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Difficulty: EasyTransport Mechanisms in Plants

In a ringing (girdling) experiment on a woody dicotyledonous stem, a outer ring of bark containing the phloem is removed while leaving the xylem intact. Which of the following observations will occur above the ring after several days?

  1. Swelling caused by the accumulation of manufactured organic nutrientsAnswer
  2. B
    Swelling caused by the accumulation of water and dissolved mineral salts
  3. C
    Immediate wilting of leaves due to the stoppage of upward water supply
  4. D
    Drying up of the stem tissues above the ring due to lack of minerals

Answer

Swelling caused by the accumulation of manufactured organic nutrients
Removing a ring of bark removes the phloem, which translocates manufactured organic food downward from the leaves. Because the xylem is intact, water and minerals continue to travel upward, but sugars cannot move past the ring, resulting in food accumulation and swelling above the ringed area.

Step-by-Step Solution

1
Identify the tissue removed and its physiological function
Ringing removes phloem tissue from the stem, which is responsible for translocating organic solutes (photosynthates).
Phloem translocates organic food manufactured in leaves downward toward the roots.
2
Identify the tissue left intact and its physiological function
Xylem vessels remain intact, maintaining upward transport of water and mineral salts from the soil to the leaves.
Xylem is situated deeper within the vascular bundle and is not removed during superficial stem girdling.
3
Deduce the resulting accumulation point
Downwards movement of sugars is blocked at the ring, leading to nutrient buildup and cell expansion (swelling) immediately above the cut.
Accumulation of organic solutes above the blockage increases local osmotic concentration and cell division.

Key Concept

Phloem Translocation and Ringing Experiment
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Topics

Transport Mechanisms in Plants

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