The subfamily Dryadoideae consists of four genera in the family Rosaceae,[.] all of which contain representative species with that host the nitrogen-fixing bacterium Frankia.[.] They are subshrubs, shrubs, or small trees with a base chromosome number of 9, whose fruits are either an achene or an compound fruit of achenes.[ It includes five genera ( Dryas, Cercocarpus, Chamaebatia, Cowania, and Purshia), all of which except the first only occur in North America.
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Taxonomic history
The subfamily has at various times been separated as its own family (Dryadaceae), or as a tribe (Dryadeae) or subtribe (Dryadinae).
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