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

1. Complex Permanent Tissues: Xylem and Phloem (basic)

In the world of plants, as they grow larger and more specialized, they require a sophisticated 'transportation network' to move essentials over long distances. Unlike simple tissues composed of a single cell type, Complex Permanent Tissues are made up of more than one type of cell that work together as a single unit to perform a common function. In highly differentiated plants, these tissues are collectively known as vascular tissues, consisting primarily of Xylem and Phloem Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.99. Together, they form vascular bundles that act as the plant's plumbing system. Xylem is the tissue dedicated to the upward movement of water and minerals absorbed from the soil by the roots. It consists of specialized structures like tracheids and vessels which form a continuous system of water-conducting channels reaching all parts of the plant Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94. On the other hand, Phloem is the plant's 'delivery service' for food. It transports the products of photosynthesis (mainly sucrose) from the leaves, where they are synthesized, to the storage organs (roots, fruits) and growing points like buds Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.96. One of the most fascinating differences between the two is their mechanism of operation. While water movement in the xylem is largely driven by physical forces like suction, food transport in the phloem is an active process. Materials like sucrose are loaded into the phloem using energy in the form of ATP. This creates osmotic pressure that forces the sap to move toward tissues with lower pressure, allowing the plant to move nutrients exactly where they are needed—for instance, moving stored sugar from roots to growing buds in the spring Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.96.

Feature	Xylem	Phloem
Primary Function	Transport of water and minerals	Transport of food (sucrose)
Direction of Flow	Unidirectional (Upward)	Bidirectional (Based on need)
Energy Requirement	Passive (Physical forces)	Active (Uses ATP)

Sources: Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.96; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.99

2. Mechanism of Water Transport (Ascent of Sap) (basic)

Hello! Today we are exploring a fascinating biological mystery: how does a tree, which lacks a heart or a pump, lift water hundreds of feet into the air against the force of gravity? This process is known as the Ascent of Sap, and it relies on two distinct biological forces that work together like a relay team. Water and dissolved minerals enter the plant through the roots and travel upward through a specialized vascular tissue called the xylem. Think of the xylem as a system of microscopic, interconnected pipes. While plant roots are strong enough to exert mechanical pressure on the earth and break apart materials Fundamentals of Physical Geography, Geography Class XI, Geomorphic Processes, p.41, they also create a biological "push" known as root pressure. By actively absorbing minerals from the soil, the roots create a concentration difference that forces water into the xylem. This mechanism is most active at night when the plant isn't losing much water to the air Science, Class X, Life Processes, p.95. However, root pressure isn't strong enough to reach the top of tall trees. During the day, the primary driver is Transpiration Pull. As water evaporates from the tiny pores in the leaves (stomata) — a process called transpiration — it creates a powerful suction or tension. Because water molecules are "sticky" and hold onto each other, this suction pulls the entire water column upward from the roots, much like drinking through a straw. Beyond transport, this process is essential for temperature regulation, helping the plant stay cool in the sun Science, Class X, Life Processes, p.95. This constant movement of water from the soil, through the plant, and into the atmosphere is a vital link in the global hydrological cycle Physical Geography by PMF IAS, Hydrological Cycle, p.325.

Feature	Root Pressure	Transpiration Pull
Nature of Force	A "Push" from the bottom	A "Pull" (suction) from the top
Primary Timing	Most important at night	Dominant during the day
Driving Event	Active mineral absorption	Evaporation from leaves

Sources: Science, Class X, Life Processes, p.95; Fundamentals of Physical Geography, Geography Class XI, Geomorphic Processes, p.41; Physical Geography by PMF IAS, Hydrological Cycle, p.325

3. Translocation of Food: The Source-Sink Relationship (intermediate)

In the previous hops, we looked at how plants pull water up from the soil. Now, let's discuss how they distribute the "food" (mainly sucrose) they've manufactured in their leaves. This process is known as translocation. Unlike the movement of water in the xylem, which is largely driven by physical forces like suction and evaporation, translocation in the phloem is a highly regulated, active process that requires energy in the form of ATP Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.95.

To understand this, we use the Source-Sink Relationship. A 'Source' is any part of the plant that produces or releases food (like a mature leaf or a storage bulb in spring), while a 'Sink' is any part that needs energy for growth or storage (like roots, developing fruits, or new buds). The fascinating part is that these roles can swap! For instance, in early spring, sugar stored in the roots (the source) moves upward to the buds (the sink) to fuel new growth Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.96. This makes phloem transport bidirectional, unlike the one-way street of the xylem.

The mechanism behind this is the Pressure-Flow Hypothesis. When sucrose is actively loaded into the phloem using ATP, it creates a high concentration of solutes. This causes water to rush into the phloem from the neighboring xylem via osmosis, building up osmotic pressure. Just as air moves from high-pressure regions to low-pressure regions Science, Class VIII (NCERT 2025 ed.), Pressure, Winds, Storms, and Cyclones, p.88, this high-pressure sap is pushed toward the 'sink' where pressure is lower. Here is a quick comparison to keep the two transport systems clear:

Feature	Xylem (Water/Minerals)	Phloem (Food/Sucrose)
Direction	Unidirectional (Upward)	Bidirectional (Source to Sink)
Energy	Passive (Physical forces)	Active (Uses ATP)
Tissue Location	Outer wood/Inner layers	Inner bark layer

A classic way to demonstrate this is the Girdling Experiment. If you remove a ring of bark from a tree (which contains the phloem), the downward flow of nutrients to the roots is cut off. While the xylem remains intact and the leaves stay green for a while, the roots eventually starve because they aren't receiving the energy-rich carbohydrates they need. This starvation leads to the eventual death of the entire tree Environment, Shankar IAS Academy (10th ed.), Chapter 13: Plant Diversity of India, p.204.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94-96; Science, Class VIII (NCERT 2025 ed.), Pressure, Winds, Storms, and Cyclones, p.88; Environment, Shankar IAS Academy (10th ed.), Chapter 13: Plant Diversity of India, p.204

4. Plant Metabolism: Photosynthesis and Root Respiration (intermediate)

To understand plant metabolism, we must view the plant as a coordinated system where energy is produced in one 'department' and consumed in another. The leaves act as the primary 'food factories' of the plant. Through photosynthesis, they capture solar energy using a green pigment called chlorophyll, located within chloroplasts Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.15. Using water (H₂O) and carbon dioxide (CO₂), they synthesize glucose (C₆H₁₂O₆), a simple carbohydrate that stores chemical energy Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.146. While photosynthesis is the 'building' phase, it only occurs in the green, light-exposed parts of the plant.

However, every living cell in a plant—including the roots buried deep underground—requires energy to stay alive and function. Since roots lack chlorophyll and are not exposed to sunlight, they cannot produce their own food. They depend entirely on the translocation of sugars from the leaves through a specialized vascular tissue called the phloem Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.150. Once the roots receive these energy-rich carbohydrates, they undergo respiration. This is the process of breaking down glucose in the presence of oxygen to release energy, which the roots then use for active transport (absorbing minerals against a concentration gradient) and cellular growth Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.149.

The relationship between these two metabolic processes is summarized below:

Feature	Photosynthesis	Respiration
Primary Location	Leaves (Chloroplasts)	All living cells (Roots, Stems, etc.)
Energy Change	Stores energy (Endothermic)	Releases energy (Exothermic)
Key Role	Synthesis of glucose	Breakdown of glucose for work

This metabolic link is vital: the leaves provide the 'fuel' (sugars), while the roots provide the 'raw materials' (water and minerals absorbed via energy-intensive processes). If the flow of sugar to the roots is interrupted, the roots lose their source of fuel, leading to a metabolic shutdown even if water and minerals are abundant in the soil.

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.15; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.146, 149, 150

5. Secondary Growth and the Anatomy of Bark (intermediate)

While primary growth helps a plant grow taller, Secondary Growth is what allows a tree to grow wider, forming the thick, woody trunks we see in nature. This process is driven by a specialized layer of living cells called the Vascular Cambium, which acts like a biological factory. This cambium layer produces new cells in two directions: it adds secondary xylem (wood) toward the inside and secondary phloem toward the outside Environment, Shankar IAS Academy (10th Ed), Plant Diversity of India, p.204. This constant addition of layers is why trees develop annual rings, allowing us to track their age and growth history.

The term "Bark" is often used loosely, but anatomically, it refers to all tissues outside the vascular cambium. It is essentially divided into two functional zones: the outer bark and the inner bark. The outer bark consists of dead cells that serve as a tough, protective shield against physical injury and water loss. The inner bark, however, is a thin layer of living tissue known as the Phloem. Its primary role is the translocation of "photoassimilates"—the energy-rich sugars (sucrose) created in the leaves during photosynthesis—to the rest of the plant, especially the roots Science, Class X (NCERT 2025), Life Processes, p.96.

Tissue Type	Location	Primary Function
Xylem (Wood)	Inner core	Transports water and minerals upward from roots.
Phloem (Inner Bark)	Just inside the bark	Transports sugars downward from leaves to roots.
Cambium	Between Xylem & Phloem	Meristematic layer that creates new growth (girth).

A critical physiological consequence of this anatomy is seen in a process called girdling (or ring-barking), where a circular strip of bark is removed. Because the phloem is located in that inner bark layer, removing it severs the food supply line to the roots. While the Xylem remains deep and intact, continuing to send water up to the leaves, the roots eventually starve because they cannot receive the energy they need to survive Science, Class X (NCERT 2025), Life Processes, p.94. Without functional roots, the entire tree eventually dries up and dies.

Sources: Environment, Shankar IAS Academy (10th Ed), Plant Diversity of India, p.204; Science, Class X (NCERT 2025), Life Processes, p.94, 96

6. The Girdling (Ringing) Experiment (exam-level)

The Girdling (or Ringing) Experiment is a classic physiological demonstration used to identify the tissue responsible for the translocation of food in plants. In this experiment, a ring of bark is carefully removed from the trunk of a tree, down to the depth of the vascular cambium. Because the phloem is located in the inner layer of the bark, this process effectively severs the downward pathway for nutrients. Interestingly, the xylem—which is located deeper in the woody part of the trunk—remains intact, allowing the upward movement of water and minerals to continue undisturbed Science, Class VII NCERT, Chapter 10, p. 150.

The immediate result of girdling is the interruption of photosynthates (mainly sucrose) moving from the leaves (the 'source') toward the roots (the 'sink'). Since the roots are below the ring and no longer receive these energy-rich carbohydrates, they eventually exhaust their stored reserves and begin to starve. You will often observe a swelling of the bark just above the ringed area, caused by the accumulation of food material that can no longer move downward Science, Class X NCERT, Chapter 5, p. 96. While the tree may look healthy for a short period because the leaves are still receiving water, the death of the roots eventually leads to the death of the entire plant.

Feature	Xylem (Inner Wood)	Phloem (Inner Bark)
Primary Function	Transport of water and minerals	Transport of food (sucrose)
Direction of Flow	Unidirectional (Upward)	Bidirectional (Source to Sink)
Status after Girdling	Intact (Water still flows)	Severed (Food flow stops)

Beyond physiology, the bark also serves as a protective layer against environmental extremes, such as excessive cold in coniferous forests Physical Geography by PMF IAS, Chapter: Climatic Regions, p. 469. This experiment highlights why damage to the bark, whether by animals or human activity, can be fatal to even the largest trees.

Sources: Science, Class VII NCERT, Life Processes in Plants, p.150; Science, Class X NCERT, Life Processes, p.96; Physical Geography by PMF IAS, Climatic Regions, p.469

7. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize your knowledge of plant anatomy with the functional roles of vascular tissues. You have learned that Xylem is located deeper within the wood and transports water upwards, while Phloem is situated in the inner layer of the bark and is responsible for the translocation of organic nutrients. When the bark is removed in a circular fashion (a process known as girdling), you are effectively severing the phloem while leaving the xylem largely intact. As explained in Science, Class X (NCERT 2025 ed.), the phloem is the vital pathway for moving energy-rich sugars from the leaves down to the roots.

The logical progression to the correct answer involves identifying which life process is halted first. Since the xylem remains functional, water can still reach the leaves initially, ruling out Option (A). However, because the phloem is gone, the roots no longer receive the glucose produced during photosynthesis. Without this constant supply of food, the Roots are starved of energy, leading to their eventual death. Once the roots fail, they can no longer absorb water or provide structural support, causing the entire tree to dry up and die. This specific mechanism is highlighted in Science-Class VII, NCERT (Revised ed 2025) as a primary cause of tree mortality in girdling incidents.

UPSC frequently uses "distractor" options to test the depth of your conceptual clarity. Option (C) regarding microbes and Option (D) regarding oxygen are secondary factors or entirely unrelated to the physical act of removing bark. While a wound might eventually attract infection, it is the metabolic failure of the root system due to nutrient deprivation that is the direct and immediate cause of death. By remembering that bark equals phloem and phloem equals food transport, you can confidently navigate through these traps to arrive at the Correct Answer: (B).