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.