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Alnus (alder; family Betulaceae) A genus of deciduous trees which are common in wet places. The roots have nitrogen-fixing nodules. The leaves are simple; the flowers are unisexual and borne in catkins, the female flowers developing into a woody, persistent, cone-like structure. The trees are useful for timber and as ornamentals. There are 35 species, occurring in the north temperate zone, extending south to the Himalayas and Andes. See also FRANKIA.