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(Alismidae) A subclass of the monocotyledons containing aquatic or semiaquatic herbaceous plants. General features of the group are: unfused carpels, in contrast to other monocotyledons in which the gynoecium is usually syncarpous; trinucleate pollen, as opposed to the binucleate pollen common to most other monocotyledons; absence of a starchy endosperm; and absence of vessels in the stem and often also in the roots. There are usually two subsidiary cells associated with each stoma. Four orders are commonly recognized: the Alismatales (or Alismales); the Hydro-charitales; the Najadales; and the Triuridales. The Triuridales is sometimes placed in the subclass Liliidae. The families of the Alismatidae are usually small, some containing only one genus (e.g. Najadaceae: Najus, naiads; Ruppiaceae: Ruppia, ditch grasses; Scheuchzeriaceae: Scheuchzeria) while the Butomaceae includes only one species, Butomus umbellatus (flowering rush). The most important family is the Potamogetonaceae (pondweeds).
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