Debra and Dave Vanderlaan
Photograph - Photography
A common misconception is that flowering sunflower heads
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.
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.
The uniform alignment of sunflower heads in a field
might give some people the false impression
that the flowers are tracking the sun.
The uniform alignment results from heliotropism
in an earlier development stage, the bud stage,
before the appearance of flower heads (anthesis).
The buds are heliotropic until the end of the bud stage,
and finally face East.
Their heliotropic motion is a circadian rhythm,
synchronized by the sun, which continues
if the sun disappears on cloudy days.
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 by the sun takes time.
The heliotropic motion of the bud is performed
by the pulvinus, a flexible segment just below the bud,
due to reversible changes in turgor pressure,
which occurs without growth.
January 6th, 2013
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