Chaga or King Alfred's cakes

Chaga has no cap. What you see on the birch is a growth 10–30 cm across, occasionally up to 40 cm, or sometimes a narrow strip up to a metre long running along a crack in the trunk. It's irregular and knobbly, bursts out through the bark and sits firmly on the tree with a broad base.

The surface is black, matt, dry and very hard, broken by deep cracks into small facets, like charcoal or burnt wood. The black colour comes from the pigment melanin. Around the edges, where the growth pulls away from the bark, the rusty brown interior often shows through.

The fruit body, called a stroma, is hemispherical or cushion-shaped, 2 to 8 cm across, and attached firmly to the wood by a broad base, with no stem. Several often fuse into a larger mass.

Young stromata are reddish brown to chestnut, at first dusted with pale asexual spores. Mature ones are black, matt at first and later almost varnished and slightly shiny. The surface is smooth or finely cracked into a network, and under a hand lens you can see tiny bumps where the spore chambers open.

The growth has no underside with pores or gills: it's a sterile mass of fungal threads and produces no spores.

The true fruiting body appears once the tree, or part of it, has died, often on the opposite side of the trunk. It spreads under the bark as a thin crust 0.5–1 m long, whitish at first and then brown, made of tubes. Eventually the bark splits open and exposes a brown layer of pores. It is rarely seen.

There are no gills or pores. The spores ripen in tiny chambers just below the surface and are shot out, leaving a black deposit on the wood up to about 3 cm around the ball.

Cut one open to see the key feature: alternating dark brown and silver-grey concentric bands beneath a thin black outer layer. The spore print is black.

There's no stem. The growth comes straight out of the trunk, usually at an old wound or broken branch, and is fused to the wood so firmly that only an axe will remove it.

There's no stem. The stroma sits directly on the wood on a broad base.

Inside, chaga is dense, corky and woody. The colour is rusty brown, turning yellowish orange towards the wood, often with pale streaks. This bright interior is what sets chaga apart from other black growths on birch. It doesn't change colour when broken.

Inside it's dense and fibrous to woody, with concentric bands. Old stromata become light, brittle and charcoal-like.

Chaga has no distinctive smell, and descriptions of the species don't mention one. Identify it by appearance instead: a black, cracked surface and a rusty yellow interior on a living birch.

The smell isn't distinctive and doesn't help with identification.