Alismaceae, the Water Plantain Family
Marsh plants, with scape-like stems; flowers with 3 green sepals, 3 white petals, 6 or more stamens, and several separate pistils.
| 1a. Ovaries in a ring; flowers in panicles (2-8 dm. high, summer) | Water Plantain, Alisma Plantago-aquatica. |
| 1b. Ovaries in a head; flowers in racemes or umbels — 2. | |
| 2a. Flowers all perfect, in a single umbel of 2-8 flowers; stamens 9 (leaves lanceolate; 15 cm. high or less; summer) | Dwarf Water Plantain, Echinodorus tenellus. |
| 2b. Flowers in a raceme of 3-flowered whorls, the lower pistillate, the upper staminate; stamens usually more than nine (1-10 dm. high, summer) (Arrow-head) — 3. | |
| 3a. Leaves ovate to linear, not sagittate at base — 4. | |
| 3b. Leaves broad or narrow, sagittate at base — 5. | |
| 4a. Pistillate (basal) flowers sessile or nearly so (2-8 dm. high, summer) | Arrow-head, Sagittaria heterophylla. |
| 4b. Pistillate flowers with obvious pedicels | Arrow-head, Sagittaria graminea. |
| 5a. Basal lobes of the leaf conspicuous, triangular, almost or quite as long as the terminal portion — 6. | |
| 5b. Basal lobes small, short, linear — 4b. | |
| 6a. Beak of the achene very short and erect; rare species | Arrow-head, Sagittaria arifolia. |
| 6b. Beak of the achene sharp, incurved at right angles to the body; common species | Arrow-head, Sagittaria latifolia. |