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The Nucellus and Chalaza in Monocotyledons: Structure and Systematics

Paula J. Rudall
Botanical Review
Vol. 63, No. 2 (Apr. - Jun., 1997), pp. 140-181
Published by: Springer on behalf of New York Botanical Garden Press
Stable URL: http://www.jstor.org/stable/4354294
Page Count: 42
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The Nucellus and Chalaza in Monocotyledons: Structure and Systematics
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Abstract

The majority of monocotyledons are crassinucellate, including some early-branching taxa (sensu Chase et al., 1995a, 1995b) such as Tofieldia, although Araceae are predominantly tenuinucellate. The tenuinucellate condition occurs in a taxonomically wide range of monocotyledons, and 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 stricto and Hypoxidaceae, possibly two tenuinucellate lilioid lineages, and 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 (Zuleitungsbahn), haustoria, postaments, podia, and perisperm. In some cases they may represent the same tissues at different developmental stages; in general the last three are seed structures. For example, a postament may be a resistant conducting passage from which the surrounding dermal cells have degenerated, or alternatively a resistant hypostase, although both are nucellar in origin. Such terminological confusions cause problems in establishing homologies. Several characters relating to the nucellus are outlined.

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