Nucellus(redirected from nucellar)
Also found in: Dictionary, Thesaurus, Medical.
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.