Q.Discuss the factors responsible for ascent of xylem sap in plants.

Several physical factors together account for how water rises from the roots to the tops of even very tall trees, chiefly explained by the cohesion-tension (transpiration pull) theory:

• Transpiration pull: Evaporation of water from the mesophyll cell walls in the leaves generates a tension (negative pressure) that is the primary driving force pulling the water column upward.
• Cohesion of water molecules: Strong hydrogen bonding between water molecules keeps the water column continuous and unbroken under tension, so the pull at the top is transmitted all the way down to the roots.
• Adhesion of water to xylem walls: Water molecules adhere to the cellulose walls of the narrow xylem vessels/tracheids, which helps support the water column against the pull of gravity. …

The upward movement (ascent) of water and dissolved minerals through the xylem — sometimes over distances of many tens of metres in tall trees — is explained by a combination of physical factors, most comprehensively unified in the cohesion-tension theory proposed by Dixon and Jolly.

Factors responsible:

• Transpiration pull (the main driving force): Water continuously evaporates from the moist walls of mesophyll cells into the sub-stomatal air spaces and diffuses out through open stomata. This loss of water generates a negative pressure (tension) at the evaporating surface within the leaf, which acts as a pull on the water immediately below it in the leaf veins (xylem), and this pull is propagated down the entire length of the xylem to the roots.
• Cohesion of water molecules: Because water molecules are polar and form extensive hydrogen bonds with one another, they exhibit strong mutual attraction (cohesion). This property allows the water inside the very narrow xylem vessels and tracheids to remain as a continuous, essentially unbroken column, even under considerable tension, without the column snapping or forming air bubbles (cavitation) under normal conditions.
• Adhesion of water to xylem walls: Water molecules are also attracted to the hydrophilic cellulose walls of the xylem vessels (adhesion). This adhesive force helps to support the weight of the water column against the downward pull of gravity, working alongside cohesion to keep the column intact and moving.
• Root pressure: A secondary, comparatively minor contributor, root pressure arises from the active accumulation of mineral ions in the root xylem (drawing water in osmotically), building up a modest positive pressure that can push the water column upward, chiefly during periods of low transpiration such as at night. It is not sufficient by itself to explain ascent in tall trees but can supplement the transpiration pull, especially in smaller plants. …

List the physical factors in order of importance — transpiration pull as the main driving tension, cohesion and adhesion as the properties that let that tension act on an unbroken column, and root pressure plus capillarity as mi …

• Treating root pressure as the main mechanism of ascent in tall trees, when it is explicitly a minor, insufficient contributor for that purpose.
• Failing to distinguish cohesion (water-to-water) from adhesion (water-to-vessel-wall) — both are needed and play different roles. …