Q51. A sign is hammered into a tree trunk 2 meters above the tree’s base. If the tree is 10 meters tall and elongates 1 meter each year, how high will the sign be after 10 years?
The sign remains at 2 meters above the tree’s base after 10 years because tree elongation occurs via primary growth at the tips, not by pushing up existing trunk sections where the sign is nailed.
Problem Breakdown
A sign hammered into a mature tree trunk stays fixed relative to the base. The tree’s stated “elongation” of 1 meter per year refers to apical growth (new height added at the crown), while secondary growth thickens the trunk radially without lifting embedded objects upward. After 10 years, the tree reaches 20 meters tall, but the sign’s position does not change.
Option Analysis
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(A) 12 meters: Assumes the sign moves up with total growth (2m initial + 10m added). Incorrect, as growth is not uniform along the trunk.
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(B) 7 meters: Might subtract growth from initial height (2m initial – something), but illogical since no downward force acts on the sign.
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(C) 4 meters: No clear basis; perhaps halves the growth or misapplies partial lift. Wrong, as the sign stays embedded at original height.
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(D) 2 meters: Correct. The sign remains 2 meters from the base, as confirmed by tree biology principles.
Tree Growth Science
Primary growth elongates shoots from meristems at tips, increasing total height without affecting lower trunk. Secondary growth (cambium layer) expands girth, pushing nails outward but not up. Real trees show nails/screws staying at fixed heights over decades.
Introduction to Sign Hammered into Tree Elongates Puzzle
Ever wonder what happens to a sign hammered into a tree trunk when the tree elongates 1 meter each year? This classic biology riddle tests understanding of plant growth. A tree 10m tall with a sign at 2m base height sparks debate: does the sign rise to 12m or stay put? Perfect for GATE Life Sciences prep, here’s the detailed solution.
Core Concept: How Trees Grow
Trees elongate via primary growth at apical meristems (tips), adding height anew without shifting trunk bark or wood below. A mature 10m tree’s lower trunk is fixed; annual 1m gain happens at the crown. Secondary growth thickens diameter via vascular cambium, moving nails sideways only.
Example: Forestry pros note old nails on oaks remain base-relative despite decades of height gain.
Step-by-Step Solution
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Initial setup: Sign at 2m on 10m tree base.
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Yearly elongation: +1m at top, tree becomes 11m (year 1), 20m (year 10).
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Sign position: Fixed in trunk tissue, unaffected by apical extension.
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Final height: Still 2m above base on 20m tree.
Why Each Option Fails
| Option | Height | Why Wrong |
|---|---|---|
| (A) 12 meters | 2m + 10m growth | Ignores growth location (tips only). |
| (B) 7 meters | Arbitrary subtraction | No basis in biology; trunk doesn’t contract. |
| (C) 4 meters | Halved growth? | Misconstrues radial vs. vertical growth. |
| (D) 2 meters | Original position | Matches primary/secondary growth facts. |
Exam Tips for GATE Life Sciences
This Q51-style question appears in biology competitive exams, emphasizing plant anatomy (meristems, cambium). Memorize: Embedded objects stay height-fixed. Relate to real scenarios like arborist nails or scars.
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