Common Script Lichen Identification Guide

The Setting and the First Impression

This lichen belongs to smooth bark in humid shade. The classic scene is an old beech wood on a damp morning, or a shaded hazel coppice, a stand of holly, a young ash, rowan, hornbeam or alder along a stream. Look at trunks and larger limbs where the bark is still tight and unfissured, particularly on the shaded north or east flank and in the lower and middle trunk zone. Deeply ridged bark on oak or pine is the wrong place; the species needs a flat, smooth canvas.

The first impression is exactly the one the common name promises. Against a pale, faintly greenish-grey film on the bark there are dozens of fine black marks, each a millimetre to a few millimetres long, curved, forked and irregular, scattered and overlapping like handwriting in an unknown alphabet, or like fine cuneiform pressed into clay. From two metres away the patch reads as a light stain on the bark with black scribbles across it. Up close the scribbles resolve into raised, slit-like structures.

Reading the Thallus

Work on the background before the marks.

Form. The thallus is crustose, meaning it is fused to the bark and cannot be prised off as a sheet. It forms a thin, continuous to slightly cracked film, sometimes so thin that it looks like a bleached area of bark and sometimes thick enough to be clearly raised and slightly rough.

Colour. Whitish, pale grey, greenish-grey, ivory or faintly olive, deepening and greening when wet and drying to almost chalk white.

Surface. Smooth to minutely uneven or finely cracked. It does not produce soredia or isidia, so it has no powdery or granular patches and no coral-like outgrowths.

The prothallus. This is a valuable and often overlooked character. Around the edge of a thallus, and along the line where two neighbouring thalli meet, look for a thin, dark grey to black border line, as though the patch had been outlined in pencil. Adjacent individuals of the same species often meet along a sharp dark seam, dividing the bark into a mosaic of pale tiles.

Reading the Lirellae

The black marks are the fruiting bodies, and in this group they are called lirellae: elongated, slit-shaped ascomata rather than round discs.

Dimensions. Typically 0.5 to 4 mm long, occasionally longer, and only about 0.2 to 0.3 mm wide. They sit level with or slightly raised above the thallus.

Outline. Straight to strongly curved, and frequently branched, sometimes repeatedly, giving forked and star-like figures. They are usually scattered and randomly oriented, though on some trunks they align loosely with the grain of the bark.

The slit and the lips. Use a good hand lens. Each lirella is bordered by two black lips that press together over a narrow disc, so the fertile surface is largely hidden, showing only as a fine dark line. In this species the lips are smooth to only faintly and finely furrowed. They are not conspicuously ridged with multiple parallel grooves. Older lirellae open slightly, exposing a narrow, dark, sometimes thinly grey-pruinose disc.

Colour. Uniformly black or blackish-brown. There is no white or pale rim of thallus tissue raised around them in typical material, although a thin thalline covering may be present at the base.

What Replaces a Spore Print

A spore print is a technique for gilled fungi and has no meaningful equivalent here, since the spores are shot from asci embedded within these slits in quantities far too small to catch on paper. What you do instead is make a preparation.

The squash mount. With a razor blade, shave one lirella off the bark, taking a thin sliver rather than a chunk. Place it in a drop of water on a slide, cover it, and press gently with the flat of a pencil until it spreads. At 400x you should find cylindrical asci, each containing eight colourless spores lying in a row.

Spore characters. The spores are elongate-ellipsoid to fusiform, roughly 20 to 50 micrometres long by 6 to 11 wide, and divided by transverse cross walls into a row of chambers, commonly three to eleven of them. They are hyaline, meaning colourless, even at maturity. Crucially, they are not muriform: there are no vertical walls creating a brick-wall pattern, only a single file of compartments.

The iodine reaction. Run a drop of dilute iodine under the cover slip. The spores and ascus apex stain violet-blue, an amyloid reaction. This is one of the most useful confirmations available for this group and takes seconds.

Spot tests. Typical material gives negative reactions across the board: potassium hydroxide, bleach and para-phenylenediamine all produce no colour change on thallus or medulla, and the thallus is UV-negative. A strongly reactive thallus points elsewhere.

Substrate, Season and Condition

There is no fruiting season. This is a perennial organism that persists for years or decades on the same trunk, and it can be examined in any month. What changes is legibility. In dry weather the thallus pales, hardens, and can be difficult to distinguish from bleached bark. After rain it swells, greens, and the contrast with the black lirellae is at its sharpest, so a damp overcast day is the best time to work.

Substrate preference is narrow and useful. Smooth bark of broadleaved trees is the rule: beech above all, then hazel, holly, ash, rowan, hornbeam, birch, willow and young oak. It favours humid, sheltered, shaded woodland with stable conditions, is frequent in old woodland and along wooded watercourses, and thins out in dry, exposed or heavily disturbed sites. It occurs widely across Europe, temperate Asia and eastern North America.

Separating the Look-Alikes on Structure

Graphis elegans is the standard field confusion. Its lirellae are longer, more regular, and their lips are strongly and conspicuously striate, carrying several parallel raised ridges along each side so that under a lens each mark looks grooved or fluted rather than smooth. It also tends to sit on a thicker, more clearly delimited thallus. If the lips are visibly multi-ridged, move to that species.

Phaeographis species have similar elongated marks but the discs are usually broader, more open, often coated with a whitish or grey pruina, and their spores are brown at maturity rather than colourless. A squash mount separates them instantly.

Opegrapha species produce shorter, narrower, often more scattered and more strictly linear lirellae, frequently on a darker, browner or barely visible thallus. Their spores are smaller, and mature spores in many species become brownish, while their ascus structure differs. As a field rule, Opegrapha marks look more like short scratches and less like flowing script.

Arthonia species have irregular, blotchy, rounded to angular ascomata that are not truly slit-like, and they often lack any clear thalline outline.

Xylographa and similar wood-dwelling genera carry comparable elongate ascomata but grow on bare decorticated wood such as old fence rails and stumps rather than on living smooth bark.

Bark cracks and insect galleries deceive beginners surprisingly often. Test with a lens: a genuine lirella is a raised, glossy, carbon-black structure with defined lips sitting on a pale thallus, while a crack is a depression with ragged edges and no consistent width.