Common Inkcap
Coprinopsis atramentaria
Also called inky cap, common ink cap
A clustered grey-brown inkcap whose egg-shaped cap opens into a ribbed bell and then digests itself from the rim inward, dripping away as black ink. It rises in dense tufts from buried wood in gardens, verges and woodland edges.
Overview
The Common Inkcap is one of the most widespread of the deliquescent, or self-digesting, gilled fungi. It belongs to Coprinopsis, a genus in the family Psathyrellaceae that was carved out of the old catch-all genus Coprinus once molecular work showed the inkcaps were not a single natural group. Only the Shaggy Mane and its close relatives remain in Coprinus proper; the great majority of grey and brown inkcaps, this species included, now sit in Coprinopsis, Coprinellus or Parasola.
What sets the inkcaps apart is a radical solution to a mechanical problem. Most gilled fungi need space between their gills so that released spores can fall clear. Inkcaps instead pack their gills together like the pages of a book, then dissolve them. Enzymes break down the gill tissue from the bottom edge upward, so the cap liquefies progressively from the margin toward the centre while the spore-bearing surface above stays intact and continues to mature. The result is the familiar black drip, and a fruiting body that can go from a firm pale egg to a puddle of ink in little more than a day.
In the field the species reads as a mid-sized, sober-coloured inkcap: grey to grey-brown, faintly scaly at the crown, ribbed along the sides, on a tall smooth white hollow stem, and almost always growing in a crowded cluster rather than singly. It is common across Europe, temperate Asia and North America, and follows human disturbance closely, so it turns up on lawns, along path edges, in flower beds and around long-removed stumps.
How to identify it
Cap
3-7 cm tall and about as wide when open, at first a pale grey-brown egg or elongated ovoid, then expanding to a bell shape and finally flaring and splitting at the rim. The surface is grey-brown to clay-brown, usually a little warmer and darker over the crown, where a scattering of small brownish scurfy scales persists. Radial grooves run from the margin toward the centre, deepening as the cap expands, and the rim frequently tears into ragged lobes before it liquefies.
Gills
Free or only narrowly attached to the stem, extremely crowded and thin, white at first, then passing quickly through grey to inky black. Deliquescence begins at the cap margin and works inward, so a single fruiting body will often show white gills near the stem, grey gills mid-way and a black dripping edge at the same moment.
Stipe
7-17 cm tall and 0.8-1.5 cm thick, white, smooth to very finely silky, hollow, straight and slightly tapering upward. There is no true ring, but many specimens show a faint ridge or narrow rim of veil tissue low on the stem, sometimes described as a rudimentary basal collar. Several stems commonly fuse at the base where they emerge from buried wood.
Flesh & spore print
Flesh is thin, whitish and watery, unchanging where cut in young specimens. The spore print is black, and mature spores are smooth and dark with a central germ pore, easily seen as a black smear on anything the cap touches.
Where and when it grows
This is a fungus of buried and decaying hardwood rather than open soil, though the wood is often invisible. Dense tufts push up from stumps, from the ground above old root systems, from timber buried during landscaping, and from the compacted verges of paths and driveways. Gardens, parks, hedge bottoms, roadsides and woodland edges all suit it, and it tolerates heavily disturbed and compacted ground better than most woodland fungi.
Fruiting is unusually long-seasoned for a gilled fungus, running from spring through to late autumn wherever rain follows warmth, with the heaviest flushes in early summer and again in autumn. It is widespread throughout Europe, across temperate Asia, and throughout much of North America, and has travelled with horticulture to temperate parts of the Southern Hemisphere.
Ecological role
The Common Inkcap is a saprotroph, drawing its nutrition from dead wood and woody debris in and just below the soil surface. Its mycelium colonises buried hardwood roots and stumps, breaking down cellulose and lignin and returning the carbon locked in root systems to the soil long after the visible tree has gone. Because so much of its substrate is underground, fruiting clusters can appear in the middle of a lawn with no wood in sight.
The deliquescent life cycle is the species' defining ecological trick. Rather than shedding spores gradually from widely spaced gills over many days, it builds a tightly packed spore factory, then dismantles it from the edge inward over a matter of hours so that each layer of gill in turn is exposed to open air as it ripens. The resulting ink carries spores away on rain splash and on the bodies of the flies and beetles that gather at the dissolving margin, giving the fungus a dispersal route that does not depend on wind alone.
Similar species
Coprinus comatus, the Shaggy Mane, is far taller and narrower, with a white cylindrical cap covered in shaggy upturned scales and a loose ring that slides freely on the stem; it also dissolves, but from a white rather than grey starting colour. Coprinellus micaceus is smaller and warmer in colour, tawny to date-brown, with the young cap dusted in glistening mica-like granules, and it fruits in even denser swarms directly on visible stumps.
Coprinopsis lagopus is more delicate and shorter-lived, its young cap covered in a coat of soft white to grey hairs that gives it a rabbit's-foot look before the cap flares and rolls back. Coprinellus disseminatus is tiny by comparison, only 1-2 cm tall, in swarms of hundreds, and its pleated caps fade and collapse rather than producing much ink. Among these, the Common Inkcap is picked out by its combination of medium size, grey-brown scurfy crown, tall smooth hollow white stem and tufted habit on buried wood.