Explain why the removal of a ring of bark and phloem from around the trunk of a tree eventually leads to the death of the roots, but does not immediately kill the leaves.

OCR A-Level Biology A (H420) — 3.3 Transport in plants · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A technique called ring-barking (girdling) involves cutting away a complete ring of bark and phloem tissue from around the circumference of a tree trunk, leaving the xylem intact. Foresters have observed that ring-barked trees continue to produce new leaves for several weeks after the procedure, but eventually die as the roots deteriorate first.

Model answer (4 marks)

Phloem is the tissue that carries sucrose and other organic solutes from the leaves (source) to the roots (sink). When the bark and phloem are removed, this transport pathway is blocked, so the roots no longer receive the sugars they need for respiration and growth. Without a supply of organic solutes, root cells cannot maintain their metabolism and eventually die.

The xylem remains intact, so water and mineral ions can still move from the roots up to the leaves for a short period. The leaves are the source of photosynthate and already contain stored carbohydrates, so they can continue to function for several weeks after girdling. However, as the roots deteriorate and can no longer supply water via the xylem, the leaves will eventually lose water and die.

Examiner tips

  • Mention phloem’s role in transporting sugars to roots
  • Explain that root death is due to lack of sugars, not water
  • State that xylem remains intact so water transport continues temporarily
  • Note that leaves survive initially because they produce their own sugars and have reserves

Common mistakes

  • Confusing phloem with xylem as the main transport tissue
  • Saying the leaves die immediately instead of after a delay
  • Ignoring the role of stored carbohydrates in the leaves

Mark scheme (4 marks)

  1. Phloem is the tissue responsible for transporting sucrose/organic solutes from leaves to roots
  2. Without a supply of sucrose/organic solutes, root cells cannot respire/grow and eventually die
  3. Xylem is left intact so water and mineral ions continue to be transported from roots to leaves in the short term
  4. Leaves do not die immediately because they are the source of photosynthate and have existing stores, but will eventually die once roots deteriorate and can no longer supply water via xylem

Key terms in this question

phloem

Related

More Transport in plants questions

▶ Try answering this question with AI marking (free) →