Sunflowers always turn to follow the sun
Not wrong, but our understanding became significantly more precise.
TaughtSunflowers track the sun across the sky all day, every day, throughout their lives.
NowOnly young sunflowers track the sun. Once mature, the flower stops moving and faces permanently east. The tracking is driven by a circadian rhythm, shown in a 2016 study in Science.
What actually happened
Walk past a field of mature sunflowers at noon and they will all be facing east, away from the sun. The tracking behavior, heliotropism, belongs to the growing stage, and it works by elongating the east side of the stem during the day and the west side at night so the head swings back overnight to face dawn.
Stacey Harmer's group at UC Davis showed in 2016 that this is controlled by an internal circadian clock, not simply a response to light: plants moved into constant light kept swinging on roughly a 24-hour cycle for several days. When the stem stops growing, the motion stops with it, leaving the head fixed eastward. That turns out to be functional. East-facing heads warm faster in the morning, and warm heads attract significantly more pollinators.
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Who was taught this
Still standard through 2016, so anyone who finished school between 1950 and 2016 learned the earlier version.