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Difficulty: MediumTransport Mechanisms in Plants

A complete ring of bark including the phloem tissue is carefully removed from the trunk of a woody dicotyledonous plant while keeping the xylem intact. After several weeks, a distinct swelling is observed in the bark region immediately above the cut ring. Which of the following processes explains the formation of this swelling?

  1. The downward translocation of manufactured organic food is blocked, causing solutes to accumulate above the ring.Answer
  2. B
    The upward transport of water and mineral salts is obstructed, forcing fluids to collect in the stem tissue above the ring.
  3. C
    The bryophyte vascular bundles are severed, preventing the redistribution of inorganic minerals to lower tissues.
  4. D
    Oxygen gas produced during dark photosynthetic reactions is trapped and inflates the outer cortex above the girdle.

Answer

The swelling above the ringed stem is caused by the accumulation of organic food substances translocated downward through the phloem from the leaves.
In girdling (ringing) experiments, removing the phloem severs the transport route for organic compounds (mainly sucrose) synthesized during photosynthesis in leaves. Because xylem remains intact, leaves continue to receive water and produce photosynthates. As manufactured food moves down the stem via phloem, it meets the blocked ring and accumulates immediately above it, causing localized tissue swelling due to organic accumulation and cell division.

Step-by-Step Solution

1
Identify the tissues present in the removed ring of bark.
Removing bark removes the phloem (sieve tubes and companion cells) but leaves the inner xylem vessels intact.
Phloem lies in the outer vascular region of dicot stems, while xylem lies towards the inner core.
2
Determine the transport function and direction of phloem versus xylem.
Phloem translocates manufactured organic food (sucrose, amino acids) bidirectionally/downward from leaves to sink tissues, whereas xylem conducts water and mineral salts upward from roots.
Leaves photosynthesize organic nutrients that must travel downward to feed stem and root cells.
3
Analyze the physical effect of severing phloem tissue.
Organic solutes flowing down from the canopy are blocked at the upper boundary of the cut ring, accumulating and swelling the tissue above the girdle due to localized cell growth and osmotic water intake.
The intact xylem continues supplying water to the leaves, maintaining photosynthesis and translocation down to the point of interruption.

Key Concept

Phloem translocation of organic solutes in plants
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Topics

Transport Mechanisms in Plants

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