SYSTEMATICS OF ACORUS - OVULE AND ANTHER

This article identifies and discusses the critical characters in the s ystematics of Acorus, a significant taxon among ''basal'' angiosperms. New data are presented an tapetum development and ovule structure, an d old characters are revisited to establish their homologies. Many of the character states found in Acorus (such as trimerous flowers, succe ssive microsporogenesis, orthotropous ovules, secretory anther tapetum . ethereal oil cells, and lack of raphides) occur both in other monoco tyledons and ''primitive'' dicoryledons. Several characters indicate c lose monocot affinities but provide little further evidence of relatio nships: P2-subtype sieve-element plastids (Behnke 1995), seedling stru cture (Tillich 1995), and ensiform leaves. Two characters that potenti ally 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 tax on to all other monocotyledons.