The vertical conduction of water from root to aerialparts of plant is called ascent of sap. The water molecules remain joined to each other due to a force of attraction called cohesion force. This attraction is due to the presence of hydrogen bonds between them. The magnitude of this force is very high therefore, the continuous water column in the xylem cannot be broken easily due to the force of gravity or other obstructions offered by internal tissues in the upward movement of water. This adhesive property of water, ie, attraction between the water molecules and the walls of xylem ensures the continuity of water column in xylem. Due to transpiration water evaporates to outer atmosphere through stomata. As a result leaf cells develop low water potential and water from leaf veins moves into leaf cells. The xylem vessel in turn draw water from xylem of main stem. A pressure (pull) is thus, exerted by all the leaves on the stems, which is called transpiration pull. This is strong enough to pull up the column of water to a great height.
The rupture and fractionation do not usually occur in the water column in vessel/tracheids during the ascent of sap because of
NEET UG 2008 — Botany Anatomy of Flowering Plants
The rupture and fractionation do not usually occur in the water column in vessel/tracheids during the ascent of sap because of
Held on 30 Apr 2008 · Verified 9 Jul 2026.
Options
- A
lignified thick walls
- B
cohesion and adhesion
- C
weak gravitational pull
- D
transpiration pull
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