Split Fibrecap
Inocybe rimosa
Also called straw-coloured fibrecap, conical fibrecap
A slender ochre-brown woodland fibrecap whose sharply umbonate cap splits radially into coarse straw-coloured fibres. One of the commonest and most recognisable members of a large, difficult genus.
Overview
Split Fibrecap is among the most frequently encountered of the fibrecaps, a huge group of small to medium brown-spored agarics characterised by silky or fibrous caps, dull clay to snuff-brown spore deposits, and a strong tendency to grow with trees. It has long been known as Inocybe rimosa, and older literature uses the name Inocybe fastigiata for the same fungus; modern classifications place it in the segregate genus Pseudosperma, along with other smooth-spored, non-bulbous relatives.
What makes the species distinctive in the field is the combination of a pale, warm straw to ochre-brown colour and the dramatic radial splitting of the cap surface. As the conical cap expands, the fibrous cuticle cannot stretch to keep pace and tears from the margin inwards, producing deep cracks that expose the paler flesh beneath and leave the cap looking like a torn parasol. A sharp central umbo almost always remains.
The genus as a whole is notoriously hard to work at species level, and many fibrecaps can only be separated with a microscope. Split Fibrecap is one of the few that a careful observer can name with reasonable confidence on field characters alone, which makes it a useful reference point for learning the group.
How to identify it
Cap
Usually 3–8 cm across, at first sharply conical or bell-shaped, expanding to broadly conical with a persistent pointed umbo. The surface is pale straw-yellow, ochre or warm tan, dry and covered in fine appressed radial fibrils. With expansion the margin splits deeply and repeatedly towards the centre, often becoming ragged and upturned; the paler flesh shows in the cracks.
Gills
Adnexed to almost free, moderately crowded, pale grey-buff or dirty cream when young and maturing through clay to dull olive-brown. The gill edges are finely fringed and stay noticeably paler than the faces, giving a whitish rim visible with a hand lens.
Stipe
3–9 cm tall and 4–10 mm thick, cylindrical and roughly equal, sometimes slightly thickened at the base but without an abruptly rimmed bulb. It is whitish to pale buff, longitudinally fibrillose and often browning where handled. There is no ring, though faint silky veil fibrils may be seen near the apex on very young fruitbodies.
Flesh and spore print
Flesh whitish to pale straw, unchanging or only slowly discolouring. The odour is faintly earthy to spermatic, a general trait of the group. The spore print is a dull clay to snuff-brown; under a microscope the spores are smooth, ellipsoid to bean-shaped, without nodules or angles.
Where and when it grows
Split Fibrecap grows from soil in broadleaf and mixed woodland, and is particularly frequent along woodland tracks, path edges, grassy rides and in short turf beneath scattered trees. It favours beech, oak, lime, hazel and birch but also appears in parkland and on well-drained calcareous ground, and occasionally in conifer plantations. Fruitbodies are typically scattered to gregarious rather than clustered.
The main season runs from early summer through to late autumn, with a peak in late summer. It is widespread and common across Europe and much of temperate North America and Asia, and is one of the first fibrecaps to appear each year.
Ecological role
Like nearly all fibrecaps this species is ectomycorrhizal, forming sheathing root partnerships with living trees. The fungal mycelium wraps the fine root tips of its host in a mantle of hyphae and grows between the outer root cells, trading soil-derived nitrogen, phosphorus and water for sugars made by the tree. Its broad host tolerance helps explain why it turns up in so many kinds of woodland.
Because it colonises disturbed, compacted ground readily, Split Fibrecap is often one of the pioneer mycorrhizal fungi along newly cut paths and around young planted trees. The fruitbodies are short-lived, decaying within days, and are heavily used by fungus gnats and other invertebrates whose larvae develop in the gills.
Similar species
Inocybe maculata, the Frosty Fibrecap, has a much darker chestnut to date-brown cap and characteristically retains whitish felted veil remnants over the umbo, giving a frosted look that Split Fibrecap lacks. Inocybe asterospora is darker and more rufous, and has a stipe that is powdery along its whole length and ends in an abruptly rimmed, marginate bulb rather than a simple tapering base.
Small Hebeloma species can look superficially similar in colour but have smooth, slightly greasy caps that do not split into radial fibres, and their gills are broadly sinuate with beaded edges. Species of Conocybe and Pholiotina are far more fragile, with rusty-cinnamon rather than clay-brown spore prints and hollow, brittle stipes.