Leptogium burgessii is a species of foliose lichen in the family Collemataceae. The species was originally described by Carl Linnaeus in 1774 and transferred to Leptogium by Camille Montagne in 1840. It forms leafy greyish to brownish patches distinguished by a velvety lower surface covered with short cream-coloured hairs composed of spherical cells. In Europe, L. burgessii has an oceanic Atlantic distribution, occurring from the Canary Islands through north-western Iberia to south-western Norway, where it grows on tree bark and mossy rocks in humid forests. The lichen requires high air humidity and is typically found in well-preserved old deciduous woodland, often growing over mosses on broad-leaved trees such as ash. Although the species has been reported from other continents including Africa, Asia, and South America, molecular studies indicate that many extra-European records may represent closely related but distinct species.
Taxonomy The taxonomy of Leptogium burgessii traces back to the Swedish taxonomist Carl Linnaeus, who first described the species as Lichen burgessii in the 13th edition of his 1774 work Systema Naturae. In his brief Latin diagnosis he characterised it as a foliose lichen with somewhat overlapping, crisp, translucent lobes and "shields" (apothecia) that are raised, roughened and crinkled, with flat, shining discs and a crisped margin. Linnaeus also cited "Burgess. mss.", indicating that he based the species on material or a manuscript name communicated by Rev. John Burgess, an 18th-century Scottish clergyman and lichen enthusiast who collected and described the species and sent it to Linnaeus. Later work on the Linnaean lichen herbarium has clarified how this name was established and typified. Per Magnus Jørgensen and co-authors showed that many lichen specimens and annotations there are by Linnaeus's son, Carl Linnaeus the Younger, and that all eight lichens sent by Burgess in 1771 are annotated by the son, including Lichen burgessii. In Systema Naturae Linnaeus provided a new Latin diagnostic phrase for the species, but based it directly on the phrase-name Burgess had used in a letter dated 20 May 1771. The corresponding specimen was located on sheet LINN 1273.91, originally labelled "crispus" and later altered to "burgessii"; on the back of the sheet is a phrase-name close to Burgess's original wording. Jørgensen and co-authors designated the left-hand specimen on this sheet as lectotype, thereby fixing the application of the name Lichen burgessii to the taxon now known as Leptogium burgessii (L.) Mont.; thin-layer chromatography of the type material showed no detectable lichen secondary metabolites. Subsequent lichenologists transferred the species through several genera before it reached its current placement. Erik Acharius moved Lichen burgessii to Parmelia in 1803, and later to Collema in 1810. Samuel Frederick Gray then treated it as Mallotium burgessii in 1821. The currently accepted combination dates from 1840, when Camille Montagne transferred Linnaeus's species to Leptogium as Leptogium burgessii in Webb and Berthelot's Histoire naturelle des Îles Canaries. In that treatment he provided a detailed Latin description under the new name, listed earlier uses of the epithet as Lichen burgessii in John Lightfoot's Flora Scotica and in English Botany, and cited its subsequent recombinations in Parmelia and Collema by Acharius, Sprengel, Hoffmann and Hooker. Montagne based his account on material collected by Jean Marie Despréaux on tree bark in the woods of Las Mercedes on Tenerife, and Leptogium burgessii is the name now generally accepted. A modern revision by Marcos Kitaura and Marcelo Marcelli places Leptogium burgessii in section Mallotium, a group of Leptogium species characterised by short hairs composed of spherical cells on one or both thallus surfaces. In that study they re-examined type material of L. burgessii and related taxa, and followed earlier authors in treating the Mexican species Leptogium inflexum as a synonym of L. burgessii, but did so "reluctantly", noting anatomical differences and the possibility that L. burgessii may be confined to Europe while L. inflexum occurs in the Americas. The same study rejected the earlier view that the African species Leptogium ethiopicum is the same species as L. burgessii, pointing to differences in lobe margins and apothecial anatomy.
Description Leptogium burgessii forms relatively large, leafy thalli usually up to about 7–10 cm across, arranged as rounded tufts or rosettes that are often slightly raised in the centre. The thallus is divided into rounded, wavy lobes mostly 3–5 mm wide that overlap irregularly and are attached to the substrate at scattered points. When dry, the surface is bluish grey to dark blackish brown or blackish olive-green, with a dull, matt appearance; under low magnification it looks smooth to only slightly wrinkled. The lobe tips are rounded and slightly rolled inwards, and the margins are gently undulating. Numerous small secondary lobes (folioles or lacinules) with rounded tips arise both from the lobe edges and around the rims of the fruiting bodies, giving the margins a distinctly frilled outline. The thallus lacks isidia. The lower surface is paler, grey to yellowish, and has a velvety texture due to a dense covering of short, cream-coloured hairs. These hairs, which are made up of only a few spherical cells, are abundant on the underside and may also occur near the apothecia on the upper surface; they anchor the lichen to the bark and are characteristic of the group of Leptogium species placed in section Mallotium. Fruiting bodies (apothecia) are fairly common. They are laminal and slightly stalked, usually 1–4 mm in diameter, with a cup-shaped, red-brown to blackish-brown disc surrounded by a thick thalline margin. The margin and the outer wall of the apothecium (the amphithecium) bear numerous small lobules, which remain free and do not form a complete collar around the disc. In vertical section the apothecia have a tall spore-bearing layer and a conspicuously thick outer zone composed of many layers of small, brick-like fungal cells (paraplectenchymatous tissue). The ascospores are fusiform, 30–40 × 12–18 μm, with 5–7 transverse and several longitudinal septa; they are divided internally into a muriform, brick-like pattern.
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