/About lichens

About lichens


The Rocks of Ages exhibit in the NBGW has examples of rocks taken from all over Wales, from the Pre-Cambrian era to the Carboniferous period.

Lichens have colonised all of these rocks, but each type of rock has its own characteristic lichen flora.

A study of saxicolous lichens (which grow on rocks and substances such as concrete) will therefore tell us a lot about the type of rock.

The Rocks of Ages display in the NBGW is a good opportunity to observe the differences in lichen diversity on a range of rock types all in one place and to learn about the relationship between the rocks and the lichens that grow on them.

Over seventy one different sorts of lichen have been identified from the Rocks of Ages display.

Many of the rocks probably had no lichens on them when placed in the Botanic Garden.

Others appear to have come complete with a covering of lichens that are probably still adjusting to life in a new area.

Lichens can be found almost everywhere, in towns as well as in the countryside and along the coast.

Each lichen is made up of two (rarely three) completely different kinds of organisms, a fungus (the mycobiont) and one or two photosynthesizing partners (the photobionts), which are either a green alga or a cyanobacterium.

The photobiont is completely surrounded by the fungus to form an organism – the lichen – which is very different in appearance from each partner when they live separately.

The algae gain protection from desiccation and strong sunlight and in turn they provide the fungus with the carbohydrates it cannot manufacture itself.

In Britain there are at least 1,800 species, 1,300 of which are found in Wales.

This is an impressive 71% of the British total in 11% of the area.

Some lichens are excellent indicators of air quality whilst others contain unique chemicals that may be of service to us in the future.

Size and Shape

The lichen body is called a thallus.

There are three main types of lichens.

  1. ‘Crustose’ lichens grow as a thin crust that cannot be removed from the substrate without damage.
  2. ‘Foliose’ lichens have a lobed, almost leafy structure and can be removed readily from the surface on which they grow.
  3. ‘Fruticose’ lichens are branched and shrubby.

They are attached to the surface by a small plate called a holdfast.

Crustose and foliose lichens grow radially from a small propagule to form a more or less circular patch or thallus.

The circular shape is often lost over time as parts of the lichen are eaten by invertebrates, meet up with a neighbouring lichen or encounter an unsuitable area of habitat.

Lichens grow slowly

Crustose lichens in particular grow extremely slowly. Many grow less than 0.5mm a year, with the zone of growth restricted to the margin of the colony.

Eventually, some lichens may reach the size of a dinner plate, although most are much smaller, a few centimetres or less across.

Reproduction

Lichens reproduce and spread by a number of different means.

Like other, non-lichenised, fungi they can sexually reproduce by means of spores.

These are produced in fruit bodies that are disc-shaped (apothecia), flask-shaped (perithecia) or elongated and slit-like (lirellae).

Spores are spread by wind, rain or animals.

In order to grow into a new individual, a spore not only has to land in a suitable place, but also has to encounter the correct photosynthesizing partner soon after germinating.

A lichen fungus cannot survive for long by itself.

Many lichens also reproduce vegetatively. Small fragments (soredia or isidia) form on the surface of the lichen. These small fragments are made up of fungal hyphae and photobiont cells.

They are readily detached from the parent lichen and, provided they land on a suitable surface, can grow into new lichens.

Where do lichens grow?

Lichens can grow and colonise a wide range of natural and man-made substrates including trees, rocks, soil, sawn wood and metal.

They can survive in the Arctic tundra, on the tops of mountains, in hot deserts and tropical rain forests.

They are found along rocky coasts, a few on rocks and even on the shells of barnacles at or below high water mark.

Although some species of lichen can grow on a wide range of surfaces, others are restricted to particular habitats.

Saxicolous lichens are those that grow on rocks and rock-like man-made substances such as concrete.

Naturally occurring rocks have been formed throughout Earth’s history and are composed of many different chemicals, which give each kind of rock its own characteristics.

Some rocks, such as granites, are chemically acid, whereas limestones and chalk are alkaline, or basic, in reaction.

Download a survey report on the lichens on these rocks [PDF], with information about each rock display.


Slate roof with lichen
Lichen growing on a slate roof

Parmelia saxatilis: crottle
Parmelia saxatilis: crottle

Moss on Devonian Red Sandstone
Moss on Devonian Red Sandstone

Xanthoria parietina
Xanthoria parietina

lichens on spotted dolerite
Lichens on spotted dolerite

The chewing gum lichen
The chewing gum lichen

Lichen on top of the large, standing limestone block.
Lichen on top of the large, standing limestone block

Lichens on dolomite
Lichens on dolomite