1. Stipular sheaths of submersed leaves free from leaf blade base, or if adnate, then adnate portion less than 1/2 length of stipule; leaves both submersed and floating or all submersed, the submersed pellucid, not channeled, flattened; peduncles stiff, if long enough, projecting inflorescence above surface of water.....................................................................1. Potamogeton

1. Stipular sheaths of submersed leaves adnate to leaf blade base for 2/3 or more of stipule length; leaves all submersed, opaque, channeled, turgid; peduncle flexible, not projecting inflorescence above surface of water....................... 2. Stuckenia