DIVERSITY OF LIFE
THE LICHENS

LICHENS ARE CHIMERICAL ENTITIES AND, THEREFORE, HAVE NO NATURAL LINES OF DESCENT

Lichens (LI-kins) is formed from a Greek root that means lick (leicho -λείχω).  The reference may be to their use as medicinals.  At that time, lichens included some mosses, liverworts, as well as lichens.

INTRODUCTION TO THE LICHENS

By convention, the lichens are classified as members of the Kingdom Fungi.  However, because the fungus (the mycobiont) and the alga (the phycobiont) can live separately in a free-living state, both components should have separate taxonomic designations.  The difficulty is compounded by the lichen itself.  The symbiotic chimeroid structure called the lichen truly is a different organism from either of its fungal or algal components thus it should have its own "taxonomic" designation, one based on form rather than phylogeny.  I have adopted the convention of the fungal classification, but we include a description of the Form-Phylum Lichens in recognition of the uniqueness of the particular symbiosis.  Certainly, lichens as a taxon follow the dictum of deux veritas.

Peltigera-membranacea-wwu.jpg (49298 bytes)

A. Peltigera, a foliose lichen.

Cladonia-macilenta-uvic.jpg (178650 bytes)

B. Cladonia, a fruticose lichen.

Parmelia-sulcata-wwu.jpg (41853 bytes)

C. Parmelia, a foliose lichen.

Usnea-rigida-wwu.jpg (55860 bytes)

D. Usnea, a fruticose lichen.

Pyrenula-duke.jpg (203210 bytes)

E. Pyrenula, a crustose lichen.

Calicium-cas.jpg (18324 bytes)

F. Calicium, the spike lichen.

Opegrapha-uni-bayreuth.jpg (36411 bytes)

G. Opegrapha, a crustose lichen.

Lepraria-uio.jpg (149943 bytes)

H. Lepraria, a crustose lichen. Some others in this group are squamulose.

Images taken from:
A: http://www.ac.wwu.edu/~fredr/Images/Peltigera%20membranacea.jpg
B: http://web.uvic.ca/ail/Cladonia%20macilenta.jpg
C: http://www.ac.wwu.edu/~fredr/Images/Parmelia%20sulcata.jpg
D: http://www.ac.wwu.edu/~fredr/Images/Usnea%20rigida.jpg
E: http://www.biology.duke.edu/dnhs/pics/Pyrenula.jpg 
F:
http://www.cas.cz/ziva/cisla/0502/04.jpg
G:  http://www.uni-bayreuth.de/departments/planta2/ass/robert/lichens/
H:
http://www.toyen.uio.no/botanisk/lav/Photo_Gallery/Lepraria/

SYNOPTIC DESCRIPTION OF THE LICHENS

The following description comes from Margulis and Schwartz (1988), Hale (1979), Ahmadjian (1967), and Nash (1996).

I. SYNONYMS: lichens

II. NUMBERS: >18,000 species.

III. PHYLUM CHARACTERISTICS:

A. ASEXUAL REPRODUCTION:The lichen can reproduce itself by the production of diaspores, lichenized structures in which the algal and fungal units function together as subunits of the lichen thallus; diaspores may be fragments of the thallus or specialized structures such as soredia and isidia.

B. SEXUAL REPRODUCTION:Sexual reproduction typically occurs only in the mycobiont by ascocarps or basidiocarps.

C. VEGETATIVE HYPHAE:The lichen thallus is a primarily composed of the mycobiont which grows as a typical mycelium until it incorporates a particular phycobiont (usually Trebouxia) into the thallus. The resulting chimera is different from both the mycobiont and phycobiont individually. Since lichens have a photosynthetic component, they can grow on barren areas such as rock outcrops. They are also common on bark and soil. They usually form one of four thallus types:

1. FOLIOSE LICHENS - The thallus is flattened and leaf-like, with the upper and lower surfaces differing in color or surface features.

2. FRUTICOSE LICHENS - In fruiticose lichens the branches of the thallus are round in cross section, with few differences between top and bottom.  They may be bushy, hairy, or strap-shaped.

3. CRUSTOSE LICHENS - The thallus of crustose lichens is appressed to the surface of the substrate, and is sometimes in the substrate, in which case the outer margin is delimited by a dark line or color difference. Some small foliose lichens can be confused with crustose species. Crustose lichens are common on rocks and tree bark.

4. SQUAMULOSE LICHENS - These lichens are characterized by the squamules, small lobe-like structures which lack a lower cortex.

See Bold et al. (1987) for structures of different thallus types.

D. CELL WALLS: Usually chitin and glucan from the mycobiont and cellulose from the phycobiont.

E. ECOLOGY: They are found in almost all terrestrial habitats from exposed rock to mud and from arctic to tropical latitudes.

ARTIFICIAL HIERARCHICAL CLASSIFICATION OF THE LICHENS. 

The taxonomy of this "phylum" is artificial since both the alga and fungus also can exist as separate entities. The form-phylum has 3 form-classes (based on the type of mycobiont: Ascomycota, Basidiomycota, or Deuteromycota) and 8 form-orders. The taxonomy is roughly based on that of Margulis and Schwartz (1988).

FORM-CLASS ASCOLICHENOMYCETES

Sexual fruiting bodies contain asci.

  FORM-SUBCLASS ASCOMYCETIDAE

Asci unitunicate, regularly arranged in a hymenium with free, unbranched paraphyses.

FORM-ORDER LECANORALES

The fruiting bodies are apothecia; this is the largest group of lichens.

Peltigera, Cladonia, Parmelia, Usnea.

FORM-ORDER SPHAERALES

Fruiting bodies are perithecia.

Pyrenula.

FORM-ORDER CALCIALES

Fruiting bodies are mazaedia, an ascocarp in which the asci degenerate at maturity and the ascospores are liberated free in the hymenium.

Calicium.

  FORM-SUBCLASS LOCULOASCOMYCETIDAE

Asci bitunicate, regularly or irregularly arranged in an ascostroma (pseudothecium) with branched pseudoparaphyses.

FORM-ORDER MYRANGIALES

Pseudothecia poorly differentiated, asci irregularly distributed.

Dermatina.

FORM-ORDER PLEOSPORALES

Pseudothecia well delimited, resembling perithecia, asci more or less regularly arranged in the stromatic layer.

Melanotheca.

FORM-ORDER HYSTERIALES

Pseudothecia well delimited, round and resembling apothecia, lirelliform, or irregular in outline.

Opegrapha.

FORM-CLASS BASIDIOLICHENOMYCETES

Fruiting bodies contain basidia

FORM-ORDER CORACALES

Which has the characters of the Form-Class.

Cora, Omphalina.

FORM-CLASS DEUTEROLICHENOMYCETES

Fruiting bodies are unknown; thallus crustose to squamulose, poorly differentiated. 

FORM-ORDER LEPRARIALES

Lepraria.

LITERATURE CITED

Ahmadjian, V. 1967. The Lichen Symbiosis. Blaisdell Publishing Co. Waltham, Mass. 152 p.

Hale, M. E. 1979. How to Know the Lichens. 2nd ed. Wm.C. Brown Publishers. Dubuque, IA.

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York.

Nash, T. H., III, ed. 1996. Lichen Biology. Cambridge University Press. New York. 303 pp.

By Jack R. Holt.  Last revised: 03/10/2013