THE NUCELLUS AND CHALAZA IN MONOCOTYLEDONS - STRUCTURE AND SYSTEMATICS

The majority of monocotyledons are crassinucellate, including some ear ly-branching taxa (sensu Chase et al., 1995a, 1995b) such as Tofieldia , although Araceae are predominantly tenuinucallate. The tenuinucellat e condition occurs in a taxonomically wide range of monocotyledons, an d there is some congruence between this character and existing monocot topologies at higher levels. For example, present evidence indicates a few tenuinucellate asparagoid clades, including Alliaceae sensu stri cto and Hypoxidaceae, possibly two tenuinucellate lilioid lineages, an d at least two tenuinucellate commelinoid lineages. Proximal nucellar structures arise from a multi-layered region of the ovule and include hypostase, enlarged dermal cells and conducting passage (Zuleitungsbah n), haustoria, postaments, podia, and perisperm. In some cases they ma y represent the same tissues at different developmental stages; in gen eral the last three are seed structures. For example, a postament may be a resistant conducting passage from which the surrounding dermal ce lls have degenerated, or alternatively a resistant hypostase, although both are nucellar in origin. Such terminological confusions cause pro blems in establishing homologies. Several characters relating to the n ucellus are outlined.