Heliotropic
The tendency of some plants to move their leaves either perpendicular to the rays of the sun (diaheliotropic) or "edge on" to the rays of the sun (paraheliotropic).
Additional Information
Sunflower heads are heliotropic, which means that the heads face East in the morning and follow the sun to the West in the evening. During the night the heads return to face East by morning.
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