THOMAS Gray, in his famous churchyard Elegy, paints a graphic picture of a rural graveyard, in which:
Beneath those rugged elms, that yew tree's shade,
Where heaves the turf in many a mould'ring heap,
Each in his narrow cell for ever laid,
The rude Forefathers of the hamlet sleep.
Later he invites us to read the epitaph of a youth to fortune and to fame unknown, "grave on the stone, beneath yon aged thorn". This, however, might well prove rather difficult: old grave-stones are frequently disfigured by large growths irregular in shape and appearing in many pastel shades, that cover the lettering on the stonework and make it hard to read. But if they are aesthetically questionable, lichens, as they are called, are at least a sign that the surrounding air is pure.
Lichens are a strange and unique form of life comprising a symbiotic pairing of a fungus and an alga. They form most readily on the barks of trees, on rocks, and, as we have seen, on gravestones. Individuals may grow by as little as a millimetre a year, but they can survive for centuries, and even, for all we know, millennia.
More than 1,700 species of lichens inhabit the islands of Ireland and Britain, and they are effective barometers of the level of pollution in the atmosphere. Although they can endure the most extreme climatic conditions, lichens are very sensitive to any impurities in the air, and a thriving population is an indication that the local air is clean. Conversely, their decline or absence signifies the very opposite. Indeed, in recent years in Britain it has been found that certain species of lichen that had been thought to be extinct are re-emerging, a development thought to be connected to the decline in levels of sulphur dioxide pollution in the atmosphere.
Other countries have more exotic varieties of these strange organisms than we might be familiar with, and the locals have been inventive in their use of them. Leconora esculenta, for example, is a rather flaky lichen which peels easily from the rocks in the arid areas where it thrives, to blow freely in the wind. In 1829, during a war between the Russians and the Prussians, a severe famine affected areas near the Caspian Sea: during a violent wind-storm, the countryside became littered with leconora esculenta, and the local populace, noticing that their sheep could eat it without ill effect, gathered the lichen, ground it down to flour, and made it into what was apparently a very palatable bread.