How It Works: The Transpiration-Cohesion-Tension Theory

How It Works: The Transpiration-Cohesion-Tension Theory

Water travels from roots to leaves without any pump — the plant uses physics alone. Here is the chain of events:

1. Transpiration pull: Water evaporates from the surfaces of mesophyll cells inside the leaf. It then diffuses through air spaces and exits via open stomata. This continual loss creates a region of lower water potential at the top of the xylem.
2. Tension: As water leaves the xylem at the top, it creates a negative pressure (tension) that acts like suction, pulling the water column upward.
3. Cohesion: Water molecules are attracted to each other by hydrogen bonds — they "stick together." Because of this cohesion, the entire column of water moves as a continuous thread rather than breaking apart under tension.
4. Adhesion: Water also sticks to the inner walls of xylem vessels. This adhesion prevents the water column from pulling away from the walls and helps support the column against gravity.
5. Root uptake: As water is pulled out of root xylem, the lowered water potential draws more water in from soil by osmosis through root hair cells — maintaining a continuous stream.

Crucially, this entire process requires no metabolic energy from the plant. It is powered entirely by solar energy driving evaporation at the leaf surface.