Editing Pollen (section)

=== Formation ===
Pollen is produced in the [[microsporangia]] in the male cone of a conifer or other [[gymnosperm]] or in the anthers of an [[Flowering plant|angiosperm]] [[flower]].

{{stack|float=left|[[File:Coenocytic Tetrad.gif|thumb|left|Pollen microspores of ''[[Lycopersicon esculentum]]'' at coenocytic tetrad stage of development observed through oil immersion microscope; the chromosomes of what will become four pollen grains can be seen.]]}}
In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, fertile sporogenous cells, the '''archespore''', form within the anther. The sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. The archespore cells divide by mitosis and differentiate to form '''pollen mother cells''' (microsporocyte, [[meiocyte]]).

In a process called '''microsporogenesis''', four haploid [[microspore]]s are produced from each diploid pollen mother cell, after [[meiotic division]]. After the formation of the four microspores, which are contained by [[callose]] walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.

Two basic types of microsporogenesis are recognised, simultaneous and successive. In simultaneous microsporogenesis meiotic steps I and II are completed before [[cytokinesis]], whereas in successive microsporogenesis cytokinesis follows. While there may be a continuum with intermediate forms, the type of microsporogenesis has systematic significance. The predominant form amongst the [[monocots]] is successive, but there are important exceptions.<ref name=Furness2001>{{cite journal |last1=Furness|first1=Carol A. |last2=Rudall|first2=Paula J.|author-link2=Paula Rudall|title=Pollen and anther characters in monocot systematics|journal=Grana|date=January 2001|volume=40|issue=1–2|pages=17–25|doi=10.1080/00173130152591840|doi-access=free|bibcode=2001Grana..40...17F }}</ref>

During microgametogenesis, the unicellular microspores undergo mitosis and develop into mature [[microgametophyte]]s containing the gametes.<ref>{{Cite web |url=http://www2.le.ac.uk/departments/biology/people/twell/lab/pollenis/development |title=Pollen Development — University of Leicester<!-- Bot generated title --> |access-date=2013-12-12 |archive-date=2014-10-06 |archive-url=https://web.archive.org/web/20141006193005/http://www2.le.ac.uk/departments/biology/people/twell/lab/pollenis/development/ |url-status=dead }}</ref> In some flowering plants,{{which|date=February 2017}} [[germination]] of the pollen grain may begin even before it leaves the microsporangium, with the generative cell forming the two sperm cells.