White Brain

White Brain (Exidia thuretiana) is a pale, watery jelly fungus that begins as small separate blisters on dead attached branches and gradually swells and coalesces into elongated, folded masses running along the underside of the wood. Fresh material is almost colourless and glassy, catching the light like a drop of clear gel.

Exidia is a genus of the Auriculariales, distinguished microscopically by basidia divided by lengthwise cross-walls into four cells. Unlike Tremella, most Exidia species are straightforward wood-rotters, feeding directly on the dead wood rather than parasitising another fungus, so they can appear on branches with no other visible fungal growth.

The species is named for the French phycologist Gustave Thuret. Its combination of pure white to faintly greyish colour, smooth surface without warts, and habit of forming long confluent bands separates it from the black and amber members of the genus that share the same habitat.

Fruiting body

Starts as small rounded cushions or blisters 5–15 mm across which expand and fuse, eventually forming irregular gelatinous bands up to 10 cm long and a couple of centimetres wide, with a folded, lobed or convoluted upper surface. The texture is soft, watery and quite fragile, collapsing readily if handled.

Surface

Smooth and glistening, without the tiny dark pimples that stud the surface of Exidia glandulosa. In dry weather the whole mass shrinks to a thin, whitish to greyish film on the bark that is easy to walk past unnoticed.

Colour

White, milky white or very pale translucent grey when fresh, sometimes taking on a faint cream, buff or pinkish tinge with age or where the tissue is thickest. Old and drying specimens may become dingy grey or yellowish.

Flesh and spore print

Flesh uniformly gelatinous and translucent, with no fibrous core, ring, volva or stipe, and no colour change on cutting. Spore print white; spores are shed from the whole exposed surface.

White Brain grows on dead but still attached branches of broadleaved trees, with beech the classic host and ash, hazel, oak and sycamore also frequent. It is most often found on the shaded underside of branches within fallen crowns or on brash piles, where humidity stays high.

The main fruiting season is autumn into winter, and fruiting bodies revive after each period of rain rather than appearing all at once. It is widespread and locally common across temperate Europe and reported more sparsely from other parts of the Northern Hemisphere.

Exidia thuretiana is a saprotroph that causes white rot in the dead wood of broadleaved trees, breaking down lignin as well as cellulose and leaving the residue pale and fibrous. It colonises branches that have died in the canopy and continues to fruit after they fall, making it part of the early to middle stages of branch-wood decomposition.

The gelatinous fruiting body is a water-management device. By absorbing rain and swelling, then drying to a thin film without dying, the fungus can extend spore release across many separate wet spells rather than committing to a single short fruiting. This strategy suits the exposed, intermittently wet microhabitat of an attached dead branch.

Myxarium nucleatum (Crystal Brain) is the closest match, also whitish and translucent, but it contains tiny opaque white grains embedded in the jelly that are visible when a lobe is held against the light; White Brain has clear, grain-free tissue. Young, very pale forms of Tremella species can look similar but usually develop yellow or brown tones as they mature.

Exidia glandulosa (Black Witch's Butter) shares the cushion-fusing habit but is olive-black with a warty, pimpled surface. Various white resupinate crust fungi coat branches in a comparable way, but they are thin, dry and firmly attached to the bark rather than swollen, watery and detachable.