Editing Endosperm (section)

== Double fertilization ==
{{main|Double fertilization}}
An endosperm is formed after the two [[sperm]] nuclei inside a [[pollen]] grain reach the interior of a female gametophyte or megagametophyte, also called the [[embryonic sac]]. One sperm nucleus fertilizes the [[egg cell]], forming a [[zygote]], while the other sperm nucleus usually fuses with the binucleate central cell, forming a primary endosperm cell (its nucleus is often called the ''triple fusion nucleus''). That cell created in the process of [[double fertilisation|double fertilization]] develops into the endosperm.  Because it is formed by a separate fertilization event, the endosperm is a separate entity from the developing embryo, and some consider it to be a separate [[organism]]. 

About 70% of angiosperm species have endosperm cells that are [[polyploid]].<ref name=Olsen2007>{{cite book | first = Odd-Arne | last = Olsen | name-list-style = vanc | year = 2007 | title = Endosperm: Developmental and Molecular Biology | publisher = Springer | url=https://books.google.com/books?id=TFEA8Gg5_hAC| isbn = 9783540712350 }}</ref> These are typically [[triploid]] (containing three sets of [[Chromosome|chromosomes]]), but can vary widely from [[diploid]] (2n) to 15n.<ref name="Baroux_2002">{{cite journal | vauthors = Baroux C, Spillane C, Grossniklaus U | title = Evolutionary origins of the endosperm in flowering plants | journal = Genome Biology | volume = 3 | issue = 9 | pages = reviews1026 | date = August 2002 | pmid = 12225592 | pmc = 139410 | doi = 10.1186/gb-2002-3-9-reviews1026 | doi-access = free }}</ref>

One flowering plant, ''[[Nuphar polysepala]]'', has diploid endosperm, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei. The same is supposed for some other basal angiosperms.<ref>{{cite journal | vauthors = Williams JH, Friedman WE | title = Identification of diploid endosperm in an early angiosperm lineage | journal = Nature | volume = 415 | issue = 6871 | pages = 522–6 | date = January 2002 | pmid = 11823859 | doi = 10.1038/415522a | bibcode = 2002Natur.415..522W | s2cid = 4396197 }}</ref> It is believed that early in the development of angiosperm lineages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endosperms with a 2:1 maternal to paternal genome ratio.<ref name=Friedman2003>{{cite journal | vauthors = Friedman WE, Williams JH | title = Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants | journal = Evolution; International Journal of Organic Evolution | volume = 57 | issue = 2 | pages = 216–30 | date = February 2003 | pmid = 12683519 | doi = 10.1111/j.0014-3820.2003.tb00257.x | s2cid = 24303275 | doi-access = free }}</ref>

Double fertilisation is a characteristic feature of [[angiosperms]].