The rocks on Haeckel Hill near Whitehorse are splotched with crusty growths of black, brown, white, grey, garish orange, and multiple shades of green.

The splotches are crustose lichens, also commonly called rock lichens, and they are among the oddest organisms in the Yukon.

They're also among the oldest. Some of those colourful splotches grow as slowly as 11 millimetres a century, says Val Loewen, a biologist with Yukon Environment. That means that a plate-sized lichen could be hundreds of years old.

Lichens are a hugely successful life form, with about 14,000 species distributed around the world, from tropical shorelines to the peaks of northern mountains. They range in shape from thin crusts on surfaces like rock or wood, to elaborate structures resembling leafy plants or vines.

Despite their wide distribution, lichens remain something of a puzzle.

For one thing, lichens aren't one organism, but two and sometimes three. They are a combination of a fungus and algae living in a mutually dependent, or symbiotic, arrangement. Some fungi live together with two quite different kinds of algae, both green and blue-green.

"They're very remarkable creatures really, when you think about it," says Loewen.

They're also hardy and adaptable. Rock lichens can colonize inhospitable locations like mountain rock faces because they don't make their living off the surface they grow on.

"They take their nutrients from the air, which makes them good indicator species for pollution," Loewen explains.

Strictly speaking, only the algae partners in the lichens take nutrients from the air. The fungi live off the work of the algae. Using the energy of sunlight, algae turn carbon dioxide and water into sugars -- enough sugars to nourish both the algae and the fungus in which they live.

In return for the nourishment provided by the algae, scientists theorize, the fungus provides shelter from excessive exposure to the sun and a fairly stable supply of moisture, both beneficial to the algae.

The process the algae use, called photosynthesis, is one that lichens share with plants. However, lichens are not plants in the definition used by botanists.

Fungi, algae, and blue-green algae (cyanobacteria) are all separate organisms related to plants. Lichens themselves are composites and unique in the world of vegetation.

One lichen expert, Trevor Goward, called them "fungi that have discovered agriculture."

Exactly how and why they discovered agriculture is a bit of a mystery. In fact, scientists are still puzzled about what makes a lichen arise, what trigger causes fungus and algae to adopt each other and create a new kind of organism.

It's possible to separate the fungus and the algae that make up a particular lichen and grow them individually in the laboratory. The resulting growths bear little resemblance to the original lichen.

But mix the two growths together under the right conditions and they will form a new association that looks like the original lichen. It appears that the fungus carries the genetic information to form the shape of the lichen, but can't actually do it without some triggering mechanism supplied by the algae. Researchers are still working toward an understanding of that mechanism.

In the meantime, lichens flourish throughout the Yukon and throughout the world, adding unexpected daubs of colour and texture in even the most difficult environments.

For more information about lichens, contact Val Loewen, Yukon Environment, at (867) 667-5281.