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| 1 Scales of the pistillate catkins persistent; leafy involucre absent; fruit a small winged nut; [subfamily Betuloideae]. |
| ..2 Pistillate scales woody, forming a persistent conelike catkin; plant a shrub, < 4 m tall (except A. glutinosa) |
| ..2 Pistillate scales deciduous with or soon after the fruits; plant a tree, > 10 m tall at maturity |
| 1 Scales of the pistillate catkins caducous; leafy involucre present, conspicuous; fruit an unwinged nut; [subfamily Coryloideae]. |
| ....3 Nut spherical, 1-1.5 cm in diameter, closely enveloped by the involucre |
| ....3 Nut ovoid, 0.4-0.6 cm long, loosely or not at all enveloped by the involucre. |
| ......4 Infructescence bracts flat, 1-3 lobed, not enclosing the nut; bark gray, smooth; trunk moderately to strongly fluted; buds 4-angled |
| ......4 Infructescence bracts inflated, loosely enclosing the nut; bark brown, shreddy; trunk not fluted; buds not 4-angled |