Editing Common sunflower (section)

==== 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]]