ALISMATIDAE

subclass of herbaceous monocotyledonous flowering plants found in aquatic or wet regions. There are three or four orders within the subclassAlismatales (or Alismales), Hydrocharitales, Najadales, and Triuridalescontaining 16 families and more than 400 species. The plants' flowers vary from large and showy to small and inconspicuous. The flowers may be regular or irregular and are arranged in inflorescences, generally as a raceme or spike. The perianth is organized in multiples of three sepals and three petals or is reduced to the point of absence in some members of the subclass. The leaves are simple and are arranged alternately along the stem, though they may be opposite or whorled in some species. The leaf blade is parallel-veined and often sheathed at the base, though the blade may be reduced or completely lost in some members. The orders Alismatales, Hydrocharitales, and Najadales are considered to be closely related, but the order Triuridales is unique and, in some classifications, of uncertain affiliation. The subclass as a whole is thought by some to be the most archaic member of the monocotyledonous angiosperms. subclass of flowering plants belonging to the class Lilopsida. Additional reading R.R. Haynes and L.B. Holm-Nielsen, A Generic Treatment of the Alismatidae in the Neotropics with Special Reference to Brazil, Acta Amazonica, 15(Supp. 12):153194 (1985), and Speciation of Alismatidae in the Neotropics, in L.B. Holm-Nielsen, I.C. Nielsen, and H. Balslev (eds.), Tropical Forests: Botanical Dynamics, Speciation, and Diversity (1989), pp. 211219; P.J.M. Maas and T. Rbsamen, Triuridaceae, in P.J.M. Maas et al., Saprophytes pro parte, pp. 155 (1986).