The process that exerts the pull on water molecules-relayed from leaf to root via cohesion-is called transpiration pull , which is a central component of the cohesion-tension theory.
How It Works
- Transpiration is the evaporation of water from the stomata (tiny pores) on the leaf surface.
- As water evaporates from the leaf, it creates a negative pressure (tension) in the leaf's air spaces and xylem vessels.
- This negative pressure pulls on the next water molecules in the chain, thanks to the cohesive property of water (water molecules stick together via hydrogen bonds).
- The pull is transmitted down the continuous column of water molecules through the xylem, all the way to the roots
- Adhesion (water sticking to xylem walls) helps maintain the integrity of this column against gravity.
Summary Table
Process| Description
---|---
Transpiration Pull| Negative pressure from water evaporation at leaves pulls
water upward through the plant
Cohesion| Water molecules stick together, forming an unbroken column
Adhesion| Water molecules stick to the xylem walls, supporting upward movement
against gravity
In essence, the "pull" on water molecules that is relayed from leaf to root via cohesion is created by transpiration at the leaf surface, and this mechanism is known as transpiration pull, as described by the cohesion-tension theory.
