Heliotropism Misconception
A common misconception is that sunflower heads track the Sun across the sky. This old and chronic misconception was debunked already in 1597 by the English botanist John Gerard, who grew sunflowers in his famous herbal garden: " 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 they are tracking the sun, but the heads are actually pointing in a fixed direction (East) all day long. However, the uniform alignment does result 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 (no growth).
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