Answer: Auxin accumulating on the lower side, inhibiting root elongation there.
- A Auxin accumulating on the upper side, accelerating elongation there instead
- B Auxin accumulating on the lower side, inhibiting root elongation there
- C Uniform auxin distribution across the root cap with no lateral asymmetry
- D Cytokinin redistribution within statocytes sensing the gravity vector
Correct answer: B. Auxin accumulating on the lower side, inhibiting root elongation there
Explanation: In roots, statoliths (starch grains) settle by gravity, signaling asymmetric auxin distribution. Auxin accumulates on lower side, inhibiting growth there, so root curves downward.
Geometric growth (where both daughter cells keep dividing) produces this classic S-shaped curve: a slow lag phase, a rapid exponential phase, and a levelling-off stationary phase as nutrients or space become limiting - the same fundamental pattern seen in population growth.
Concept context
Phases of plant growth, the five classical plant hormones plus newer ones, photoperiodism, vernalization, and seed dormancy.