UPSC Prelims 2011 · GS Paper I

Question 18: Science and Technology

When the bark of a tree is removed in a circular fashion all around near its base, it gradually dries up and dies because

aWater from soil cannot rise to aerial parts

bRoots are starved of energy

cTree is infected by soil microbes

dRoots do not receive oxygen for respiration

AnswerThe answer is (b).

Removing bark in a ring, called girdling, cuts the phloem, which lies in the bark. Phloem carries the sugars made in the leaves down to the roots. The xylem, deeper in the wood, goes on carrying water upward for a while, but the roots can no longer get food, so they starve, stop absorbing water and minerals, and the whole tree gradually dies. Remember: girdling breaks food transport in the phloem, not water transport in the xylem.

Why each option is right or wrong

(a) Water moves up the xylem, which is inside the wood and is not removed with the bark.
(b) Right: the phloem is cut, so food cannot reach the roots and they starve.
(c) Infection may follow but is not the reason for death.
(d) Roots still take oxygen from the soil air; starvation is what kills them.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2011, General Studies Paper I, Series A. The paper on upsc.gov.in.