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The oval central mass of tissue in the ovule; contains the embryo sac.



the central part, or nucleus, of the ovule in seed plants; it is homologous to the megasporangium of ferns (Pteridophyta). Inside the nucellus, the megaspore mother cell is differentiated. The nucleus of the megaspore mother cell divides, forming four nuclei, from which subsequently arise four megaspores. Three of the megaspores atrophy, and one develops into the female gametophyte (in gymnosperms, the primary endosperm; in angiosperms, the embryo sac). In some flowering plants the tetranuclear cell divides into two binuclear cells or does not divide at all; the gametophyte originates from one binuclear cell (the second one atrophies) or from the entire tetranuclear cell. The nucellus preserved in the seed is called the perisperm.

References in periodicals archive ?
This is due to the fact that the species investigated to date did not have pollen chambers formed front nucellar breakdown.
Anatomical observations on the nucellar apex of Welwitschia mirabilis and the chemical composition of the micropylar drop.
a nucellar beak, despite looking specifically for this structure.
spermatogenesis have not revealed a nucellar beak structure (Norstog,
The nucellar beak in Ginkgo apparently has a function relating to
first established by the nucellar beak in contact with the
Ginkgo is similar, but the former lack participation of a nucellar beak
tenuinucellate), but the nucellar epidermis later further divides to form a nucellar cap, so that the megagametophyte becomes deep-seated.
Following the earlier literature and current conventions, Dahlgren and Clifford (1982) identified two characters relating to the micropylar region of the nucellus in monocotyledons: (1) presence or absence of parietal cell (sometimes called "tapetal cell," or Deckzellen) and (2) presence or absence of a multiseriate nucellar cap derived from cell divisions in the nucellar epidermis.
In Convallaria and Canna the archesporial cell divides into a proximal megasporocyte and a distal parietal cell, which then repeatedly divides anticlinally to form a single layer of cells between the megasporocyte and the nucellar epidermis.
Table I Micropylar nucellar structures in some families of Liliales
In Poaceae the ovule is always either crassinucellate or tenuinucellate with a nucellar cap (Dahlgren & Clifford, 1982).