Hemithecium

Hemithecium is a genus of lichen-forming fungi in the family Graphidaceae. These lichens form pale grey-white to yellow-olive crusts on tree bark and are characterized by straight to weakly curved fruiting bodies with charcoal-black walls that create sharp contrasts against the surface, along with distinctive spine-tipped filaments lining their interior chambers. The genus has a pantropical distribution, growing on shaded tree trunks and large branches in primary or lightly disturbed evergreen forests, where their marked sensitivity to canopy opening makes them practical indicators of intact, moisture-rich woodland habitat.

Taxonomy

The genus was circumscribed by Vittore Benedetto Antonio Trevisan de Saint-Léon in 1853.

Description

Hemithecium forms a pale grey-white to yellow-olive crust (thallus) that lacks a true Template:Lichengloss and is often dotted with minute crystals. Its Template:Lichengloss are straight to weakly curved, frequently Template:Lichengloss, and bordered by thick, charcoal-black walls—the Template:Lichengloss is Template:Lichengloss throughout, causing the script-like slits to contrast sharply with the thallus. The clear hymenium is non-Template:Lichengloss and lined with short, spine-tipped Template:Lichengloss; these Template:Lichengloss filaments are a hallmark of the genus. Eight hyaline ascospores develop in each Graphis-type ascus; they become transversely septate to strongly Template:Lichengloss, measure roughly 30–100 × 10–25 μm, remain iodine-negative (I–), and are enveloped by a thin halo that persists in potassium hydroxide solution. Most species contain norstictic acid or stictic acid, occasionally together with trace depsidones of the protocetraric series.

The coexistence of fully carbonised lirellae, spinulose periphysoids, a non-inspersed hymenium and large, I– muriform spores sets Hemithecium apart from superficially similar script lichens. In Acanthothecis the margins remain pale; Fissurina lacks spinulose filaments; and Carbacanthographis shares the black excipulum but possesses minute, often iodine-positive spores. Within Hemithecium itself two informal species groups are recognised: a "large-spored" complex (e.g., H. radicicola) with spores exceeding 60 μm long, and a "medium-spored" set (e.g., H. flabillense) with spores 30–50 ΜM long, yet both uphold the diagnostic character combination described above.

Ecology

The genus is pantropical, extending from lowland Amazonian and West-African rainforests through Southeast Asian dipterocarp stands to montane cloud forests of New Guinea and north-eastern Australia. All known species are corticolous, occupying shaded trunks and large branches in primary or only lightly disturbed evergreen forest; their marked sensitivity to canopy opening means several taxa disappear rapidly after logging, making Hemithecium a practical indicator of intact, moisture-rich woodland habitat.

Species

As of June 2025, Species Fungorum (in the Catalogue of Life) accepts 22 species of Hemithecium.

• Hemithecium alboglaucum Template:Au
• Hemithecium amboliense Template:Au
• Hemithecium andamanicum Template:Au
• Hemithecium argopholis Template:Au
• Hemithecium canlaonense Template:Au
• Hemithecium duomurisporum Template:Au
• Hemithecium endofuscum Template:Au
• Hemithecium flavoalbum Template:Au
• Hemithecium flexile Template:Au
• Hemithecium fulvescens Template:Au
• Hemithecium himalayanum Template:Au
• Hemithecium kodayarense Template:Au
• Hemithecium lamii Template:Au
• Hemithecium nagalandicum Template:Au
• Hemithecium nakanishianum Template:Au
• Hemithecium oshioi Template:Au
• Hemithecium pulchellum Template:Au
• Hemithecium radicicola Template:Au
• Hemithecium rimulosum Template:Au
• Hemithecium scariosum Template:Au
• Hemithecium staigerae Template:Au
• Hemithecium verrucosum Template:Au