Hooded Tube Lichen

Hooded Tube Lichen (Hypogymnia physodes) is an extremely common foliose lichen of the family Parmeliaceae, forming pale grey rosettes and sprawling patches 3-10 cm across on twigs, branches, trunks and timber. It is often the dominant lichen on birch and conifer branches across northern temperate and boreal regions.

Its defining structural feature is that the lobes are hollow. Rather than being solid sheets, each lobe is a closed inflated tube, so a lobe pressed between finger and thumb collapses and a cut lobe reveals an empty chamber with a dark inner wall. This single character separates Hypogymnia from almost every other grey foliose lichen.

The second signature feature gives the species its common name: the lobe tips turn upward and burst open at the end, and the ruptured underside curls back like a hood or a cowl, spilling out coarse grey-white soredia.

Thallus and lobes

The thallus is foliose, loosely attached, and forms irregular rosettes or sprawling mats 3-10 cm across. Lobes are 1-4 mm wide, strap-shaped, overlapping, and conspicuously swollen and inflated — hollow tubes rather than flat plates.

Lobe tips and soredia

The tips of many lobes turn upward and split open. The lower cortex peels back and recurves like a hood, and from inside this ruptured tip masses of coarse granular grey to greyish-white soredia are released. Finding these hooded, sorediate tips is diagnostic.

Colour and surfaces

The upper surface is pale grey to greenish-grey, smooth and often slightly shiny; the lower surface is black, wrinkled, and completely lacks rhizines — the thallus grips only at scattered points, so lobes lift freely from the bark. The hollow interior is dark-walled.

Fruiting bodies

Apothecia are uncommon: reddish-brown discs raised on short stalks. The species relies overwhelmingly on soredia for reproduction.

Hooded Tube Lichen grows on the bark and twigs of conifers — spruce, pine, larch and fir — and of birch, alder, oak and other broadleaves, and readily colonises fence rails, gate posts, dead wood and, occasionally, siliceous rock. It favours acidic bark and well-lit but not baking situations: conifer plantations, birch woodland, heathland scrub, moorland fences and forest edges.

The species is widespread and often abundant throughout the boreal and temperate northern hemisphere, including much of Europe, northern Asia and North America. It is relatively tolerant of general atmospheric conditions and is often one of the few large foliose lichens persisting in more built-up landscapes. Perennial and evergreen, it can be examined at any time of year.

Hypogymnia physodes is an epiphyte that gains nothing from the host plant beyond an anchoring point; the green algal partner photosynthesises within the thin upper layers while the fungus forms the hollow architecture, and mineral nutrition comes entirely from precipitation and airborne dust. The inflated hollow lobes are thought to help the thallus stay gas-exchange-efficient and dry out and rewet quickly.

Its abundance is driven by copious soredia production from the split lobe tips, which are readily washed and blown onto neighbouring twigs, so it colonises young branches and new timber rapidly. Because it absorbs directly from the air over a large surface, it is widely used in biomonitoring studies that measure accumulated airborne elements in lichen tissue.

Other Hypogymnia species share the hollow lobes and rhizine-free black underside; Hypogymnia tubulosa is separated by soredia forming in a rounded head at the very tip of the swollen lobe rather than bursting from a split, recurved hood, and by more cylindrical, less flattened lobes. Hypogymnia inactiva and relatives differ in lobe form and chemistry.

Parmelia sulcata is grey and similarly common but its lobes are solid, its upper surface bears a white reticulate ridge network, and its black underside is densely rhizinate. Platismatia glauca is larger, with wavy crisped margins, mottled undersides and solid lobes. Pseudevernia furfuracea is grey-white but fruticose, forming free tufts of flattened branches with black undersides rather than appressed rosettes.