Editing Pollen (section)

==Structure and formation== <!-- Redirect from "microsporogenesis" goes to this section, so don't change title without changing the redirect! -->
Pollen itself is not the male gamete.<ref name="facts_and_practice_for_a_level">{{Cite book | last1 = Johnstone | first1 = Adam | title = Biology: facts & practice for A level | year = 2001 | publisher = Oxford University Press | isbn = 978-0-19-914766-3 | page = [https://archive.org/details/biologyfactsprac0000john/page/95 95] | url = https://archive.org/details/biologyfactsprac0000john/page/95}}</ref> It is a [[gametophyte]], something that could be considered an entire organism, which then produces the male gamete. Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell. In flowering plants the vegetative tube cell produces the [[pollen tube]], and the generative cell divides to form the two sperm nuclei.

Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see [[electron micrograph]], right). Pollen grains of [[pine]]s, [[fir]]s, and [[spruce]]s are winged. The smallest pollen grain, that of the [[forget-me-not]] (''Myosotis'' spp.),{{which|date=February 2017}} is 2.5–5&nbsp;[[micrometre|μm]] (0.005&nbsp;mm) in diameter.<ref name="sporomex">{{Cite web | url=http://www.sporomex.co.uk/technology/51-pollenspores |title = Spores and Pollens}}</ref> Corn pollen grains are large, about 90–100&nbsp;μm.<ref>{{Cite journal| pmc = 59743 | doi = 10.1073/pnas.211287498 | year = 2001 | title = Corn pollen deposition on milkweeds in and near cornfields | volume = 98 | issue = 21 | pages = 11919–24 | pmid = 11559840 | journal = Proceedings of the National Academy of Sciences of the United States of America | last1 = Pleasants| first1 = J. M.| last2 = Hellmich| first2 = R. L.| last3 = Dively| first3 = G. P.| last4 = Sears| first4 = M. K.| last5 = Stanley-Horn| first5 = D. E.| last6 = Mattila| first6 = H. R.| last7 = Foster| first7 = J. E.| last8 = Clark| first8 = P.| last9 = Jones| first9 = G. D. | bibcode = 2001PNAS...9811919P | doi-access = free}}</ref> Most grass pollen is around 20–25&nbsp;μm.<ref name="sporomex"/> Some pollen grains are based on [[geodesic polyhedra]] like a [[soccer ball]].<ref>{{cite journal |first1=Kleber |last1=Andrade|first2=Sara |last2=Guerra |first3=Alexis |last3=Debut |title=Fullerene-Based Symmetry in Hibiscus rosa-sinensis Pollen |journal=PLOS ONE |date=2014 |volume=9 |issue=7 |pages=e102123 |doi=10.1371/journal.pone.0102123 |pmid=25003375 |pmc=4086983 |bibcode=2014PLoSO...9j2123A |doi-access=free }} See also [https://www.newscientist.com/article/mg25934510-500-time-for-your-close-up-vivid-images-of-nature-loom-larger-than-life/ this picture] by Igor Siwanowicz of a [[morning glory]] pollen grain.</ref>

<gallery heights="160" widths="200" caption="Micrographs of different types of pollen">
File:Oenothera speciosa pollen 200x.jpg|Triporate pollen of ''[[Oenothera speciosa]]''
File:Lilium auratum - pollen.jpg|Pollen of ''[[Lilium auratum]]'' showing single sulcus (monosulcate)
File:Arabis voch1-4.jpg|''[[Arabis]]'' pollen has three colpi and prominent surface structure.
File:Жизнеспособность пыльцы 13.jpg|Apple pollen
File:Fire lily pollens on an insect's hair.jpg|Pollen of ''[[Lilium bulbiferum]]'' on an insect's hair
File:Gewoon sneeuwklokje.jpg|Pollen of ''Galanthus nivalis''
</gallery>

=== 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.

=== Structure ===
[[File:Tulip Stamen Tip.jpg|thumb|upright|Tulip [[anther]] with many grains of pollen]]
[[File:Cactus flower pollen.jpg|thumb|Closeup image of a cactus flower and its stamens]]
[[File:Jasper - Pollen storm - 20140624B.jpg|thumb|Pollen storm in [[Jasper National Park]]]]
Except in the case of some submerged aquatic plants, the mature pollen grain has a double wall. The vegetative and generative cells are surrounded by a thin delicate wall of unaltered [[cellulose]] called the '''endospore''' or '''intine''', and a tough resistant outer cuticularized wall composed largely of [[sporopollenin]] called the '''exospore''' or '''exine'''. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual.

The spines may be less than a micron in length (spinulus, plural spinuli) referred to as '''spinulose''' (scabrate), or longer than a micron (echina, echinae) referred to as '''echinate'''. Various terms also describe the sculpturing such as '''reticulate''', a net like appearance consisting of elements (murus, muri) separated from each other by a lumen (plural lumina). These reticulations may also be referred to as brochi.

The pollen wall protects the sperm while the pollen grain is moving from the anther to the stigma; it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall, which prevents the pollen grain from shrinking and crushing the genetic material during desiccation,{{citation needed|date=June 2022}} is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin.

Pollen apertures are regions of the pollen wall that may involve exine thinning or a significant reduction in exine thickness.<ref name="Furness-2004">{{Cite journal|last1=Furness|first1=Carol A.|last2=Rudall|first2=Paula J.|date=2004-03-01|title=Pollen aperture evolution--a crucial factor for eudicot success?|journal=Trends in Plant Science|volume=9|issue=3|pages=154–158|doi=10.1016/j.tplants.2004.01.001|pmid=15003239|citeseerx=10.1.1.462.5084}}</ref> They allow shrinking and swelling of the grain caused by changes in moisture content. The process of shrinking the grain is called harmomegathy.<ref>{{cite journal |last1=Katifori |first1=Eleni |last2=Alben |first2=Silas |last3=Cerda |first3=Enrique |last4=Nelson |first4=David R. |last5=Dumais |first5=Jacques |title=Foldable structures and the natural design of pollen grains |journal=Proceedings of the National Academy of Sciences |date=27 April 2010 |volume=107 |issue=17 |pages=7635–7639 |doi=10.1073/pnas.0911223107 |pmid=20404200 |pmc=2867878 |bibcode=2010PNAS..107.7635K |doi-access=free}}</ref> Elongated apertures or furrows in the pollen grain are called colpi (singular: colpus) or sulci (singular: [[Sulcus (morphology)|sulcus]]). Apertures that are more circular are called pores. Colpi, sulci and pores are major features in the identification of classes of pollen.<ref>{{cite web|url=http://www.geo.arizona.edu/palynology/ppapertr.html|author=Davis, Owen|title=Aperture|work=geo.arizona.edu|access-date=2009-02-16|archive-url=https://web.archive.org/web/20090203035127/http://www.geo.arizona.edu/palynology/ppapertr.html|archive-date=2009-02-03}}</ref> Pollen may be referred to as '''inaperturate''' (apertures absent) or '''aperturate''' (apertures present).

The aperture may have a lid ([[Operculum (botany)|operculum]]), hence is described as '''operculate'''.<ref>{{cite journal|last1=Furness|first1=Carol A.|last2=Rudall|first2=Paula J.|author-link2=Paula Rudall|title=Apertures with Lids: Distribution and Significance of Operculate Pollen in Monocotyledons|journal=International Journal of Plant Sciences|date=November 2003|volume=164|issue=6|pages=835–854|doi=10.1086/378656|bibcode=2003IJPlS.164..835F |s2cid=84766627}}</ref> However the term inaperturate covers a wide range of morphological types, such as functionally inaperturate (cryptoaperturate) and omniaperturate.<ref name=Furness2001/> Inaperaturate pollen grains often have thin walls, which facilitates [[pollen tube]] germination at any position.<ref name="Furness-2004" /> Terms such as '''uniaperturate''' and '''triaperturate''' refer to the number of apertures present (one and three respectively). Spiraperturate refers to one or more apertures being spirally shaped.

The orientation of furrows (relative to the original tetrad of microspores) classifies the pollen as '''sulcate''' or '''colpate'''. Sulcate pollen has a furrow across the middle of what was the outer face when the pollen grain was in its tetrad.<ref name=Spor72/> If the pollen has only a single sulcus, it is described as '''monosulcate''', has two sulci, as '''bisulcate''', or more, as '''polysulcate'''.<ref name=Simp11>{{cite book |chapter-url=https://books.google.com/books?id=dj8KRImgyf4C&pg=PA453 |first1=Michael G. |last1=Simpson |title=Plant Systematics |chapter=Palynology|pages=453–464|publisher=Academic Press |year=2011 |isbn=978-0-08-051404-8 |access-date=6 January 2014}}</ref><ref name=Singh1>{{cite book|chapter-url=https://books.google.com/books?id=In_Lv8iMt24C&pg=142|page=142|chapter=Palynology|access-date=23 January 2014|title=Plant Systematics: An Integrated Approach |isbn=9781578083510|last1=Singh|first1=Gurcharan|date=2004|publisher=Science Publishers }}</ref> Colpate pollen has furrows other than across the middle of the outer faces, and similarly may be described as '''polycolpate''' if more than two. '''Syncolpate''' pollen grains have two or more colpi that are fused at the ends.{{sfn|Kaltenrieder|von Ballmoos |2003}}<ref name=Spor72>{{Cite journal| author=Sporne, Kenneth R. | year=1972 | title=Some Observations on the Evolution of Pollen Types in Dicotyledons | journal=New Phytologist | volume= 71 | issue=1 | pages=181–185 | doi=10.1111/j.1469-8137.1972.tb04826.x| doi-access=free| bibcode=1972NewPh..71..181S }}</ref> [[Eudicots]] have pollen with three colpi ('''[[tricolpate]]''') or with shapes that are evolutionarily derived from tricolpate pollen.<ref>{{Cite journal|author1=Judd, Walter S. |author2=Olmstead, Richard G. |name-list-style=amp |year = 2004|title = A survey of tricolpate (eudicot) phylogenetic relationships|journal = American Journal of Botany| volume = 91|pages = 1627–1644|doi = 10.3732/ajb.91.10.1627|pmid=21652313|issue = 10|doi-access = free}}</ref> The evolutionary trend in plants has been from monosulcate to polycolpate or polyporate pollen.<ref name=Spor72/>

Additionally, [[gymnosperm]] pollen grains often have air bladders, or vesicles, called '''sacci.''' The sacci are not actually balloons, but are sponge-like, and increase the [[buoyancy]] of the pollen grain and help keep it aloft in the wind, as most gymnosperms are [[Anemophily|anemophilous]]. Pollen can be '''monosaccate''', (containing one saccus) or '''bisaccate''' (containing two sacci). Modern [[pine]], [[Picea|spruce]], and [[Podocarpus|yellowwood]] trees all produce saccate pollen.<ref>{{Cite book |title=Paleopalynology |author=Traverse, Alfred |date=1988 |publisher=Unwin Hyman |isbn=978-0045610013 |oclc=17674795}}</ref>