Common Misconceptions

Misconception: "Plants grow towards light because they 'want' or 'need' more light for photosynthesis."

Reality: While growing towards light is indeed beneficial for photosynthesis, the mechanism is not driven by photosynthetic need. It is a purely physical hormone response: auxin redistributes to the shaded side of the shoot, causing differential cell elongation that results in bending. The plant does not "decide" to grow towards light — the response is automatic and chemical.

Misconception: "Roots grow towards water because they detect it and respond."

Reality: Roots show positive hydrotropism (growth towards water), but this is a much less well-understood response than gravitropism. For AQA GCSE, you only need to know that roots show positive gravitropism (towards gravity) and that the mechanism involves auxin inhibiting elongation on the lower (high-auxin) side. Do not confuse hydrotropism with gravitropism in exam answers.

Misconception: "Auxin has the same effect in roots and shoots."

Reality: This is a critical distinction. In shoots, high auxin concentration promotes cell elongation (more auxin = more growth). In roots, the same concentration of auxin inhibits cell elongation (more auxin = less growth). Roots are far more sensitive to auxin than shoots. This different sensitivity is what causes roots to curve downward while shoots curve upward when both are treated with auxin — same hormone, opposite responses.

Misconception: "Weedkillers kill all plants equally."

Reality: Selective weedkillers are made from synthetic auxins that cause weeds (broad-leaved plants) to grow abnormally fast and die, while narrow-leaved crop plants (grasses, cereals) are largely unaffected because they metabolise the synthetic auxin differently. The weedkiller exploits the auxin response mechanism selectively — it is a targeted tool, not a general plant poison.