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birch family of flowering plants, usually placed in the order Fagales; some authorities, however, have placed the family in the order Betulales. The family contains six genera and 120-150 species. It can be divided into two subfamilies: Betuloideae, with the genera Betula (birch) and Alnus (alder); and Coryloideae, with the genera Carpinus (hornbeam), Corylus (hazel, or filbert), Ostrya, and Ostryopsis.