Country: The litmus plant

It is best not to say it to Frank Dobson, one-time crack pistol shot (British team, Rome Olympics, 1960) and erstwhile part-time skipper of a Thames River police launch, but lichens are really quite boring in a way.

He is one of the leading lights in the 600-member British Lichen Society, and they are a truly dedicated bunch. The objects of their interest are what's called lower, or prim-itive, biota. Telling the species apart is as difficult as spotting which sphagnum moss is which. They are, it may be said, more varied than the grasses, the ferns, or even the fungi (or mushrooms). All the same, they are, to misquote Hamlet, neat but not quite gaudy enough to be really popular.

Plantlife, the group that aims to do for botany what the RSPB does for birds, is rather plaintive in its announcement of the launch of the Red Data Book for lichens, which takes place next Wednesday at the Natural History Museum in London. "Few people are aware of lichens, yet they grow all around us," it murmurs. Red Data Books flag up conservation anxieties surrounding different bits of our wildlife. This latest one will have first to make people interested at all.

But it is typical of the discreet charm of the lichens (all 1,500-odd species of them in Britain) that they do - so to speak - grow on you. They are complicated things. In a sense, they are almost as much an event as a thing. They are, explains Mr Dobson, an association of an alga and a fungus. "Under some definitions seaweeds are lichens, really," he says. They are mostly ignored by these specialists. "The field of study," writes Mr Dobson in his definitive Lichens: An Illustrated Guide to the British and Irish Species, "is circumscribed by tradition rather than by science."

Few people are first drawn to botany by a purely scientific curiosity. With lichens, one gets the feeling that those who love them come to the study of these splashes of usually subdued colour on a rock or tree after years of preoccupation with the more obviously glamorous plants. But in a dark and dank wood, the lichens that love old bark are often almost luminous in the delight they bring to the eye. On coastal clifftops, their hues of russet or eau de nil or grey are a subtle relief.

The obstinacy of lichens is very appealing: they are the most numerous species in Antarctica, they manage on the most blasted rock face, and the most heavily drenched stream-side. They can also be charmingly fussy: according to Peter Lambley, English Nature's specialist, some of them are happiest in woodland with 400 years under its girths. Nick Stewart, who researched the Red Data Book, says that one species is to be found in only four sites in Britain, and on a total of as few as 50 trees.

Yet the picture is far from gloomy. Lichens are good indicators of air quality. The pollution in our cities is now so much alleviated that some species are reappearing after many years' absence. One such has popped up in the gardens of Buckingham Palace. Some others, it is true, are suffering a little, especially in the countryside, probably because traffic is now spreading some nitrous oxides and thus low-level ozone far and wide. Contradictorily, says Peter Lambley, some species manage quite well beside busy roads. "Others seem not to like the agro-chemicals, and suffer on trees next to fields," he adds.

Some lichen species that positively thrived in the dirty years are now finding life difficult. One liked mineral slag heaps; its extinction there can perhaps be mourned - but its revival not accommodated. Several lichens are part of what is now called a "suite" of species which add up to making a case for preserving this or that woodland or heathland.

The lichen, with its toughness, attractiveness, sensitivity and becoming modesty, is not likely to set many hearts on fire next week. The complexity of its conservation position - neither dire nor unthreatened - will make few headlines. But the more we learn about its habits and tastes, and track them, the better we can understand some of the minutiae of habitat and pollution. Lichens are not merely lovely, they are a sort ofliving litmus paper monitoring our stewardship.

Plantlife: 0171-938 9111