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Bracken (Pteridium aquilinum) is a conspicuous fern that forms large clonal colonies in a variety of habitats. The large, more or less triangular leaves develop from fiddleheads that develop widely spaced along the branches of an extensive subterranean rhizome that may reach nearly 400 m in length. The taxonomy of the genus remains controversial, but most botanists currently favor a classification involving five or more species. In this sense, Pteridium aquilinum is distributed widely in mostly the northern hemisphere, in both the New and Old Worlds.
Bracken produces a pharmacopeia of toxic compounds, including: thiaminase (which breaks down the amino acid thiamine and results in vitamin B deficiency), ecdysomes (hormones that stimulate uncontrolled early molting in insects), tannins (which bind to proteins and other compounds), and hydrogen cyanide, and also produces carcinogenic compounds. The combination of chemicals renders the plants toxic to most animals, both invertebrates and vertebrates, although some insect specialists ingest bracken tissue to become poisonous to their predators. Humans have long eaten the fiddleheads (emerging young leaves) of bracken, but over-ingestion of fresh or dried fronds has been linked to stomach and esophageal cancers.
Bracken is an aggressive colonizer of disturbed and other successional habitats and has been considered an invader of pastureland. It has been shown to be allelopathic (to produce compounds that inhibit the growth of other plant species) and can for dense monocultures. It is difficult to eradicate or control and, because it is toxic, renders such pasturage unfit for grazing. Pteridium aquilinum is considered a noxious weed, especially in portions of Great Britain and mainland Europe.