Here Dr M takes a look at some of the foliose lichens that students examined in the lab under the expert guidance of botanical colleague Fay Newbery.
The thallus (the main body of the lichen) is lobed and leaf-like with a separate upper and lower cortex of fungal hyphae, with the algal cells in the upper levels of a loose “medulla” region. The whole lichen is usually adpressed (adhered flat) to the surface by means of rhizines, sometimes only loosely, sometimes more firmly, but can usually be quite easily removed from the substrate (wood or rock) and invariably remain intact when removed with a knife etc.
The descriptions of the species below are modified from Frank Dobson’s remarkable book: “Lichens an illustrated guide to the British and Irish Species”.
Evernia prunastri. The genus name meaning “branched”, this lichen appears fruticose but is a flattened, branched foliose lichen as indicated by the different colours of the upper (grey-green) and lower (white) surfaces (upper and lower cortex). The thallus is strap-shaped and the upper surface can become sorediate in older specimens. Very common on deciduous trees, more tolerant of pollution than Ramalina (the fruticose lichen which Evernia resembles).
Flavoparmelia caperata. The genus name means “yellow Parmelia” from the colour of the thallus, which can be large and up to 20 cm across, yellow-grey when dry becoming apple green when wet. Lobes to 1 cm wide expanding at the edges, transversely wrinkled and very contorted when mature and the surface becoming covered in coarse soredia. Lower surface black with simple rhizines. Very common on acid-barked deciduous trees in the south, also on well-lit rocks and roofs and on mosses.
Hypogymnia physodes. The genus name meaning “naked below” due to lack of rhizines on the under surface. Thallus grey-green and hollow (inflated) with black dot-like pycnidia towards the tips. Under-surface light brown at the margin. Lobe ends often upturned and split with farinose soredia. Very common on trees, rocks and amongst mosses and heather in heathland.
Hypogymnia tubulosa. Thallus grey-green and hollow (inflated), with erect, tubular, finger-like lobes with farinose soredia at the tips, distinguishing it from H.physodes, above). Very common on trees, rocks and amongst mosses and heather in heathland.
Melanelixia sp. (synonyms include Melanelia). The genus name means “black” due to the dark colour of the dry thallus. Thallus characteristically dark-brown, but becoming green when wet. The centre of the thallus with densely packed isidia which can obscure the lobes. The commonest species are M. fuliginosa which has a rather glossy surface and M. subaurifera with a more matt surface. Very common on trees.
Parmelia sulcata. Parmeliaceae is a family of larger foliose lichens with wider thalli. The type genus name means “embedded fruit bowl” due to the nature of the (often rare) lecanorine apothecia). Thallus grey with overlapping ridged lobes with an often coarse white network of white lines (pseudocyphellae) along which powdery soralia develop which may spread to cover the centre of the thallus. Lower surface black with simple rhizines. Very common on trees and rocks in the lowlands.
Parmetrema perlatum. Thallus light pearl-grey, loosely attached to the substrate, with undulating margins with soralia. Under-surface black with simple rhizines. Common on trees and rocks in the south and west.
Physcia aipolia. Thallus to 6 cm diameter, white, pale grey or blue-grey, lobes with distinct pale pseudocyphellae, under surface with dark rhizines. Lecanorine apothecia numerous and large (to 3 mm diameter) prominent with a thick margin and dark brown to black disc, becoming convex, sometimes pruinose. Common on well-lit nutrient rich trees and shrubs.
Physcia adscendens/tenella. Physciaceae is a family of smaller foliose lichens with narrow thalli. Two similar species with pale-grey thallus lobes to 1 mm wide with few to many pseudocyphellae, lobe tops raised with long marginal cilia. Under-surface white with scattered darker tipped rhizines. In P.adscendens The lobe ends become hood-shaped with soredia along the lower surface. Fertile forms have lecanorine apothecia to 2 mm diameter with black disc often pruinose. P.tenella is very similar but the lobe ends are not hood-shaped but split and turn back forming lip-like structures with coarse soredia. Common on trees and twigs. Some consider these to be one species.
Punctelia subrudecta. The genus name means “dotted” due to the pointy shape of the pseudocyphellae. Thallus grey, with rather thick often “flowing” appearance (unlike the sometimes similar-looking Parmelia sulcata which has a more “thin and smooth” appearance). Thallus orbicular, reaching about 7 cm across with smooth rounded lobes to 1cm wide, sprinkled with delimited soralia containing farinose soredia arising from white punctiform (point) pseudocyphellae. Under surface light brown with simple rhizines. Common on well lit trees in the south.
Xanthoria parietina. Genus name meaning “golden-yellow”. Thallus bright orange in sunny sites to greenish-grey in shaded sites. Lobes to 7 mm wide, wrinkled, overlapping with upturned edges. Under-surface pale to almost white with a few pale rhizines. Apothecia lecanorine usually abundant towards the centre of the thallus, disc orange with paler margin. One of the commonest lichens on nutrient-rich trees, rocks and walls and a species which really enjoys bird-perching sites! One of the most pollution tolerant of the foliose species.
Xanthoria polycarpa. Thallus greenish-grey to yellow-orange, orbicular and ca. 2 cm diameter with very narrow lobes (1 mm or less – this helps distinguish it from X.parietina with which it is often confused by beginners desperate to find a new species!). Lobes are convex and incised at the tips, forming short, finger-like projections often obscured by the numerous crowded and stalked apothecia to 4mm diameter.
Glossary of terms related to Foliose lichens
Apothecium – disc-shaped fruiting body containing the spores.
Disc – the central upper surface of the apothecium.
Isidium (plural isidia) – detachable asexual outgrowth on the thallus containing the photobiont and cortex, detached isidia can grow into a new genetically identical lichen.
Photobiont – the photosynthetic partner in the lichen, maybe a green alga or cyanobacterium.
Pruinose – Fine white powdery bloom e.g. on the disc of the apothocium in some lichens, e.g. Physcia sp.
Pseudocyphellae – Pale patch, dot or line where the cortex is thin or absent (e.g. in Parmelia sulcata).
Pycnidia – Flask-like body on the thallus which produce conidia – asexual spores.
Rhizines – root-like hold-fast structures arising from the lower cortex, simple or branched depending on species.
Soralium (plural soralia) – structure producing soredia.
Soredia – small powdery propagule containing a few algal cells and fungal hyphae and acting as asexual propagules.
Thallus – the main body of the lichen. containing both fungal 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!