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Cyatheaceae A family of ferns, most of which are massive tree-ferns, with gradate sori arising at first marginally, and then deflecting to the under-side of the fronds, where they are protected by a pair of indusial flaps. There are two genera and 265 species, Cyathea and Dicksonia occurring mainly in the tropics and moist, warm, temperate southern hemisphere. See also THYRSOPTERIDACEAE.