20,000 or so types of lichen are spread throughout almost every environment on Earth - including many where nothing else will grow. When conditions become too harsh even for lichens, they simply go dormant, awakening again when conditions improve. You will also find lichens in environments surrounded by other plants, on the north side of trees where moss does not crowd it out, growing on tree branches or rotting logs in the woodpile, clinging to rocks beside a stream.

Lichens derive their structure from a fungus, which forms the main body, or thallus. The fungus is nourished by food photosynthesized by algae cells or cyanobacteria which lie suspended in the areas of the thallus exposed to light. It is not clear what the algae gets out of the relationship; however, some species of algae are found only in lichens, and so perhaps they benefit somehow (protection from the elements?)

Scientists classify lichens according to one of three forms:

  • Flat Crustose lichens clinging to rocks,
  • Fruticose lichens growing up like little mini-trees, or dangling beard-like, from tree branches,
  • Foliose lichens curling up from soil and tree bark.

Sometimes, an intermediate Squamulose form of lichen will appear; these appear scaly or pebbly.

Areas of lichens will sometimes exhibit powdery granules called soredia, which become detatched, and generate new lichens. Other lichen fungi distribute spores, and the fungi that form from them capture algae cells opportunistically.


Swiss botanist Simon Schwendener's hypothesis of lichens' being a parasitic relationship between a fungus and an alga may be well-accepted today, but it was quite controversial when he introduced it at the Swiss Naturalists' Club in 1867. Established lichenologists were so offended that they went to special trouble to ridicule the idea. However, over the next decade or so, other botanists duplicated his results. Amateur naturalists had their hand in as well, the most famous being Beatrix Potter, who was a strong supporter of the theory. Her uncle managed to read a paper of hers on the subject before the Linnean Society in 1896. She was not permitted to attend, however. Ms. Potter was treated so poorly by the botanical boys' club that she eventually gave up botany for writing children's books. So much of a hubbub over something Carolus Linnaeus didn't even feel worth the trouble of classification!

Change in science happens one funeral at a time. Eventually, Schwendener's hypothesis became established scientific fact, so that H. G. Wells and Julian Huxley's popular 1929 biology book The Science of Life was able to state

"A lichen is no more a single organism than a dairy farm is a single organism."

Lichens appeared during the Devonian period, surprisingly recent for something so simple. I recall lichens figured into one of the last wild theories of Life on Mars before all hopes were dashed by Viking. As successful as they are, lichens are too advanced to live anywhere but Earth.


Much help from

"Lichens - a case of kidnapping", Susan J. Tweit, Wild Lives
http://www.zianet.com/snm/Lichen.htm

"Beatrix versus the Botanists" excepted from Liaisons of Life: From Hornworts to Hippos, How the Unassuming Microbe Has Driven Evolution by Tom Wakeford, John Wiley & Sons, at
http://www.nytimes.com/books/first/w/wakeford-liaisons.html

Lichen Terminology, Steven B. Selva, The Lichen Museum, Univesity of Maine at Fort Kent
http://academic.umfk.maine.edu/lichens/terms.htm

Something that should be noted: since lichens don't have the same protection mechanisms that the terrestrial plants have (waxy cuticle, stomata) they are very succeptible to airborne pollution. The pendulous lichen (Old Man's Beard) are very sensitive, and will not thrive if the air anything less than pure. On the other hand, there is a yellow lichen (xanthoria) that seems to thrive in urban areas where the air can be quite polluted.

If the tolerance levels of a particular lichen are known, then surveys can be taken which can give a rough estimate of the amount of air pollution in a given area. Also, it has been noted that lichen diversity (the number of lichen species in a particular area) will decrease when there is an increase in air pollution.

1. Richardson, D. H. S. Pollution monitoring with lichens.; Richmond Publishing Co. Ltd.:Slough, 1992

Li"chen (?), n. [L., fr. Gr. .]

1. Bot.

One of a class of cellular, flowerless plants, (technically called Lichenes), having no distinction of leaf and stem, usually of scaly, expanded, frond-like forms, but sometimes erect or pendulous and variously branched. They derive their nourishment from the air, and generate by means of spores. The species are very widely distributed, and form irregular spots or patches, usually of a greenish or yellowish color, upon rocks, trees, and various bodies, to which they adhere with great tenacity. They are often improperly called rock moss or tree moss.

A favorite modern theory of lichens (called after its inventor the Schwendener hypothesis), is that they are not autonomous plants, but that they consist of ascigerous fungi, parasitic on algae. Each lichen is composed of white filaments and green, or greenish, rounded cells, and it is argued that the two are of different nature, the one living at the expense of the other. See Hyphae, and Gonidia.

2. Med.

A name given to several varieties of skin disease, esp. to one characterized by the eruption of small, conical or flat, reddish pimples, which, if unchecked, tend to spread and produce great and even fatal exhaustion.

© Webster 1913.

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