liliopsida n comprising seed plants that produce an embryo with a single cotyledon and parallel-veined leaves: includes grasses and lilies and palms and orchids; divided into four subclasses or superorders: Alismatidae; Arecidae; Commelinidae; and Liliidae Synonym(s) Monocotyledones class Monocotyledones Monocotyledonae class Monocotyledonae Liliopsida class Liliopsida