For which one of the following is capillarity not the only reason ?

1. Surface Tension and Intermolecular Forces (basic)

To understand how a massive tree transports water hundreds of feet into the air, we must first look at the microscopic world of intermolecular forces. All matter is composed of particles that are held together by attractive forces, known as interparticle attractions. The strength of these forces depends on the nature of the substance and the distance between its particles; even a tiny increase in distance can cause these forces to drop significantly Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.101. In liquids like water (H₂O), these forces manifest in two critical ways: Cohesion (the attraction between similar molecules, like water to water) and Adhesion (the attraction between different molecules, like water to the walls of a plant vessel).

Surface tension is a direct result of these cohesive forces. Inside a body of liquid, a molecule is pulled in all directions by its neighbors. However, at the surface, the molecules lack neighbors above them, so they are pulled inward, creating a tension that makes the surface behave like a stretched elastic membrane. This is why water forms spherical droplets rather than spreading flat on certain surfaces. For instance, when water is placed on an oiled or waxed surface, the cohesive forces within the water are stronger than the adhesive forces between the water and the oil, causing the water to pull itself into a neat, round drop Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.162.

When these forces act within very narrow spaces, we observe capillarity (or capillary action). This is the same principle that allows blotting paper to soak up ink or cotton fibers to draw up water—the liquid is literally pulled into the tiny pores by the combined power of adhesion and cohesion. In the world of plants, these forces are essential for maintaining a continuous column of water within the xylem, though as we will see in later steps, they are just one part of a much larger hydraulic system.

Force Type	Definition	Role in Water Transport
Cohesion	Attraction between like molecules (H₂O to H₂O).	Keeps the water column continuous and unbroken.
Adhesion	Attraction between unlike molecules (H₂O to Cell Wall).	Helps water "stick" to the sides of narrow tubes to resist gravity.

Sources: Science, Class VIII (NCERT 2025 ed.), Particulate Nature of Matter, p.101; Science, Class VIII (NCERT 2025 ed.), Light: Mirrors and Lenses, p.162

2. Capillarity: Principles and Daily Applications (basic)

Capillarity (or capillary action) is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. This phenomenon is driven by two fundamental intermolecular forces: adhesion (the attraction between the liquid molecules and the surface of the tube) and cohesion (the attraction between the liquid molecules themselves). When the adhesion to the walls is stronger than the internal cohesion of the liquid, the liquid is drawn upward, forming a curved surface known as a meniscus.

In our daily lives, we see capillarity in action everywhere. It is the reason why a sponge absorbs water, why a cotton towel wicks moisture away from your skin, and how a candle wick draws melted wax upward to keep the flame burning. Similarly, in the soil, capillarity allows moisture to move through the tiny pores between soil particles, ensuring that water reaches the root zone of plants even when the water table is slightly lower.

In plant anatomy, this principle is vital but has its limits. Water enters the narrow conducting tubes called xylem, where capillary action helps it begin its upward journey. However, it is a common misconception that capillarity alone can move water to the top of a tall tree. Because the force of gravity eventually offsets the upward pull of capillarity, it can only lift water a short distance. To reach the high foliage, plants must combine capillarity with root pressure and the much more powerful transpiration pull—a suction force created when water evaporates from the leaves Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 95.

Force	Description	Role in Capillarity
Adhesion	Attraction between different substances (e.g., Water & Cellulose)	Pulls the liquid "up" the sides of the tube.
Cohesion	Attraction between similar molecules (e.g., Water & Water)	Holds the liquid column together so it moves as one.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93-95

3. Soil Moisture and Underground Water Movement (intermediate)

To understand how plants survive, we must first understand the "pantry" they draw from: the soil. Soil moisture refers to the water held within the tiny pore spaces between soil particles. While it represents a tiny fraction of Earth's total water (roughly 0.005%), it is the primary reservoir for terrestrial life and actually contains more water than all the world's river channels combined Majid Hussain, Environment and Ecology, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.22.

Water moves through the soil via two primary, opposing forces: Percolation and Capillary Action. Percolation is the downward movement of water driven by gravity; its speed depends heavily on soil texture—sandy soils percolate quickly, while clayey soils hold water tightly Majid Husain, Geography of India, Soils, p.4. Conversely, capillary action is the ability of water to move upward or sideways against gravity through narrow pores. This happens because of adhesion (water sticking to soil surfaces) and cohesion (water molecules sticking to each other). In dry climates, this process can pull deep groundwater to the surface where it evaporates, often leaving behind mineral crusts known as hardpans or calcium carbonate nodules called kanker NCERT Class XI, Fundamentals of Physical Geography, Geomorphic Processes, p.45.

For a plant, the amount of moisture is a "Goldilocks" problem. Too little water leads to moisture stress, a common challenge in dryland farming where rainfall is less than 750mm Shankar IAS Academy, Environment, Agriculture, p.359. However, too much water (water-logging) is equally dangerous. Excessive water displaces the air in soil pores, starving roots of oxygen and triggering the formation of toxic compounds that stunt plant growth Majid Husain, Geography of India, Agriculture, p.18. Thus, healthy plant physiology relies on a well-drained soil structure that balances water retention with aeration.

Process	Direction	Primary Driver	Key Result
Percolation	Downward	Gravity	Recharges groundwater/aquifers
Capillary Action	Upward/Lateral	Surface Tension (Adhesion/Cohesion)	Brings moisture (and salts) to the surface

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.22; Geography of India, Majid Husain, Soils, p.4; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT), Geomorphic Processes, p.45; Geography of India, Majid Husain, Agriculture, p.18; Environment, Shankar IAS Academy, Agriculture, p.359

4. Plant Anatomy: The Xylem Conductive System (basic)

In complex plants, the transport of water and minerals is managed by a specialized vascular tissue called Xylem. Imagine the xylem as a series of interconnected 'pipes'—specifically vessels and tracheids—that span from the roots, through the stem, and into the leaves Science, Class X, Life Processes, p. 94. Unlike our circulatory system which uses a heart to pump blood, plants rely on physical forces and concentration gradients to move fluids upward against gravity. This system is strictly unidirectional, moving raw materials from the soil toward the foliage where they are needed for photosynthesis Science, Class X, Life Processes, p. 94.

The movement starts at the roots through a process of active ion uptake. Root cells use energy to absorb minerals from the soil, creating a concentration difference between the root and its environment. To balance this, water naturally moves into the root, creating root pressure that pushes water slightly upward Science, Class X, Life Processes, p. 94. For taller plants, however, this pressure isn't enough. They utilize capillarity—the ability of water to climb narrow tubes due to its 'stickiness' (adhesion and cohesion)—but even this only works over short distances.

The 'engine' that drives water to the top of the tallest trees is transpiration pull. As water evaporates from the pores (stomata) in the leaves, it creates a suction force. Because water molecules are cohesive (they stick together like a chain), this suction pulls the entire column of water upward from the roots to the leaves. To keep this system clear, it's helpful to compare it to the plant's other transport system:

Feature	Xylem System	Phloem System
Primary Cargo	Water and dissolved minerals	Food (sucrose) and amino acids
Direction	Upward (Unidirectional)	Upward and Downward (Bidirectional)
Main Force	Transpiration pull and root pressure	Pressure-driven translocation

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.94; Science, Class X (NCERT 2025 ed.), Life Processes, p.95

5. Transpiration Pull and the Ascent of Sap (intermediate)

To understand how a tree taller than a skyscraper moves water from its roots to its highest leaves without a mechanical pump like a heart, we must look at the Ascent of Sap. This process is primarily driven by Transpiration Pull. Think of the water inside a plant's xylem vessels as a continuous, unbroken chain. When water evaporates through the stomata in the leaves—a process called transpiration—it creates a 'suction' or negative pressure at the top of this chain. This suction pulls the entire column of water upward to replace what was lost Science, Life Processes, p.95. This mechanism is so powerful that it can overcome the massive force of gravity in even the tallest redwood trees.

While the 'pull' comes from the top, the water column stays intact because of two critical physical properties: cohesion (water molecules sticking to each other) and adhesion (water molecules sticking to the walls of the xylem). This is known as the Cohesion-Tension Theory. While capillarity (the tendency of liquid to rise in narrow tubes) assists this movement over very short distances, it is insufficient on its own for the scale of a whole plant. Instead, it is the constant evaporation of H₂O molecules that creates the primary driving force during the day when stomata are open Science, Life Processes, p.95.

It is important to distinguish between the forces at play during different times. While transpiration pull dominates during the day, root pressure—a 'pushing' force generated by active transport of ions into the roots—plays a more significant role at night when transpiration is low. Beyond just transport, this entire process is a vital component of the hydrological cycle, as plants contribute significantly to atmospheric moisture through evapotranspiration Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.325.

Feature	Transpiration Pull	Root Pressure
Nature of Force	Suction / Pull (Negative Pressure)	Hydrostatic / Push (Positive Pressure)
Primary Timing	Daytime (Stomata open)	Nighttime (Stomata closed)
Scale	Responsible for long-distance transport	Effective only over short heights

Sources: Science, Life Processes, p.95; Physical Geography by PMF IAS, Hydrological Cycle (Water Cycle), p.325

6. Root Pressure and Osmotic Gradient (exam-level)

To understand how water climbs a 100-meter tree, we must first look at the very bottom: the roots. The journey begins with an osmotic gradient. Plants do not just wait for water to seep in; they actively take up mineral ions from the soil into the root cells. This creates a difference in concentration between the root and the soil. Because the concentration of solutes is now higher inside the root, water naturally moves into the root from the soil to eliminate this difference through osmosis Science, Class X (NCERT 2025 ed.), Chapter 5, p. 95.

This continuous inward movement of water creates a hydrostatic pressure known as Root Pressure. Think of it as a "pushing force" from the base. As water enters the xylem—the plant's dedicated water-conducting tissue—it creates a column of water that is steadily pushed upward. While this pressure is significant, and can even be strong enough to mechanically break apart earth materials as roots grow Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Chapter 6, p. 41, it has its limits. In the grand scheme of a tall tree, root pressure is usually only sufficient to move water over short distances or during the night when transpiration is low.

To reach the highest canopy, plants rely on a combination of this "push" from the roots and a "pull" from the leaves. In the xylem vessels, water molecules stick to each other (cohesion) and to the walls of the tube (adhesion/capillarity). However, root pressure ensures that the water column remains continuous and primed, preventing air bubbles from breaking the flow Science, Class X (NCERT 2025 ed.), Chapter 5, p. 95.

Feature	Root Pressure (The Push)	Transpiration Pull (The Pull)
Direction	Upward from the base	Upward from the top
Cause	Active ion uptake and Osmosis	Evaporation from leaf stomata
Dominance	Major at night/in small plants	Major during the day/in tall trees

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.95; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Chapter 6: Geomorphic Processes, p.41

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental mechanics of surface tension, adhesion, and cohesion, this question challenges you to apply those building blocks to real-world systems. Capillarity occurs when the adhesive force between a liquid and a solid surface is stronger than the cohesive forces within the liquid, causing it to rise in narrow spaces. In inanimate objects like blotting paper or cotton fabric, this physical force is the sole driver of movement. However, the UPSC often tests your ability to distinguish between a simple physical phenomenon and a complex biological process where multiple forces are at play simultaneously.

To arrive at the correct answer, (D) Rising of water from the roots of a plant to its foliage, you must recognize that while capillary action does exist within the narrow xylem vessels, it is insufficient on its own to transport water to the top of tall trees. As explained in NCERT Science Class X, this upward movement is a multi-factor process driven largely by transpiration pull—a suction effect created by evaporation at the leaves—and root pressure. Capillarity is merely a supporting contributor in this biological pump, not the only reason, which is the specific condition the question asks you to identify.

The other options are classic distractors designed to test your understanding of porous media. In (A) Blotting of ink, (B) Rising of underground water through soil pores, and (C) Spread of water on cotton cloth, the narrow gaps between fibers or soil particles act as capillary tubes. In these inanimate examples, there is no biological suction or metabolic pressure; the movement is exclusively driven by capillary action. Always look for the "biological exception" in these types of General Science questions, as it frequently points to the limit of a purely physical explanation.