1a. Carpels ≤ 28, in one ring-like whorl; receptacle flattened, disk-like; inflorescence paniculate with numerous rebranching branches... Alisma
1b. Carpels >28, usually very numerous; receptacle convex, dome-shaped; carpels in several spirally arranged whorls; inflorescence verticillate, usually branching or rebranching in whorls of three or more.... 2
2a. Achenes laterally flattened, winged and smooth, without ribs; bracts without bracteoles at the base of inflorescence branch whorls; roots septate... Sagittaria
2b. Achenes terete, ribbed but not winged; bracts and bracteoles at the base of inflorescence branch whorls; roots not septate... Echinodorus
Worldwide in distribution, Alismataceae has (potentially) three genera and 11 species in the flora. All species are rooted, herbaceous, wetland or aquatic plants with three unfused sepals and three unfused petals.