HOTS Science Primary 5 Systems

A gardener removes a complete ring of bark from the trunk of a healthy mango tree, cutting through the outer layer down to the wood. The ring of removed bark is 5 cm wide and is made at a point halfway up the trunk. The tree is well-watered and receives full sunlight throughout the experiment. After three months, which observation is most likely, and why?

A The roots will die before the leaves wilt, because the bark contains phloem which normally carries glucose from the leaves down to the roots, and removing the bark ring cuts this supply so the roots cannot respire and absorb water.
B The leaves will wilt and die before the roots are affected, because removing the bark exposes the xylem and water evaporates from the cut surface, so less water reaches the leaves.
C Both the roots and leaves will die at the same time, because the xylem and phloem are both located in the bark, so removing it cuts off all transport between the roots and the shoot.
D The tree will survive without any damage because the wood beneath the bark contains enough stored nutrients to sustain both roots and leaves until the bark grows back.
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Worked Solution

Step 1: Identify what is in the bark The bark of a tree contains phloem — the transport tissue that carries dissolved glucose (made in the leaves by photosynthesis) downward to the stem, roots and other non-photosynthetic parts of the plant. The xylem, which carries water upward from roots to leaves, is located in the wood (the inner, harder tissue) and is NOT damaged when only the bark ring is removed. Step 2: Predict the immediate effect of removing the bark ring Once the complete ring of bark (phloem) is removed, glucose made in the leaves can no longer travel downward past the cut. Glucose will accumulate above the cut and be unavailable to the roots. The roots continue receiving water upward through intact xylem in the wood, but they receive no glucose. Step 3: Reason about which part dies first Roots need glucose for cellular respiration to generate the energy required for active transport of minerals and water uptake. Without incoming glucose, root cells cannot respire adequately, lose their ability to absorb water, and eventually die. The leaves, still receiving water via the intact xylem and still making glucose by photosynthesis, will remain healthy longer. Eventually, once roots die and water uptake stops, leaves will also wilt — but roots die first. Step 4: Match to the correct option Option A correctly identifies that phloem is in the bark, that removing it cuts glucose supply to roots, and that roots die before leaves. This is the counter-intuitive result of girdling/ring-barking.

Correct answer: The roots will die before the leaves wilt, because the bark contains phloem which normally carries glucose from the leaves down to the roots, and removing the bark ring cuts this supply so the roots cannot respire and absorb water.

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