| | 1. Leaf simple or few-forked, grass-like |
| A. septentrionale |
| | 1' Leaf 1-pinnate, 1° leaflets many |
| |
| | 2. Leaf axes greenish or straw-colored above red-brown base |
| A. trichomanes-ramosum |
| | 2' Leaf axes dark red- to purple-brown to purple-black ± throughout |
| |
| | 3. Petiole < 0.6 mm wide, narrowly winged; 1° leaflets 3–6(7) mm, generally shallowly crenate on upper, outer margins |
| A. trichomanes subsp. trichomanes |
| | 3' Petiole generally > 0.6 mm wide, unwinged; 1° leaflets generally 5–12 mm, margin shallowly lobed ± throughout |
| A. vespertinum
|