Cap surface

Fibrous

A fibrous surface (fibrillose) is a particular cap texture that is neither smooth nor scaly. It looks as if hundreds of radial threads (fibrils) had been pressed in under the mushroom's thin, translucent skin, running from the center (the umbo) to the edges of the cap. Sometimes these fibers are so pronounced that the cap looks "combed," and its edges can split along these threads.

24 species in the database.

The fibrous threat: fibrecaps (Inocybe)

In mycology, the term "fibrous" has a very alarming reputation. M.Ya. Zerova and David Arora emphasize that this feature gave its name to a whole genus of extremely dangerous mushrooms — the fibrecaps (Inocybe).

Most members of this genus (the deadly fibrecap, for example) have a markedly radially fibrous cap that is conical or bell-shaped.

Why are they dangerous? Fibrecaps contain the toxin muscarine (the same one found in the fly agaric, but in fibrecaps its concentration is often tens of times higher!). Picking and eating "little fibrous mushrooms" is a direct path to intensive care.

Other fibrous mushrooms

A fibrous cap structure isn't unique to fibrecaps:

1. Webcaps (Cortinarius): Many species in this huge (and often poisonous) genus have an innately fibrous cap.

2. Charbonnier (Tricholoma portentosum): An excellent edible mushroom. Its smooth gray cap has darker radial "streaks," or fibrils, showing through the skin.

3. Woolly milk cap (Lactarius torminosus): This mushroom's cap isn't just fibrous but densely fibrous and hairy, especially along the inrolled margin.

Questions and answers

How is a fibrous surface different from a scaly one?

Scales (as on the parasol mushroom) are bits of skin that have peeled away or curled upward (you can catch them with a fingernail). Fibrils are the internal structure of the skin itself. They're embedded in the surface and lie tight and flat. The mushroom looks as if its surface were made of silk threads.

How do you avoid confusing an edible Tricholoma with a poisonous fibrecap?

Fibrecaps (Inocybe) are usually much smaller, with a thin stem, a sharp umbo in the center of the cap, and gills that turn dirty brown with age (from the spores). The Charbonnier is a large, fleshy autumn mushroom with white gills and a thick stem, and its cap is much smoother, with the "streaks" only showing through beneath the cuticle.

Sources

1.Зерова М.Я. та ін. Визначник грибів України. 1979
2.Roger Phillips. Mushrooms (Roger Phillips)
3.David Arora. Mushrooms Demystified
4.Index Fungorum