The identity of the oldest lineage of monocotyledons is a subject of debate. Alternative interpretations of morphological homologies are variously consistent with proposals that species of Alismatanae, Dioscoreales, or Melanthiales were the earliest descendants of the first monocotyledons. We present phylogenetic analyses based on DNA sequences of the plastid locus rbcL in which Acorus calamus, an herb with unspecialized floral features and of uncertain affinities, is supported as a member of the oldest extant lineage of monocotyledons. This conclusion is consistent with a substantial body of morphological, anatomical, and embryological evidence and offers an explanation for the failure to identify any close relationship between Acorus and other genera.
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