The memorial stone of Francis Septimus Hollis in the grave of his father, George Hollis, is covered in multi-coloured patches of lichen.
Headstone of Francis Septimus Hollis
The lichen, at 12 o’clock, has an off-white, greenish body (thallus) with very small black spots (approx 0.5 mm). The dots are apothecia; reproductive structures.
The lichen is probably Porpidia tuberculosa. Lichens are always referred to by their Latin names although a small number have common names which vary in different parts of the country, leading to confusion.
Let us take a closer look
A magnifying camera reveals the detail of the white thallus
A magnifying camera reveals the detail of the white thallus which has fissures (it is areolate) and black dots (apothecia) are close to the surface and have a black rim. This biological structure is embedded in the memorial stone. A chisel would be needed to remove this embedded or crustose lichen from the stone.
A picture of the lichen thallus with a millimetre scale to show the magnification of the apothecia.
What are Lichens?
Lichens are interesting; they are a relationship between a fungus and an algae. The fungus provides protection and a home for the algae whereas the algae photosynthesise and provides nutrition, in the form of sugars, for the fungus. This sort of relationship, where two organisms live together and cannot live separately, is called symbiosis. If they can exist separately, it is called commensalism or mutualism. The distinction is important to biologists who have spent time trying to get the two organisms to live separately with varying degrees of success.
Porpidia tuberculosa is a crustose lichen
Crustose lichens consist of 3 layers, an upper cortex, an algal layer, and a medulla. The medulla and the cortex are fungal, and the medulla grows in the grains of the substrate, in this case, the grains of the sandstone gravestone.
A generalised diagram of a crustose lichen.
Above is a generalised diagram of a crustose lichen. It is a magnified cross section through a thallus containing an apothecium. The apothecium is a fruiting body of the fungus; equivalent to a mushroom. It contains asci, which are spore sacs containing about eight spores. The asci are separated by sterile filaments called paraphyses. When the spores are mature, the ascus opens, and the spores are dispersed.
The substrate is the gravestone, in this case, and hyphae from the medulla grow into it. The lichen and gravestone become part of each other and if the lichen is removed, both the lichen and the gravestone will be damaged.
The medulla and cortex are the inner and outer parts of the fungus and the individual algal cells are confined to a layer between medulla and cortex. The medulla and cortex consist of a mass of microscopic strands called hyphae. Hyphae look a little like cotton wool under a microscope and mycelium is a name for a tangled mass of hyphae.
Lichens are very slow growing organisms, and they mainly reproduce asexually. However, the apothecium produces fungal spores. These spores disperse in the wind and form a new lichen with a matching algae.
John Brownbill