BPSC 70th CCE Re-exam Prelims General Studies · Question 126 of 142

Correct — B, Remain at same position. A tree does not grow the way a child does. Increase in height comes from the apical meristem, the pocket of dividing cells at the tip of the shoot, and it is added at the top: new cells are produced and elongate at the growing point, and the tissue already laid down below stays where it is. The trunk at one metre above the ground was formed years ago and does not stretch, so a nail driven into it stays one metre above the ground no matter how tall the tree becomes. What does change at that height is girth. The vascular cambium, a lateral meristem forming a cylinder between the xylem and the phloem, divides to add new wood on its inner face and new phloem on its outer face season after season, and the cork cambium adds bark outside that. So over two years the trunk thickens around the nail — often enough to swallow the head, which is why old fences and signboards are found embedded deep inside trunks, and why nails inside timber are a well-known hazard at the sawmill. The nail may end up buried, but it does not rise. Since (b) is correct, option (d) 'None of the above' fails.

• (a)Move up — The intuitive answer, and the reason the question is set. It assumes the trunk elongates along its whole length like a stretching rubber band, so that a mark on it would ride upward as the tree grows. Trees do not elongate that way — new length is added only at the tips, by the apical meristems of shoot and root, so a mark at a given height stays at that height.
• (c)Move sideways — There is no growth process that would carry a fixed object laterally through wood. Secondary growth does add tissue outward all around the cambium, so the trunk's surface moves outward past the nail and the nail becomes progressively more deeply embedded — but the nail's position relative to the ground and to the trunk's axis does not shift.
• (d)None of the above — This escape option requires all three preceding statements to be false, and the second is exactly right. Note the structure of the option set: (a) and (c) describe motion, (b) describes no motion, so the three between them exhaust the possibilities and nothing is left for 'none of the above' to name.

Plant growth is localised in meristems, tissues of permanently dividing cells, and the division of labour between them explains almost every question of this kind. Apical meristems at the tips of shoots and roots produce primary growth, which is growth in length; the region just behind the tip is where cells elongate, and below that the tissue matures and becomes fixed. Lateral meristems produce secondary growth, which is growth in girth: the vascular cambium lays down secondary xylem inward and secondary phloem outward, and the cork cambium produces the protective outer bark. In seasonal climates the cambium's activity varies through the year, producing the concentric annual rings from which a tree's age is read. The same anatomy explains a second classic result — girdling. Strip a ring of bark from around a trunk and you cut the phloem, the tissue that carries sugars downward from the leaves; the roots are then starved of food and the tree dies, even though the xylem inside is intact and water still rises.

The reasoning route is to ask where the new material is being added. If growth in length happens only at the tips, then nothing below a tip can move upward, and the question is settled in one step. It is worth testing the same idea against variants an examiner can build from it: a nail driven into a branch will move outward from the trunk only if the branch tip grows past it, which it does — but the nail itself stays where it was put; two nails driven a foot apart vertically stay a foot apart; a wire tied round a young trunk does not slide up, it strangles the tree as the girth increases. Each variant is answered by the same distinction between apical and lateral growth. And the girdling result is the natural companion fact, because it uses the other half of the same anatomy — the phloem in the bark — and UPSC has asked it directly.

• Primary growth, the increase in length, occurs only at apical meristems at the tips of shoots and roots; tissue already formed below does not elongate
• Secondary growth, the increase in girth, comes from lateral meristems — the vascular cambium, which adds secondary xylem inward and secondary phloem outward, and the cork cambium, which forms the bark
• A nail fixed one metre above the ground therefore stays one metre above the ground, while the thickening trunk closes over it — the reason embedded nails and wires are found inside old timber
• The vascular cambium's seasonal activity produces annual rings, from which the age of a tree can be counted
• Girdling — removing a ring of bark all round a trunk — severs the phloem and starves the roots of sugars, killing the tree while the xylem is still intact

• Assuming a tree elongates along its whole trunk; only the tips add length
• Confusing 'the trunk grows around the nail' with 'the nail moves' — the nail is stationary and the wood advances past it
• Forgetting that girth and height are produced by different meristems, which is the distinction the whole question rests on

BPSC asks the everyday consequence — what happens to a nail, to a mark, to a wire — and expects the anatomy to be recalled by implication rather than named. UPSC asks the mechanism directly, most famously through girdling: why a tree dies when a ring of bark is removed. Both draw on the same two facts about where a plant adds length and where it adds width.

• practice — not a real PYQ

  Increase in the girth of a tree trunk is brought about by

  • (a)the apical meristem
  • (b)the vascular cambium and cork cambium
  • (c)the intercalary meristem
  • (d)elongation of existing cells throughout the trunk

  Answer(b) the vascular cambium and cork cambium — the lateral meristems responsible for secondary growth; the apical meristem adds length at the tips.

• practice — not a real PYQ

  The age of a tree in a seasonal climate can be estimated by counting

  • (a)the number of branches
  • (b)the annual rings in a cross-section of the trunk
  • (c)the number of leaves shed each year
  • (d)the depth of the root system

  Answer(b) the annual rings in a cross-section of the trunk — produced by the seasonal variation in the activity of the vascular cambium.