Auxin distribution drives phototropism

Steps:

• Shoots bend toward light via phototropism, where auxin accumulates on the shaded side, promoting cell elongation there.
• Light is detected by the tip, which redistributes auxin downward to the growth zone.
• Barriers or alterations affecting auxin transport from tip to base disrupt bending.
• Evaluate each setup: intact tip with clear path enables response; blocks or offsets prevent it.

Why A is correct:

• The glass plate below the tip blocks auxin diffusion to the elongating cells, preventing unequal growth despite light detection (classic Boysen-Jensen experiment).

Why the others are wrong:

• B: Glass lower down allows auxin from tip to reach both sides, enabling shaded-side accumulation and bending.
• C: Replaced tip (centrally) restores normal auxin distribution for phototropic response.
• D: Offset tip mimics unilateral light, causing auxin to concentrate on one side and induce bending.

Final answer: A