Systematics of Acorus: ovule and anther

This article identifies and discusses the critical characters in the systematics of Acorus, a significant taxon among "basal" angiosperms. New data are presented on tapetum development and ovule structure, and old characters are revisited to establish their homologies. Many of the character states found in Acorus (such as trimerous flowers, successive microsporogenesis, orthotropous ovules, secretory anther tapetum, ethereal oil cells, and lack of raphides) occur both in other monocotyledons and "primitive" dicotyledons. Several characters indicate close monocot affinities but provide little further evidence of relationships: P2-subtype sieve-element plastids (Behnke 1995), seedling structure (Tillich 1995), and ensiform leaves. Two characters that potentially link Acorus with palaeoherb outgroups, anther wall formation and cellular endosperm, both require further review. Apart from the strong signal from molecular data, there is still virtually no nonmolecular evidence to confirm a taxonomic placement for Acorus as the sister taxon to all other monocotyledons.