**Chaga** refers to the sterile stage of the Clinker polypore (*Inonotus obliquus*) — large growths formed by this fungus on the trunks of living birch trees. These growths consist of a mass of fungal mycelium and host tree tissues that have proliferated under the influence of hormones and active compounds secreted by the fungus. These growths are not fruiting bodies in the strict sense, i.e., they are not specialised organs of sexual reproduction and do not produce sexual spores. In taxonomic nomenclature, the asexual stage is designated *Inonotus obliquus* f. *sterilis*, which can be somewhat misleading: the surface of these growths actively produces asexual spores — chlamydospores — which give chaga its characteristic black or dark brown colour.
The growths can reach up to 50 cm in diameter and protrude 50–100 mm from the trunk. Beneath the hard, cracked blackish-brown outer layer lies a softer, looser mass of cork-like consistency, ochre in colour with yellowish veins. Chaga can persist for many years; growths aged twenty years or more have been recorded.
The sexual stage of the fungus typically develops when the host tree begins to die. Beneath the bark of the infected tree, a new structure appears as a thin dark-brown layer of tubular hymenophore, 3–15 mm thick, often extending along the trunk for considerable distances — up to 1–2 (4) m. The tubes are single-layered, up to 10 mm long, and oriented at an angle of 20–30° to the trunk, which gave the species its scientific name (*obliquus*). Pores are 0.2–0.3 mm in diameter: rounded to oblong in young specimens (3–5 per mm), becoming angular and toothed with age. When fresh, the tissue is leathery-fleshy; when dry, it becomes hard and brittle.
The sexual stage of chaga is a rare find — one that not every mycologist can claim to have encountered — primarily because it is often completely hidden beneath the bark. Most often, overmature or dried specimens are discovered after the bark has already fallen away. The process by which the fungus emerges from beneath the bark is quite interesting: so-called "lever" plates grow along the edge of the hymenophore; as they expand, they lift the bark away from the wood, thereby creating space for effective sporulation.
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