Transport In Plants

Sap ascent in tall trees is best explained by the cohesion-tension theory described in NCERT Class 11, Chapter 11 (Transport in Plants). As water evaporates from the mesophyll cell surfaces into intercellular spaces and out through stomata, it creates a negative pressure or tension at the leaf end of the xylem. Because water molecules are strongly attracted to one another by hydrogen bonding (cohesion) and adhere to the hydrophilic walls of the narrow tracheary elements (adhesion), the continuous water columns in xylem do not break under tension. This transpiration pull is transmitted right down to the roots, drawing water upward against gravity. Root pressure is incorrect as the sole answer because it is weak, develops mainly at night, and cannot account for lifting water in trees over ten metres tall. Xylem parenchyma do not actively pump bulk water upward; xylem conduction is a passive physical process. Capillary rise alone is far too limited to raise water hundreds of metres. A consistency check shows that since transpiration creates measurable negative pressures, the pull mechanism aligns with the physics of unbroken water columns.