Dryadoideae

The Dryadoideae subfamily of the Rosaceae consists of four genera, all of which contain representative species with root nodules 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 aggregate of achenes.
Taxonomic history.
The subfamily has at various times been separated as its own family (Dryadaceae), or as a tribe (Dryadeae) or subtribe (Dryadinae).