lichen(Composite organisms formed by the symbiosis between species of fungi and an algae. They are either crusty patches or bushy growths on tree trunks, stone walls, roofs or garden paths. Because they have no actual roots they get their sustenance from the atmosphere and rainwater. Lichens play an important role in the detection and monitoring of pollution, especially sulphur dioxide, as they are highly sensitive to pollution and different species disappear if pollution reaches specific levels)