1. Pistils in a dense spherical head; uppermost flowers mostly staminate, with the stamens numerous; leaves unlobed or sagittate.

Sagittaria

1. Pistils in a single ring on a flattish receptacle or few in a tiny loose head; flowers all bisexual, with 6 (-9) stamens; leaves never lobed.

2. Plants nearly always more than 10 cm tall; achenes ca. 1.5-3 mm long, flattened.

Alisma

2. Plants less than 10 cm tall; achenes ca. 1-1.3 mm long, plump.

Echinodorus