The movement or orientation of an organism or cell in response to sunlight, either moving toward or away from it.
Directly from Greek 'helios' (sun) and 'taxis' (arrangement, order, movement). The term entered biology in the late 1800s to describe directional movement responses to light stimuli.
Single-celled organisms like algae perform heliotaxis automatically—they don't have brains or eyes, yet they can 'find' the sun like it's playing an invisible game of Marco Polo in water.
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