24 July 2010


Image: Ericd

The tenacity of living things sometimes surpasses belief. There is scarcely a square mile of the planet surface, no matter how forbidding, which does not support at least some form of life. The richness and diversity of living systems reach their astounding zenith in the tropical rain forests; their nadir is at the polar icecaps, especially that of the south.

Even in the polar wildernesses of rock and scree, and even, in places, in the ice itself, life may be yet found. The forms here are usually quite simple, single-celled animals and plants. Where conditions relent slightly, if only for an hour or two a day at midsummer, the terrain is colonized by lichens.

In fact, over great tracts of the world’s most inhospitable regions, lichens are the only multicellular organisms able to thrive. They grow abundantly in such daunting places as rocks which are alternately washed by the sea and baked by the sun. They will grow on walls, roofs, tree-trunks, on the bare soil. Some sorts even secrete acids which enable them to grow inside rocks, waiting for the surface to weather away before fruiting.

The key to their success is as strange as it is wonderful, and may be studied here in suburban Hertfordshire as well as in Antarctica or the Sahara desert.

Though given a single scientific name, a lichen is really a partnership of two separate sorts of organisms – a fungus and an alga. Fungi do not have chlorophyll, and so cannot use photosynthesis to manufacture their own food, as green plants (including the algae) do. On the other hand, most algae cannot, unaided, survive extreme conditions. By entering into partnership, the fungus receives sugars and vitamins from the alga, while the alga in turn gets minerals and protection from extremes of dryness and illumination; and both organisms are then enabled to colonize places previously out of bounds.

A handful of the familiar mushroom-type fungi undergo lichenization, as the process is called, but the vast majority belong to a more primitive group, the Ascomycetes.

So specialized have these lichen-fungi become that few are now able to survive for long without an algal partner. The algae, however, especially if conditions are favourable, can often exist independently. Free algae of the right sort must be available for the formation of a new lichen by sexual reproduction, for it is the fungal element alone which produces spores. The fungus is the senior partner, makes up the bulk of the lichen’s structure, and gives the lichen-plant or thallus its characteristic shape; the thallus usually has one of three main types of form: shrubby, leafy, or crust-shaped.

The clustered thalli can appear quite fantastic, like creatures from another world, or the vegetation dreamed up by an artistic imagination on the verge of madness, and the colours, which are always marvellously subtle and soft, come from Nature’s most ethereal paintbox. At different seasons and in different states of dryness the appearance may change dramatically, for during adverse conditions the lichen “shuts down” and waits for things to get better.

Unless conditions are exceptionally favourable, most reproduction is by vegetative, or non-sexual, means. Many sorts produce little powder-dusted pores which also help to keep the thallus aerated; the particles of powder are called soredia, each of which, when dispersed by the wind or by sticking to animals and birds, can grow into a new thallus. Even simpler and more common is dispersal by fragmentation: bits of the parent thallus get broken off, for example by being trampled, and blow away to develop elsewhere.

The pores in a lichen ensure that there is free flow of gases between the photosynthesizing algae and the outside world. Lichens are often efficient at absorbing whatever substances are in the environment, which has had dire consequences for those Lapps and Eskimoes who depend on caribou and reindeer for food.

One of the main components of the vegetation of the northern tundra is the Reindeer Moss, which, despite its name, is a lichen. It bulks large in the diet of the reindeer and caribou, and is the main source of carbohydrate in that particular food chain. Unfortunately, Reindeer Moss is exceptionally good at absorbing the radioactive fallout from atom-bomb tests which finds its way into the upper atmosphere and is then distributed all over the planet surface. High levels of radioactive caesium and strontium have accumulated in the bodies of the grazing animals, becoming even more concentrated in the bodies of the northern people.

So good are lichens at absorbing toxic substances that they have been used as indicators of atmospheric pollution. The lichen flora of, say, North Devon or rural Scotland is rich in species: 1400 in all have been recorded in the British Isles. But as you approach the industrial centres the number of species falls off rapidly, as does the luxuriance of growth of those that remain.

Our local lichen flora is badly impoverished, simply because the air here is so dirty. The main pollutant is sulphur dioxide, produced by the inefficient burning of fossil fuels. But others, in far smaller quantities, and even more dangerous, are contributing to the continuing decline in lichens: fluorides, heavy metals such as cadmium and lead, agricultural and garden fertilizers and pesticides, and a whole catalogue of other industrial wastes – the list is depressingly long.

Lichens may be regarded like the coal-miner’s canary. We ignore their death at our peril. The few species that remain to us in south-west Hertfordshire survive mainly by growing on alkaline surfaces, such as walls, asbestos roofs, limestone paving, and concrete, which offset to some extent the acidity of the rain.

Yet the ability of the lichens to colonize inhospitable terrain is undiminished. By poisoning the atmosphere, we have altered the balances that have remained unchanged for millennia, and those types which have become adapted to them will die. But already there are new and – almost – sinister types of lichen emerging, able to thrive in the new conditions. One, the aptly named Pollution Lichen, was unknown in Europe before 1860. A recently published guide chillingly gives its status thus: Widespread in industrial and densely populated parts of Europe. Abundant in England.

The writing, plainly printed in lichen thalli for anyone to see, is on the wall.

(Introduction to these pieces; see all)

1 comment:

Paul Biba said...

Fascinating post.