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Amazing Story!

When most people see lichens growing on trees, rocks and decaying wood, they believe they are plants. In fact, lichens do not belong to the plant kingdom, which includes mostly vascular plants and mosses. Lichens are not easily classified because they are formed in a special biological partnership between two entities that each has its own classification.

Lichens are the stable vegetative product of a mutually beneficial, symbiotic relationship between a fungus and another organism capable of producing food by photosynthesis. (Photosynthesis uses sunlight to convert carbon dioxide and water into sugars and other carbohydrates). The characteristic common to all lichens is how they acquire the food energy they need to grow. Dare we say "You are HOW you eat"?

Usnea trichodea.

Enlarge image.This specimen of Usnea trichodea hung in a swampy area at the head of a river in Fundy National Park, New Brunswick.

Fungi are not capable of producing their own food. In lichen, a fungus finds a food source in either green algae or cyanobacteria. These contain the green pigment chlorophyll necessary for the photosynthetic process and are referred to as the photosynthetic symbiont, or photobiont, for short. The fungus shares the food energy the photobiont makes for itself. Lichenologist Trevor Goward has described lichens as "fungi that have discovered agriculture".

Some Usnea scabrata.

Enlarge image.This Usnea scabrata grew on a spruce tree at around 1500 m in the northern Rocky Mountains by Wokkpash Lake, British Columbia.

When the two components (fungus and photobiont) associate, they undergo an amazing physical transformation. The resulting lichen does not look like either, but the fungal component has the greater influence over the lichen's appearance. There are 14,000 or so species of fungi able to associate with photobionts, and the lichens they produce are tremendously diverse. Selective pressures caused by certain habits and habitats influence how lichens appear so that a similar appearance may not mean that two species are closely related.

Lichens lack the water-repellent covering that plants rely on to help them conserve water, so a lichen can absorb water and the nutrients it contains directly through its surface rather than through a root system as plants do. Some lichens of the genus Usnea are able to keep growing even when broken off from the parent thallus and are just draped over a branch or wire.

Usnea's particularly efficient ability to absorb minerals from the air has significant implications for people. Lichens of the genus Usnea are among the organisms most sensitive to airborne pollutants, making them especially valuable as natural indicators of air quality. Absorbed pollutants can upset the delicate balance of the lichen's symbiotic relationship enough to kill it.

Polluted areas have fewer kinds of lichens because the more sensitive ones die off. The presence or absence of Usnea (where one might expect it) in a floristic survey is a strong indicator of pollution levels. Once fairly common in humid regions of Europe, Usnea longissima is now almost extinct on that continent, apparently due to widespread pollution. Lichen tissue may also be analysed in the laboratory to identify the polluting compounds. Because lichens such as Usnea are such efficient accumulators of certain pollutants, they can act as an early warning system for measuring air quality.

Usnea covered tree.

Enlarge image.In the wilderness in Quetico Provincial Park, Ontario, this Usnea lichen covers almost every branch of a tree.

Lichens produce secondary compounds that are not directly involved in their primary metabolism. Usnea lichens produce usnic acid, giving them their distinctive pale yellow colour, and protecting them from overexposure to light and from browsing invertebrates that don't like their taste. Usnic acid is medically and commercially useful too. In the 14th century, people believed that because Usnea resembled hair, it could be used to strengthen hair. Its antibiotic properties were recognized independently by many cultures.

Illustration of three Northern Parulas, by John A. Crosby.

Enlarge image.The Northern Parula hollows its nest out of a hanging bunch of Usnea lichen, often without lining or with only a few pieces of grass or other plant material or hairs.

Species of Usnea are used on almost every continent: in Chinese medicine, contemporary homeopathic medicine, and traditional medicine in the Pacific Islands and New Zealand. Lichen extracts are often an ingredient in soaps and deodorants, in part because of their antibiotic properties. Indigenous peoples of Mexico use it to make a fermented corn beverage.

The more obvious fibrous qualities of Usnea have proven useful to wildlife as well as people. Usnea has been found in the nesting material of flying squirrels and more than 50 species of birds, and the Northern Parula Warbler uses it almost exclusively. The absorptive qualities of Usnea make it useful as wound dressings, baby diapers and feminine sanitary absorbents.

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