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Lichens: lowly but precious forms of plant life

Posted in Nature, Plants on Saturday, 29 October 2011

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This edited article about botany originally appeared in Look and Learn issue number 851 published on 6 May 1978.

Lichens, picture, image, illustration

Various lichens we see every day by Eric Saunders

Most people know what lichens look like. They would probably describe them as forms of moss, grey, green or yellow in colour, that grow on such surfaces as those of tree trunks or stone walls.

In fact, lichens are not mosses. They are not any one sort of plant at all – but a combination of two, growing in a strange partnership.

Lichens are composite growths consisting of fungus and algae. The latter are a group of primitive plants, generally growing in water. They range in size from giant seaweeds with 200-metre stems to the minute cells that float as green scum on a pond, or often coat the sides of aquarium tanks.

The fungi include not only the familiar mushrooms and toadstools, but also the moulds and many microscopic forms.

A fungus, which many botanists do not classify as a true plant, normally depends for food on the dead or living plant or animal matter on which it grows.

Algae, on the other hand, though very simple in structure, are true plants, and as such they contain chlorophyll, a substance made possible by the process known as photosynthesis.

By means of photosynthesis, a plant uses the energy of the sun, carbon dioxide from the atmosphere and water drawn up through the roots to supply itself with the carbohydrates which it needs to live and grow.

The body of a lichen consists basically of a web of fungal threads, or hyphae, enclosing a number of algal cells. The fungus gives protection to the algae, and provides them with the moisture and mineral salts which are vital to them, and which they cannot obtain for themselves. In return, the algae feed the fungus with some of the surplus carbohydrates which they produce by photosynthesis.

There are three main kinds of lichen. The crustose lichens form a continuous “crust”, and are able to survive in regions of little rainfall – including both hot deserts and the Polar regions.

Foliose (leaf-like) lichens thrive in rainy climates. A common example is the leafy yellow or orange lichen seen growing on the trunks of trees in the British Isles. The foliose lichens have a comparatively complex structure, consisting of alternate layers of fungus and algae.

The third type is fruticose, or branched, lichen. Lichens of this kind have stalked, or thready, growth, and are found especially in humid and foggy areas.

Much still remains to be learnt about the way in which new lichens come into existence. However, it is known that they reproduce in several ways.

In some cases, the fungus produces spores in cup-shaped bodies. When these spores are discharged, algal cells are also ejected. As the spores germinate, the algae multiply as well. Where the growing fungus succeeds in combining with the algae, a new lichen body is born.

Some lichens reproduce by releasing what are called soredia. A Soredium consists of one or more algal cells wrapped in fungal threads. Under the right conditions, a soredium will develop into a lichen.

Other lichens, which have no soredia, may reproduce by simple fragmentation, a piece breaking off and developing as a separate body.

Though lichens are a very lowly form of life, they are remarkably hardy and long-lived. They are resistant to extremes of climate, and are found all over the world. On high mountains, at altitudes where no trees or even grasses can survive, lichens may be found growing on rocks.

Animals such as reindeer depend on lichen for nourishment in the tundra, the icy waste of the Arctic region. Even human beings have been known to keep themselves alive for short periods by using lichen for food.

Lichens have been found useful to man in various other ways. They have, for example, proved a valuable source of natural dyes. One of these plays an important part in chemical analysis, as the blue pigment in litmus paper. This is turned red by acid, and turned blue by an alkali. Lichens have also been used in the manufacture of medicines.

While many fungi prey on other forms of life as parasites, their partnership with the algae in lichen is one of mutual benefit. It is an example of what is called symbiosis – a word of Greek origin meaning “living together”.

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