How do plants bend toward light sources?
Show answer & explanation
Answer: Shaded side cells elongate faster
Shaded side cells elongate faster ✓ — Correct! When light hits one side, auxin (growth hormone) migrates to the shaded side. Auxin makes cells elongate, so the dark side grows longer and pushes the stem toward light. It's counterintuitive—the shaded side grows MORE, causing the bend. Darwin discovered this mechanism!
Light side cells elongate faster — Wrong. It's the opposite! The shaded side grows faster because auxin accumulates there. If the lit side grew faster, the plant would bend away from light. Auxin migration to the dark side causes the bending toward light.
Roots pull the stem toward light — Wrong. Roots grow downward (gravitropism) and don't pull stems toward light. The bending happens in the stem itself—auxin makes shaded-side cells elongate faster, pushing the plant toward the light source.