Oleaceae, the Olive Family
Trees or shrubs, with opposite leaves and regular flowers; sepals 4, or calyx none; petals 4, united, or none; stamens usually 2; ovary 2-celled, superior.
| 1a. Leaves simple (shrub 2-5 m. high; flowers blue or white, in showy clusters in spring) | Lilac, Syringa vulgaris. |
| 1b. Leaves compound (trees; flowers greenish, inconspicuous, in spring) (Ash) — 2. | |
| 2a. Lateral leaflets sessile | Black Ash, Fraxinus nigra. |
| 2b. Lateral leaflets stalked — 3. | |
| 3a. Twigs sharply 4-angled | Blue Ash, Fraxinus quadrangulata. |
| 3b. Twigs not distinctly angled — 4. | |
| 4a. Leaves pubescent beneath | Red Ash, Fraxinus pennsylvanica. |
| 4b. Leaves glabrous beneath — 5. | |
| 5a. Leaves pale-green beneath, obscurely serrulate | White Ash, Fraxinus americana. |
| 5b. Leaves bright-green beneath, sharply serrulate | Green Ash, Fraxinus pennsylvanica var. lanceolata. |