Alismatidae

(botany) A relatively primitive subclass of aquatic or semiaquatic herbaceous flowering plants in the class Liliopsida, generally having apocarpous flowers, and trinucleate pollen and lacking endosperm.

A relatively primitive subclass of the class Liliopsida (monocotyledons) of the division Magnoliophyta (Angiospermae), the flowering plants, consisting of 4 orders, 16 families, and less than 500 species. Typically they are aquatic or semiaquatic, with apocarpous flowers and nonendospermous seeds. They have trinucleate pollen, and the stomates usually have two subsidiary cells. The orders Alismatales, Hydrocharitales, and Najadales are closely related among themselves and have often been treated as a single order, Helobiae or Helobiales. The Triuridales differ from the other orders in being terrestrial and mycotrophic, without chlorophyll, and in having abundant endosperm in the seeds. See also Alismatales; Hydrocharitales; Triuridales; Magnoliophyta; Najadales; Plant kingdom.

The noun has one meaning:

Meaning #1: one of four subclasses or superorders of Monocotyledones; comprises about 500 species in 14 families of aquatic and semi-aquatic herbs
  Synonym: subclass Alismatidae

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