Field guide ยท Entry 01 of 06 Psilocybe semilanceata
How the liberty cap is described, and why the description is not enough
The cap, gills, stem and spore print of Psilocybe semilanceata as mycologists describe them, and why these features cannot rule out toxic species.
Mycologists describe the liberty cap by a small set of features: a conical cap with a distinct pointed tip, a long thin pale stem without a ring, gills that darken to purple-brown, and a spore print of the same dark colour. Each of these is shared with other species, and some of those species are poisonous. This entry sets out the description and then explains where it stops being reliable.
The cap
The cap is 5 to 25 mm across and 6 to 22 mm tall, shaped like a narrow bell or a cone, and topped by a small nipple-like point that mycologists call a papilla. The cap is hygrophanous: it changes colour as it dries. Wet caps range from ochre to chestnut brown, often with fine radial grooves near the edge; dry caps fade to a pale yellowish brown. In damp weather the surface is sticky because of a thin gelatinous skin, the pellicle (species description after Bas et al. 1995).
The British mycology site First Nature gives a similar range of 0.5 to 2 cm across (First Nature). Young and old caps look different, and weather changes the colour within hours, so a single photograph says little.
Gills and spore print
Under the cap sit 15 to 27 narrow gills. They start pale brown and darken to grey and then purple-brown as the spores ripen. A spore print, made by leaving a cap on paper so that the spores fall, is very dark purple-brown. Under the microscope the spores measure about 10.5 to 15 by 6.5 to 8.5 micrometres.
The spore print colour is one of the more useful features, because it separates the liberty cap from mushrooms with rust-brown or snuff-brown prints, such as Galerina and Pholiotina. It does not separate it from other dark-spored species, and a print from a damaged or immature cap can mislead.
Stem
The stem is 4.5 to 14 cm long and only 1 to 3.5 mm thick, pale and often wavy. There is no ring. Many of the dangerous lookalikes have a ring or a fragile ring zone, but rings can wash off or fall away with age, so the absence of a ring proves nothing on its own.
Bruising
All parts of the mushroom can turn bluish where handled or bruised, and older specimens may turn blue on their own. Work on the related Psilocybe cubensis showed that the blue colour forms when enzymes released by the injury turn psilocybin into psilocin and then link psilocin molecules into blue chains (Lenz et al. 2020). Bruising is not a safety test: blue staining does not show that a mushroom lacks other toxins, and a handful of small mushrooms can easily hold more than one species.
Where the description runs out
| Feature | What it tells a mycologist | What it cannot rule out |
|---|---|---|
| Pointed cap | Fits the species | Conical caps in Inocybe, Conocybe and Panaeolus |
| No ring | Fits the species | Toxic species whose ring has worn off |
| Dark purple-brown spores | Excludes rust- and snuff-brown spored genera | Other dark-spored grassland species |
| Grassland habitat | Fits the species | Toxic species that also grow in grass or nearby wood chips |
| Blue bruising | Suggests psilocin is present | A mixed collection with a toxic species in it |
Its closest relative, Psilocybe pelliculosa, is described as almost impossible to tell apart in the field; it differs mainly in having smaller spores, about 9 to 13 by 5 to 7 micrometres. Forensic scientists in Poland noted that once the mushroom is no longer fresh, identifying it by eye is "almost impossible", which is why they developed a DNA test (Adamczyk et al. 2007).
A list of features on a website is a description, never a safety check. The species that cause liver and kidney failure are small, brown and common in the same months. Only an expert examination, often with a microscope, settles an identification, and in the UK and the Netherlands picking the liberty cap to keep it is illegal in any case.
What to read next
The lookalikes entry compares the species that are most often confused with the liberty cap and the cases where that went wrong. The habitat entry explains why grassland mushrooms are so mixed, and the law entry covers what picking means under UK and Dutch law.