Psilocybe semilanceata, commonly known as the liberty cap, is a psychedelic (or "magic") mushroom that contains the psychoactive compounds psilocybin and baeocystin. Of the world's psychoactive mushrooms, it is the most common in nature, and one of the most potent. The mushrooms have a distinctive conical to bell-shaped cap, up to 2.5 cm (1.0 in) in diameter, with a small nipple-like protrusion on the top. They are yellow to brown in color, covered with radial grooves when moist, and fade to a lighter color as they mature. Their stems tend to be slender and long, and the same color or slightly lighter than the cap. The gill attachment to the stem is adnexed (narrowly attached), and they are initially cream-colored before tinting purple as the spores mature. The spores are dark purplish-brown in mass, ellipsoid in shape, and measure 10.5–15 by 6.5–8.5 micrometers.
The mushroom grows in fields, grassy meadows, and similar habitats, particularly in wet, north-facing fields that are well-fertilized by sheep and cattle feces. But unlike P. cubensis and P. coprophila, the fungus does not grow directly on dung; rather, it is a saprobic species that feeds off decaying grass roots. It is widely distributed in the cool temperate and subarctic regions of the Northern Hemisphere, particularly in Europe. However, it has also been reported occasionally from warmer locations such as India, South America, and Australasia. The earliest reliable history of P. semilanceata intoxication dates back to 1799 in London, and in the 1960s the mushroom was the first European species confirmed to contain psilocybin. Further investigations into the chemical makeup of the fungus revealed the presence of the psychotropic substances baeocystin and phenylethylamine. The mushroom has also been shown to inhibit an antibiotic-resistant form of the human pathogen Staphylococcus aureus, and it secretes antifungal compounds that help it compete for nutrients with soil microorganisms.
Psilocybe semilanceata is a saprobic fungus, meaning it obtains nutrients by breaking down organic matter. The mushroom grows solitarily or in groups on the ground, typically in fields and pastures. It is often found in fields that have been fertilized with sheep or cow dung, although it does not typically grow directly on the dung. The mushroom is also associated with sedges in moist areas of fields, and it is thought to live on the decaying root remains. Like some other grassland species such as P. mexicana, P. tampanensis and Conocybe cyanopus, P. semilanceata may form sclerotia, a dormant form of the fungus, which affords it some protection from wildfires and other natural disasters.
The first reliably documented report of Psilocybe semilanceata intoxication involved a British family in 1799, who prepared a meal with mushrooms they had picked in London's Green Park. According to the chemist Augustus Everard Brande, the father and his four children experienced typical symptoms associated with ingestion, including pupil dilation, spontaneous laughter and delirium. The identification of the species responsible was made possible by James Sowerby's 1803 book Coloured Figures of English Fungi or Mushrooms,which included a description of the fungus, then known as Agaricus glutinosus (originally described by Moses Ashley Curtis in 1780). According to German mycologist Jochen Gartz, the description of the species is "fully compatible with current knowledge about Psilocybe semilanceata."
Czechoslovakia (1973),Germany (1977),Norway (1978), and Belgium and Finland (1984). In 1965, forensic characterization of psilocybin-containing mushrooms seized from college students in British Columbia identified P. semilanceata—the first recorded case of intentional recreational use of the mushroom in Canada. The presence of the psilocybin analog baeocystin was confirmed in 1977. Several studies published since then support the idea that the variability of psilocybin content in P. semilanceata is low, regardless of country of origin.