Lichens are among the most successful symbiotic relationships between plants, the partners being an alga and a fungus which have become completely integrated. They have various characteristic forms, of which one of the commonest is the thin flat crust, like that growing on this gravestone.
Crust-forming lichens typically grow on stone or rock, often on tiles; they are the earliest colonisers in very hard growing conditions like the Arctic, being tougher than either constituent part and capable of standing extreme cold, desiccation and heat. One remarkable example, the bushy Ramalina from the Negev desert, can tolerate over 8o°C when dry, yet continue to photo-synthesise when part-frozen at —10 deg C.
The algal component of the lichen carries out
Many lichens like this are strongly coloured when growing in sunlight, often in shades of yellow and orange. These tints, which man has used for millennia as
dyes, are due to a deposit of lichen acids in the cells nearest the surface. These act as filters to reduce the strength of light reaching the photosynthetic algal cells; the same lichen growing in shade will be green.
Crusty lichens obtain their water supplies from rain or run-off water. They are thus very susceptible to any impurities in the water, and in modern times dissolved sulphur dioxide, perhaps the commonest industrial effluent, has been found to damage and finally kill them depending on its concentration. Hence lichens are widely used as indicators of the degree of industrial pollution.