Which process causes plant stems to bend toward a light source?

Which process causes plant stems to bend toward a light source?

• A. Gravitropism
• B. Thigmotropism
• C. Phototropism
• D. Hydrotropism

Answer: (C)

Phototropism is the plant’s growth response to light, causing stems to bend toward a light source. When light hits from one side, blue-light receptors in the stem trigger auxin to accumulate on the shaded side. The raised auxin level promotes longer cell growth on that side, so the shaded side lengthens more and the stem bends toward the light. This helps the plant maximize light capture for photosynthesis. The other tropisms describe responses to gravity, touch, and moisture, not light, so they don’t explain why stems curve toward a light source.

Phototropism is the plant’s growth response to light, causing stems to bend toward a light source. When light hits from one side, blue-light receptors in the stem trigger auxin to accumulate on the shaded side. The raised auxin level promotes longer cell growth on that side, so the shaded side lengthens more and the stem bends toward the light. This helps the plant maximize light capture for photosynthesis. The other tropisms describe responses to gravity, touch, and moisture, not light, so they don’t explain why stems curve toward a light source.