Lichens are usually found high up on the splash zone, Tiny algae and fungus live together. Algae live on the inside and fungus on the outside – which provides shape and shelter for the algae (It’s such a beautiful relationship!!). Algae feed by using photosynthesis and then produce sugar which is consumed afterwards by the fungus.
Lichens do not have roots, but absorb water and gases through their upper surface, and are therefore sensitive to atmospheric pollution. For this reason they are rarely found around cities and grow best on the wetter west side of the British Isles. Those on trees thrive best on the sunny, south-west aspects of trunks and branches. Disappearance of lichen species can be used to detect rising levels of air pollution.
There are three main types. The encrusting forms, including the bright orange/dark yellow ‘Xanthoria’ grow on roofs, walls, gravestones, signs and tree trunks. Leaf-like species develop flat lobes spreading over bark or stones, and shrubby forms which grow vertically from the ground or hang from trees. Few have common names.
The main body of the lichen is called a Thallus and each species quite different. Thallus absorbs water from the rain, sea spray, fog and dew. Grow from some millimetres per year to a few centimetres per year, and can live in very typical difficult conditions.
Some Typical lichens:
Leaf encrusting form
Upright or erect form
Tufted form – http://www.geograph.org.uk/photo/1060980
Granular encrusting form – http://www.geograph.org.uk/photo/1753336
Reference:
Image: http://www.geograph.org.uk/photo/1190524