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Pteridium (bracken; family Dennstaedtiaceae) A monotypic genus (P. aquilinum) of ferns with deep, creeping rhizomes, and much dissected, pinnate fronds. The sori are continuous all round the edges of those pinnae that produce them, and are protected both by the recurved margin of the pinna, and by a delicate indusial flap on their inner side. It is one of the world's most common ferns, and a pest in many pasture lands in temperate climates, since it is not usually eaten by animals and is encouraged by burning. The genus is cosmopolitan, except for the Arctic and temperate S. America.