During Ascent, water column present inside Vessels and tracheids, does not rupture due to

Question

Accepted Answer

The correct option is 4: Cohesion and adhesion.

Students, let's understand why the water column inside the xylem vessels and tracheids does not rupture, even as it moves against gravity to great heights in plants. This phenomenon is best explained by the cohesion-tension theory, which relies heavily on two crucial properties of water: cohesion and adhesion. Cohesion refers to the strong attractive forces between water molecules themselves, primarily due to hydrogen bonding. This strong intermolecular attraction allows water molecules to stick together, forming an unbroken, continuous column within the narrow xylem conduits, much like a chain. Adhesion, on the other hand, is the attraction between water molecules and the hydrophilic (water-attracting) walls of the xylem vessels and tracheids. This adhesive force helps prevent the water column from pulling away from the vessel walls, providing additional support and resisting the downward pull of gravity and the tension created by transpiration. Together, these forces give the water column a high tensile strength, meaning it can withstand significant pulling forces without breaking, ensuring an uninterrupted flow of water from the roots to the leaves.

The incorrect option is 1: Transpiration pull.
Transpiration pull is the driving force that creates the tension needed to pull water upwards. While it initiates the movement, it is the cohesive and adhesive properties of water that allow the water column to withstand this pull without rupturing. Without cohesion and adhesion, the transpiration pull would simply break the water column.

The incorrect option is 2: Weak gravitational pull.
While gravity does exert a downward force on the water column, it is not weak enough to be ignored. In fact, plants must actively overcome gravity for water ascent. The weakness of gravity alone is not sufficient to explain the integrity of the water column; rather, it highlights the strength of the cohesive and adhesive forces at play.

The incorrect option is 3: Lignified thick walls.
Lignified thick walls of the xylem vessels and tracheids provide structural support to the plant and prevent these conduits from collapsing under the intense negative pressure (tension) generated during transpiration. However, these walls do not directly prevent the water column *itself* from rupturing. Their role is to maintain the integrity of the conducting pipes, not the water inside them.