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Dr M’s liking crustose lichens

Dr M has been investigating lichen diversity with his MSc students and you can check out the other posts on liking lichen growth forms and foliose lichens, and fruticose lichens.

Here Dr M takes a look at some of the crustose lichens that students examined in the lab under the expert guidance of botanical colleague Fay Newbery.

So what are crustose lichens? Well in general terms they are crust-like growths growing on the bark of trees or on rocks and other substrates.

More specifically, crustose lichens are lichens in which the thallus (the main body of the lichen) is crust-like with an upper protective cortex of fungal hyphae, and with algal cells in a loose “medulla” region, and the whole lichen is closely adhered to the substrate (wood or rock for example). This means that crustose lichens cannot easily be removed from the substrate and invariably disintegrate when removed with a knife etc.

Dr M’s crustose lichen gallery: Below are photos of a range of crustose lichens examined by Dr M’s students, the brief descriptions have been modified from Frank Dobson’s remarkable book: “Lichens an illustrated guide to the British and Irish Species”.

There is a gallery of relevant terms at the bottom of this post.

Arthonia radiata. Thallus white, pale grey fawn-grey to brown, apothecia rounded or to stellate (star-shaped),  black and only slightly raised above the thallus with no obvious margin very common on smooth bark trees and shrubs.

Graphis scripta. The genus name means “writing” due to the script-like nature of the apothecia. The thallus is white to grey with a smooth or wrinkled surface, and the apothecia are lip-shaped (known as lirellate) up to several mm long and with pronounced, raised carbonaceous margins.  The species is common on smooth-barked trees and shrubs.

Lecanora cf chlarotera. The genus name means “beautiful small bowl” due to the beautiful small bowl-like apothecia! This is a very large genus of maybe 100 species in Britain often requiring chemical test to verify species. Thallus grey to creamy grey, smooth or warted, sometimes aereolate. Apothecia lecanorine (the beautiful small bowls, actually rather like cute tiny jam tarts!) to about 1mm diameter, disc buff to red-brown. Very common on trees in less polluted areas.

Lecidella eleochroma. Thallus smooth, often with black lines (called the prothallus) demarcating adjacent areas, often forming complex mosaics with other crustose lichens such as Lecanora spp. Apothecia lecideine (like tiny dark wine gums), disc concave with smooth black margins. Very common on smooth-barked trees, shrubs and fences and gates.

Glossary of terms related to crustose lichens

Aereolate – in which the thallus is cracked.
Apothecium (plural apothecia) – the fungal sexual reproductive structures containing the spores.
Cortex – the upper layer of the thallus.
Disc – the central part of the lecanorine or lecideine apothecium.
Lecideine – with apothecium shaped like tiny wine gums with the margin the same colour as the central disc.
Leconorine – with apothecium shaped like tiny jam tarts with the margin a different colour to the disc.
Lirellate – with apothecium lip-shaped.
Medulla – the middle layer of the thallus.
Placodiod – thallus with a lobed margin.
Thallus – the main body of the lichen with fungal hyphae and algal cells.

Getting interested in lichens? Why not check out the British Lichen Society who always welcome new members and interested beginners into their lichenological fold!

Open Air Laboratories (OPAL): Here is a nice presentation on lichens from the OPAL website


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