The absence of growth or movement response to light; the lack of tropistic orientation toward or away from illumination in organisms.
From Greek 'apo-' (away/without), 'photo-' (light), 'trop-' (turning), and '-ism' (state or condition). The term emerged in 19th-century botany to categorize plant behaviors when light had no directional effect.
Root systems show aphototropism—they grow downward into soil regardless of light direction—which reveals that plants have evolved *separate* sensory systems for different growth needs, not one unified light-detection system.
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