Answer: Auxin (IAA) from apical bud.
- A Gibberellins diffusing downward from the shoot apex to suppress lateral buds
- B Cytokinins transported upward from the root tip through the xylem stream
- C Auxin (IAA) from apical bud
- D Ethylene accumulating locally at axillary bud junctions
Correct answer: C. Auxin (IAA) from apical bud
Explanation: Auxin produced by the apical bud inhibits growth of lateral buds (apical dominance). Removing the apex allows lateral growth.
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.