Haloragidaceae, the Water Milfoil Family
Aquatic or marsh herbs, with alternate, opposite, or whorled leaves, and small, inconspicuous terminal or axillary flowers, frequently without petals (summer).
| 1a. Leaves none, or else very small and inconspicuous | Water Milfoil, Myriophyllum tenellum. |
| 1b. Leaves alternate (1-4 dm. high) | Mermaid Weed, Proserpinaca palustris. |
| 1c. Leaves opposite or whorled — 2. | |
| 2a. Leaves entire (2-4 dm. high) | Mare’s-tail, Hippuris vulgaris. |
| 2b. Leaves toothed or dissected (Water Milfoil) — 3. | |
| 3a. Flowers in the axils of foliage leaves — 4. | |
| 3b. Flowers in terminal spikes, subtended by bracts — 5. | |
| 4a. Flowers above water, subtended by toothed or entire leaves | Water Milfoil, Myriophyllum heterophyllum. |
| 4b. Flowers submerged, subtended by dissected leaves | Water Milfoil, Myriophyllum farwellii. |
| 5a. Flowers solitary or in pairs at each joint of the spike | Water Milfoil, Myriophyllum alternifolium. |
| 5b. Flowers several at each joint of the spike — 6. | |
| 6a. Bracts deeply pinnatifid | Water Milfoil, Myriophyllum verticillatum var. pectinatum. |
| 6b. Bracts entire or toothed | Water Milfoil, Myriophyllum spicatum. |