A nail is nailed into the trunk of a tree at a point 1 m from the soil level. After 2 years the nail will

Question

UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2025 Q126 — Science & Technology PYQ with Answer

State PCS 2025 Q126 — Science & Technology PYQ with Answer