Editing Ovule (section)

=== Nucellus, megaspore and perisperm ===
The ''nucellus'' (plural: nucelli) is part of the inner structure of the ovule, forming a layer of diploid ([[sporophyte|sporophytic]]) cells immediately inside the integuments. It is structurally and functionally equivalent to the [[sporangium|megasporangium]]. In immature ovules, the nucellus contains a megasporocyte (megaspore mother cell), which undergoes [[sporogenesis]] via [[meiosis]]. In the [[megaspore mother cell|megasporocyte]] of ''Arabidopsis thaliana'', [[meiosis]] depends on the expression of [[gene]]s that facilitate [[DNA repair]] and [[homologous recombination]].<ref name="Seeliger-2012">{{cite journal |vauthors=Seeliger K, Dukowic-Schulze S, Wurz-Wildersinn R, Pacher M, Puchta H |title=BRCA2 is a mediator of RAD51- and DMC1-facilitated homologous recombination in Arabidopsis thaliana |journal=New Phytol. |volume=193 |issue=2 |pages=364–75 |year=2012 |pmid=22077663 |doi=10.1111/j.1469-8137.2011.03947.x |doi-access=free |bibcode=2012NewPh.193..364S }}</ref> 

In gymnosperms, three of the four [[haploid]] spores produced in meiosis typically degenerate, leaving one surviving megaspore inside the nucellus. Among angiosperms, however, a wide range of variation exists in what happens next. The number (and position) of surviving megaspores, the total number of cell divisions, whether nuclear fusions occur, and the final number, position and ploidy of the cells or nuclei all vary. A common pattern of embryo sac development (the ''Polygonum'' type maturation pattern) includes a single functional megaspore followed by three rounds of mitosis. In some cases, however, two megaspores survive (for example, in ''Allium'' and ''Endymion''). In some cases all four megaspores survive, for example in the ''Fritillaria'' type of development (illustrated by ''Lilium'' in the figure) there is no separation of the megaspores following meiosis, then the nuclei fuse to form a triploid nucleus and a haploid nucleus. The subsequent arrangement of cells is similar to the ''Polygonum'' pattern, but the ploidy of the nuclei is different.<ref>{{citation |author1=Gifford, E.M. |author2=Foster, A.S. |year=1989  |title=Morphology and evolution of vascular plants |journal=Taxon |volume=38 |issue=4 |page=613 |publisher=W. H. Freeman and Company |location=New York  |doi=10.2307/1222641 |jstor=1222641 |bibcode=1989Taxon..38Q.613S }}</ref>

After fertilization, the nucellus may develop into the ''perisperm'' that feeds the embryo. In some plants, the diploid tissue of the nucellus can give rise to the embryo within the seed through a mechanism of [[asexual reproduction]] called [[nucellar embryony]].