within a landscape or even on a single rock or tree, occasionally within the very same thallus. For example, under some conditions a given species will produce abundant apothecia, in others the same species will produce abundant soredia or isidia; in one place it will be black inside or underneath, and in another white.

References

Stridvall’s Lichen Gallery (European lichens, including many crustose species)

Alan Silverside’s Lichen Pages (British lichens)

Irish Lichens (good coverage, many excellent photos)

Pictures of Tropical Lichens

Key to the crustose genera of California [CALS bulletin, vol. 5, no. 1, 1998]

Macrolichens of North America (extensive list of genera, species, and resources… an ongoing MO project by Chris Parrish)

Key to lichens on limestone in eastern North America

Lichen checklists (worldwide)

Lichen genera of the world   North America   Japan   Pacific Islands   Hawaii   Australia   New Zealand

Collections of lichen photos (worldwide)

Stridvalls   Timdal   British Lichens   Silverside   Irish Lichens   LichenIreland   Belgium   France   Le Naturaliste   Italy   Poumarat   Almería, Spain   Azores   Tropical Lichens   Iran   Tibet   Japan   Australia   Australia   Australia   New Zealand   Sharnoff   CNALH   Alberta, Canada   Gatineau Park   Saba   Ticolichen   Chile   Mushroom Observer   flickr