(b) (i) the structure and function of the vascular system in the roots,…

The idea

Herbaceous dicotyledonous plants rely on vascular tissue to move water and minerals upward and assimilates in either direction. In roots, xylem forms a central star-shaped arrangement with phloem between the arms; in stems, vascular bundles ring the outer edge with xylem inside and phloem outside, often with cambium between; in leaves, xylem lies above phloem in the midrib and veins. Xylem vessels are dead, lignified, hollow tubes that carry water and mineral ions from root to shoot, providing support. Phloem sieve tubes with companion cells carry sucrose and amino acids to sinks such as roots, growing tips and storage organs. Recognising tissue position in each organ lets you link structure to function in multiple-choice questions.

What earns the marks

• Xylem in roots is arranged centrally, often as a star shape, with phloem between the arms of the xylem.
• In stems, vascular bundles are arranged in a ring near the periphery, with xylem positioned internally and phloem externally.
• In leaves, xylem is located above phloem within the vascular bundle of the midrib and veins.
• Xylem vessels are dead, lignified, hollow structures that transport water and mineral ions and provide support.
• Phloem sieve tubes and companion cells transport assimilates such as sucrose and amino acids to sinks.
• The cambium between xylem and phloem allows secondary thickening in dicotyledonous plants.

Common mistakes and how to put them right

• Assuming xylem is always on the outside of a bundle.

  Put it right: In stems the xylem lies towards the inside and phloem towards the outside, so check the organ before deciding position.

• Confusing the direction of phloem transport as always upward.

  Put it right: Phloem moves assimilates bidirectionally to sinks, which may be roots or growing tips.

• Describing xylem as living cells.

  Put it right: Xylem vessels are dead and lignified, which is why they form hollow tubes for water movement.

Exam tips

• Sketch a quick labelled cross-section of root, stem and leaf before answering to fix tissue positions.
• Read each option carefully for organ-specific wording, since a true statement about a stem may be false for a root.
• Link each tissue to its transported substance and direction to eliminate distractors quickly.