What is a lichen?
A lichen (pronounced li'ken) is a dual organism consisting of a fungus and a photosynthetic alga or blue-green alga (cyanobaterium) which live in close association. The photosynthetic partner manufactures food for the whole lichen and the fungus provides a stable, protective environment for its alga. The fungus forms the main body of the lichen, and in most cases, the alga lies sandwiched between upper and lower fungal layers. Where a lichen has a green algal partner, the green algal layer can often be seen by scratching the upper surface of the lichen. Lichens are often quoted as a classic example of symbiosis - a mutually beneficial partnership between two organisms.
Other interesting facts:
- • some crust-like lichens on rocks have a 'legendary slow' growth rate, sometimes as little as 0.1 mm per year,
- • in undisturbed conditions, some rock-dwelling lichens can survive to a great age (many hundreds of years) and are among the oldest living organisms in Scotland,
- • many lichens have a remarkable tolerance to drying out, during which state they can survive extremes of heat and cold; this means that they can tolerate being scorched by the sun in summer months, yet also survive ice and snow, and are therefore able to grow higher up in the mountains than other plants.
Scottish lichens are many colours: white, grey, black, yellow, orange, sulphur, apple-green, pink or scarlet. Most grow as crusts, some are leafy (foliose), while others are shrubby. They are completely different from the mosses and liverworts with which they often grow. The majority of mosses and liverworts are green, leafy and photosynthesise their own food.