The Mystery Of Why Sunflowers Turn To Follow The Sun — Solved

Aug 9, 2016

Scientists have answered a burning question central to the charm of sunflowers: Why do young flowers move their blooms to always face the sun over the course of a day?

And then: Once sunflowers reach maturity, why do they stop tracking the sun and only face east?

In a newly-published article in Science, the researchers say the young plant’s sun-tracking (also called heliotropism) can be explained by circadian rhythms – the behavioral changes tied to an internal clock that humans also have, which follow a roughly 24 hour cycle. A young flower faces east at dawn and greets the sun, then slowly turns west as the sun moves across the sky. During the night, it slowly turns back east to begin the cycle again.

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6 comments on “The Mystery Of Why Sunflowers Turn To Follow The Sun — Solved

  • I was stopped in my tracks when I saw a similar article that stated that the sunflowers “liked” to be in this position or the other.
    After thinking about it for about ten seconds I decided the uneven stem growth was responsible for turning the plant.

    Can I have some research money?

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  • Stopped on a narrow country lane, waiting for highway fog to lift, surrounded by fields of sunflowers shrouded.

    A strange, quiet realm of sentinel blooms, not knowing where to look…

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  • Sam Millar #4
    Aug 11, 2016 at 6:50 am

    Plant “behaviour” is fascinating. Fun to contemplate Tinbergen’s four “whys” (evolution, function, development, causation) in a non-animal case!

    Plant movement is usually caused by uneven growth rates or in the case of my Venus Fly Traps, pressure of sap (or release of pressure) causing torsion on twisted elastic fibres!

    Plant movements are usually slow, so many people are unaware of leaves tracking the Sun.
    In many cases, the leaves of solar tracking plants are controlled by a specialized organ called the pulvinus. This organ is a swollen part of the petiole that may occur where it joins the stem, the leaf blade, or both.

    It contains motor cells that generate mechanical forces that control the orientation of the petiole and thus the leaf blade. The forces are produced by changes in the turgor in the pulvinus.

    The cells of this organ have highly elastic cell walls that allow them readily to change size and shape. The cells of the upper pulvinus have the capability of increasing their turgidity with water uptake, while the lower pulvinus can lose water very easily. The net effect is a force that moves the petiole.

    Another mechanism producing heliotropism is produced by small mechanical changes along the length of the petiole and by movements of the younger parts of the stem. Heliotropic plants are continuously moving their leaves, leaflets, and pinnules to readjust to prevailing light conditions. Movements occur rather rapidly, every fifteen to sixty seconds, which is just slow enough to be imperceptible to most humans.

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