Editing Common sunflower (section)

== Description ==

The plant has an erect rough-hairy stem, reaching typical heights of {{convert|3|m|ft|abbr=off|frac=2}}. The tallest sunflower on record achieved {{convert|9.17|m|ftin|abbr=on}}.<ref name="Guiness">{{cite web |title=Tallest Sunflower |url=https://www.guinnessworldrecords.com/world-records/tallest-sunflower |website=Guinness World Records |access-date=4 May 2014 |archive-date=7 June 2023<!--last good--> |url-status=dead |archive-url=https://web.archive.org/web/20230607124159/https://www.guinnessworldrecords.com/world-records/tallest-sunflower }}</ref> Sunflower leaves are broad, coarsely toothed, rough and mostly alternate; those near the bottom are largest and commonly [[heart-shaped]].<ref name="Spellenberg-2001">{{Cite book |last=Spellenberg |first=Richard |url=https://archive.org/details/nationalaudubons00spel/page/378/ |title=National Audubon Society Field Guide to North American Wildflowers: Western Region |publisher=Knopf |year=2001 |isbn=978-0-375-40233-3 |edition=rev |pages=378–379 |orig-date=1979}}</ref>

=== Flower ===
The plant flowers in summer. What is often called the "[[flower]]" of the sunflower is actually a "flower head" ([[pseudanthium]]), {{Convert|7.5-12.5|cm|frac=2}} wide,<ref name="Spellenberg-2001" /> of numerous small individual five-petaled flowers ("[[floret]]s"). The outer flowers, which resemble petals, are called [[ray flower]]s. Each "petal" consists of a [[ligule]] composed of fused petals of an asymmetrical ray flower. They are sexually sterile and may be yellow, red, orange, or other colors. The spirally arranged flowers in the center of the head are called [[disk flower]]s. These mature into fruit (sunflower "seeds").

The [[prairie sunflower]] (''H. petiolaris'') is similar in appearance to the wild common sunflower; the scales in its central disk are tipped by white hairs.<ref name="Spellenberg-2001" />{{clear|left}}

==== Heliotropism ====
[[File:SunflowerSunset2.jpg|thumb|upright=1.6|Flower heads facing east, away from the late afternoon sun]]

A common misconception is that flowering sunflower heads [[heliotropism|track the Sun]] across the sky. Although immature flower buds exhibit this behaviour, the mature flowering heads point in a fixed (and typically easterly) direction throughout the day.<ref name="Hangarter">{{cite web|url=http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/solartrack/solartrack.html|title=Solar tracking: sunflower plants |last=Hangarter |first=Roger P. |work=Plants-In-Motion website |publisher=Indiana University |access-date=22 August 2012 |quote=Many people are under the misconception that the flower heads of the cultivated sunflower (Helianthus annuus) track the sun... Immature sunflower flower heads do exhibit solar tracking and on sunny days the buds will track the sun across the sky from east to west... However, as the flower bud matures and blossoms, the stem stiffens and the flower head becomes fixed facing the eastward direction."}}</ref><ref>{{cite web|url=http://www.flowers-org.com/helianthus-sunflower.html |title=Sunflowers in the blooming stage are not heliotropic anymore. The stem has frozen, typically in an eastward orientation. |url-status=dead |archive-url=https://web.archive.org/web/20130523191033/http://www.flowers-org.com/helianthus-sunflower.html |archive-date=2013-05-23 }}</ref> This old misconception was disputed in 1597 by the English botanist [[John Gerard]], who grew sunflowers in his famous herbal garden: "[some] have reported it to turn with the Sun, the which I could never observe, although I have endeavored to find out the truth of it."<ref name="Gerard1597">{{cite book|last=Gerard|first=John|title=Herball, or Generall Historie of Plantes|publisher=John Norton|location=London|year=1597|pages=612–614|url=http://www.botanicus.org/page/1956748|access-date=2012-08-08}} Popular botany book in 17th century England</ref> The uniform alignment of sunflower heads in a field might give some people the false impression that the flowers are tracking the Sun.

This alignment results from [[heliotropism]] in an earlier development stage, the young flower stage, before full maturity of flower heads ([[anthesis]]).<ref>{{cite web|url=http://www.geochembio.com/biology/organisms/sunflower/|title=Sunflower, Developmental stages (life cycle)|work=GeoChemBio website|access-date=8 August 2012|archive-date=27 November 2012|archive-url=https://web.archive.org/web/20121127144105/http://www.geochembio.com/biology/organisms/sunflower/|url-status=dead}}</ref> Young sunflowers orient themselves in the direction of the sun. At dawn, the head of the flower faces east and moves west throughout the day. When sunflowers reach full maturity, they no longer follow the sun and continuously face east. Young flowers reorient overnight to face east in anticipation of the morning. Their heliotropic motion is a [[circadian]] rhythm, synchronized by the sun, which continues if the sun disappears on cloudy days or if plants are moved to constant light.<ref name="Atamian-2016">{{Cite journal|last1=Atamian|first1=Hagop S.|last2=Creux|first2=Nicky M.|last3=Brown|first3=Evan A.|last4=Garner|first4=Austin G.|last5=Blackman|first5=Benjamin K.|last6=Harmer|first6=Stacey L.|date=5 August 2016|title=Circadian regulation of sunflower heliotropism, floral orientation, and pollinator visits|journal=Science|language=en|volume=353|issue=6299|pages=587–590|doi=10.1126/science.aaf9793|issn=0036-8075|pmid=27493185|url=https://zenodo.org/record/889822|bibcode=2016Sci...353..587A|doi-access=free}}</ref> They are able to regulate their circadian rhythm in response to the blue-light emitted by a light source.<ref name="Atamian-2016" /> If a sunflower plant in the bud stage is rotated 180°, the bud will be turning away from the sun for a few days, as resynchronization with the sun takes time.<ref name="HäderLebert2001">{{cite book|author1=Donat-Peter Häder|author2=Michael Lebert|title=Photomovement|url=https://books.google.com/books?id=2nevsljDiCYC&pg=PA673|access-date=15 August 2010|year=2001|publisher=Elsevier|isbn=978-0-444-50706-8|pages=673–}}</ref>

When growth of the flower stalk stops and the flower is mature, the heliotropism also stops and the flower faces east from that moment onward. This eastward orientation allows rapid warming in the morning, and as a result, an increase in pollinator visits.<ref name="Atamian-2016" /> Sunflowers do not have a [[pulvinus]] below their [[inflorescence]]. A pulvinus is a flexible segment in the leaf stalks (petiole) of some plant species and functions as a 'joint'. It effectuates leaf motion due to reversible changes in [[turgor]] pressure which occurs without growth. The sensitive plant's closing leaves are a good example of reversible leaf movement via pulvinuli.

[[Image:SunflowerModel.svg|thumb|upright=.9|Vogel's model for ''n''=1&nbsp;... 500]]

==== Floret arrangement ====
Generally, each floret is oriented toward the next by approximately the [[golden angle]], 137.5°, producing a pattern of interconnecting [[spiral]]s, where the number of left spirals and the number of right spirals are successive [[Fibonacci number]]s. Typically, there are 34 spirals in one direction and 55 in the other; however, in a very large sunflower head there could be 89 in one direction and 144 in the other.<ref>{{Cite book |last1=Adam |first1=John A. |year=2003 |title=Mathematics in Nature: Modeling Patterns in the Natural World |url=https://books.google.com/books?id=2gO2sBp4ipQC&q=large-sunflower+spirals+144+89&pg=RA1-PA217 |via=[[Google Books]] |access-date=31 January 2011 |location= Princeton, New Jersey |publisher= Princeton University Press | page=217 |isbn=978-0-691-11429-3 }}</ref><ref>{{cite web |url=http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fibnat2.html#demos |title= Fibonacci Numbers and Nature - Part 2 |first1= Ron |last1= Knott |website=Department of Computer Science |publisher = [[University of Surrey]] |date=12 February 2009 |access-date=31 January 2011 |url-status=dead |archive-url=https://web.archive.org/web/20090916234127/http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fibnat2.html#demos |archive-date=16 September 2009 }}</ref><ref>{{cite web |url=http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fibnat.html |last1=Knott |first1=Ron |title= Fibonacci Numbers and Nature |website=Department of Computer Science |publisher = University of Surrey |date=30 October 2010 |access-date=31 January 2011 |url-status=dead |archive-url=https://web.archive.org/web/20090907063800/http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fibnat.html |archive-date=7 September 2009 }}</ref> This pattern produces the most efficient packing of seeds mathematically possible within the flower head.<ref>{{Cite book|url=https://books.google.com/books?id=f_VMeAToefwC&q=fibonacci+packing+efficiency&pg=PA154|title=Introduction to landscape design |last1=Motloch |first1=John L. |year=2000 |access-date=31 January 2011 |location = New York, USA | publisher= John Wiley & Sons, Inc. |page=154 |isbn=978-0-471-35291-4 }}</ref><ref>{{Cite book|url=https://archive.org/details/phyllotaxissyste0000jean/page/185 |url-access=registration |page= 185 |quote=fibonacci packing efficiency. |title=Phyllotaxis |access-date=2011-01-31|isbn=978-0-521-40482-2|last1=Jean|first1=Roger V|year=1994|publisher=Cambridge University Press }}</ref><ref>{{cite web|url=http://www.geocities.com/capecanaveral/lab/5833/cycas.html |title=Parastichy pair(13:21) of CYCAS REVOLUTA (male) florets_WebCite |url-status=dead |archive-url=https://web.archive.org/web/20091003194946/http://www.geocities.com/capecanaveral/lab/5833/cycas.html |archive-date=3 October 2009 }}</ref>

A model for the pattern of [[floret]]s in the head of a sunflower was proposed by H. Vogel in 1979.<ref>{{Cite journal |last=Vogel |first=H. |title=A better way to construct the sunflower head |journal=Mathematical Biosciences |volume=44 |issue=3–4 |pages=179–189 |year=1979 |doi=10.1016/0025-5564(79)90080-4 }}</ref> This is expressed in [[polar coordinates]]
:<math>r=c \sqrt{n},</math>
:<math>\theta=n \times 137.5^{\circ},</math>
where θ is the angle, ''r'' is the radius or distance from the center, and ''n'' is the index number of the floret and ''c'' is a constant scaling factor. It is a form of [[Fermat's spiral]]. The angle 137.5° is related to the [[golden ratio]] (55/144 of a circular angle, where 55 and 144 are Fibonacci numbers) and gives a close packing of florets. This model has been used to produce computer generated representations of sunflowers.<ref>{{cite book |last1=Prusinkiewicz |first1=Przemyslaw |author-link=Przemyslaw Prusinkiewicz |last2=Lindenmayer |first2= Aristid |author-link2=Aristid Lindenmayer |title=The Algorithmic Beauty of Plants |publisher=Springer-Verlag |year=1990 |url= https://archive.org/details/algorithmicbeaut0000prus/page/101 |pages=101–107 |isbn=978-0-387-97297-8 }}</ref>

{{gallery|mode=packed
|Sunflower macro wide.jpg|Detail of disk florets
|Solros - Sunflower (Helianthus annuus) - Ystad-2024.jpg|After flowering, the  seeds are visible.
|Sunflower (Helianthus annuus) seeds.jpg|Sunflower seeds
}}

=== Genome ===
The sunflower [[genome]] is [[diploid]] with a base [[chromosome]] number of 17 and an estimated [[genome size]] of 2,871–3,189&nbsp;million [[base pair]]s.<ref>{{cite web |title=''Helianthus annuus'' (common sunflower) Genome Project |publisher=NCBI |url=https://www.ncbi.nlm.nih.gov/genome?term=txid4232 |access-date=2012-02-20}}</ref><ref>{{NCBI taxid|4232|''Helianthus annuus''}}</ref> Some sources claim its true size is around 3.5&nbsp;billion base pairs (slightly larger than the [[human genome]]).<ref>{{cite web |title=Sunflower Genome Holds the Promise of Sustainable Agriculture|website=ScienceDaily |date=14 January 2010 |url=https://www.sciencedaily.com/releases/2010/01/100112121930.htm}}</ref>