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 [synonyms: Monocotyledones, class Monocotyledones, Monocotyledonae, class Monocotyledonae, Liliopsida, class Liliopsida].