Auxin's Site of Production and Transport

Steps:

• Recall auxin as a plant hormone primarily synthesized in shoot apical meristems.
• Note auxin's polar transport from shoot tips downward via phloem and parenchyma.
• Evaluate each option against known auxin biology: production site, distribution, and effects on growth.
• Identify the option matching auxin's basipetal movement from tips.

Why D is correct:

• Auxin (IAA) is produced in shoot tips and transported basipetally, spreading through vascular tissues to influence growth below.

Why the others are wrong:

• A: Auxin distribution is polar, not equal, due to directed transport.
• B: Auxin promotes cell elongation in shoots for tropisms, though it inhibits in roots.
• C: Primary synthesis occurs in shoot tips, not the stem itself.

Final answer: D