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Nature

A Lichen on the Tilley family Gravestone

On the grave of the Tilley family, Alfred and Mary Adelaide, (Area NES, Row 3, Plot 9) there are a couple of patches of a yellow lichen called Caloplaca flavescens. They could also be Caloplaca aurantia; the species are often confused, and DNA analysis is necessary to be sure. Both species are described by Paul Whelan (2012) in his excellent ‘Lichens of Ireland’.

The Tilley family headstone, Heene Cemetery, showing the lichen Caloplaca flavescens.
The Tilley family headstone, Heene Cemetery, showing the lichen Caloplaca flavescens.
Magnification of Caloplaca flavescens, Heene Cemetery.
Magnification of Caloplaca flavescens, Heene Cemetery.

Above is a magnified photograph showing the crustose, yellow lichen with a placodioid edge. Crustose means the lichen is flat to the surface of the gravestone and it has grown into the surface so you can only get if off with a chisel. The edge is called placodioid because it has radiating peripheral lobes.

Closer magnification of Caloplaca flavescens, Heene Cemetery.
Closer magnification of Caloplaca flavescens, Heene Cemetery.

Let us look (above) at the lobes in more detail. A human needs a chisel to get this lichen off the gravestone but slugs and snails graze away damaging the thallus or body of the lichen. This helps us to look inside the thallus.

Lichens consist of two organisms, a fungus and an alga living together in a mutually beneficial relationship called symbiosis. The graze damage lets us see the white fungus enclosing the green, single celled algae. The fungus provides a home for the algae and the algae photosynthesises, providing nutrition for the fungus. The photograph also shows two or three fruiting structures, called apothecia that produce fungal spores for the fungal component of the lichen. These are circular and look like jam tarts; well perhaps lemon curd tarts.

Magnification of the edges of Caloplaca flavescens, Heene Cemetery.
Magnification of the lobe edges of Caloplaca flavescens, Heene Cemetery.

Looking at the magnified edge of the lobes (above), you can see the green algae through the white fungus. The rest of the lichen is covered in a yellow pigment which is thought to give the lichen protection against ultra-violet light. Some scientists say that it protects the lichen from snail grazing; however, the Heene snails are not deterred.

Peripheral lobe of Caloplaca flavescens under a microscope.
Peripheral lobe of Caloplaca flavescens under a microscope.

Let us look (above) at a peripheral lobe of Caloplaca under a microscope. This fragment is about 2mm by 1mm and includes an apothecium – can you see it? It looks like a jam tart. The surface looks like it has been sprinkled with icing sugar. We describe that as pruinose and again think the crystals function to deter grazers.

Underside of Caloplaca flavescens under a microscope.
Underside of Caloplaca flavescens under a microscope.

if we turn it over, we can clearly see (above) the white fungal fibres or mycelium with a layer of green algae and a layer of yellow pigment. In the centre of the lobe there is a hollow region. If you look closely at the green algae you can probably make out that they are single celled; each cell looking like a green ball. They are individual members of the Trebouxia genus or family.

Microscope photograph of peripheral lobes of Caloplaca flavescens showing white fungal mycelium.
Microscope photograph of peripheral lobes of Caloplaca flavescens showing white fungal mycelium.
Microscope photograph of peripheral lobes of Caloplaca flavescens showing a layer of green algae.
Microscope photograph of peripheral lobes of Caloplaca flavescens showing a layer of green algae.

Two further microscope photographs (above) of peripheral lobes of Caloplaca show white fungal mycelium above and below a layer of green algae. All this is below a layer of the yellow pigment, parietin.

If you are interested in finding out more about lichens, take a look at the British Lichen Society website. It is full of information, learning materials and photographs: https://britishlichensociety.org.uk/.

John Brownbill