Geotropism describes a plant's growth response to which stimulus?

• A. Light
• B. Water
• C. Gravity
• D. Temperature

Answer: (C)

Geotropism is the plant’s growth response to gravity. Gravity serves as the directional cue that tells the plant which way is down, and the plant uses this cue to orient its growth. This involves uneven growth caused by the hormone auxin redistributing within the tissues in response to gravity. In roots, higher auxin on the lower side slows growth there, so the root bends downward—positive geotropism. In shoots, higher auxin on the lower side speeds up elongation, bending the shoot upward—negative geotropism. This mechanism is distinct from responses to other stimuli: phototropism is toward light, hydrotropism toward water, and thermotropism toward temperature, each driven by different environmental cues.