UPSC Prelims 2011 Q18: Science & Technology – Biology
When the bark of a tree is removed in a circular fashion all around near its base, it gradually
dries up and dies because
- (a) Water from soil cannot rise to aerial parts
- (b) Roots are starved of energy
- (c) Tree is infected by soil microbes
- (d) Roots do not receive oxygen for respiration
Answer: (b) Roots are starved of energy
Explanation
Why ring-barking (girdling) a tree kills it
When the bark is cut away in a ring around the trunk, it removes the layer that includes the phloem.
| Option | Verdict | Reason |
|---|---|---|
| (a) Water from soil cannot rise to aerial parts | Incorrect | Water travels through the xylem, which lies deeper in the wood and is not cut by removing bark |
| (b) Roots are starved of energy | Correct | The phloem, in the bark, carries sugars made by photosynthesis down to the roots; cutting it stops this supply, and the roots eventually die of starvation, killing the tree |
| (c) Tree is infected by soil microbes | Incorrect | Ring-barking is a mechanical, not infectious, cause of death |
| (d) Roots do not receive oxygen for respiration | Incorrect | Oxygen reaches roots through soil air spaces, unrelated to bark removal |
Xylem vs phloem:
| Tissue | Carries | Direction |
|---|---|---|
| Xylem | Water and minerals | Roots → leaves (upward) |
| Phloem | Sugars (food) | Leaves → roots and other parts (downward, mainly) |
Answer: (b) Roots are starved of energy.
Why each option is right or wrong
- (a) Incorrect. Water transport uses the xylem, which is not removed by stripping the bark.
- (b) CORRECT. Removing the bark cuts the phloem, so roots stop receiving food and starve.
- (c) Incorrect. This is a mechanical injury, not a microbial infection.
- (d) Incorrect. Root oxygen supply comes from soil air, unaffected by bark removal.
Key point
Ring-barking (girdling) cuts the phloem in the bark, which carries food (sugars) down to the roots; roots starve and die, even though water (xylem, deeper in the trunk) still rises for a while.
Remember
Ring-barking cuts the phloem, starving the roots of food.
Common mistakes
- Assuming bark removal stops water transport; that is the xylem's job, deeper in the trunk.
- Blaming an infection for what is really a mechanical, structural injury.
- Mixing up the direction of flow: phloem mostly carries food downward from leaves.
Exam tip
Bark = phloem (food transport). Wood/xylem is deeper and carries water. Ring-barking always points to a food-starvation answer.
Sources
- NCERT Class XI Biology — Transport in Plants
- Peter H. Raven, Ray F. Evert and Susan E. Eichhorn — Biology of Plants (W. H. Freeman)