The pathway of water
Water absorbed by the root hairs follows this route: soil → root hair → root cortex → xylem → stem → leaf. It moves through the xylem as a continuous column, then evaporates from the leaf cells and diffuses out through the stomata as water vapour.
Transpiration and the transpiration pull
Transpiration is the loss of water vapour from a plant, mainly through the stomata of the leaves. As water evaporates from the leaf, it creates a suction force that pulls the whole column of water upward through the xylem. This is the transpiration pull, and the continuous flow of water from root to leaf is the transpiration stream.
The water column does not break because of two properties of water:
- Cohesion — water molecules stick to one another by hydrogen bonds, so the column is pulled up as a single thread.
- Adhesion — water molecules cling to the walls of the narrow xylem vessels.
Root pressure provides a smaller push from below, and capillary action in the very narrow vessels also helps, but transpiration pull is the main driving force.
Key idea
Mineral salts do not have their own transport system — they travel dissolved in the water that moves up the xylem in the transpiration stream.
Factors affecting the rate of transpiration
- Light — opens the stomata, so transpiration is faster in bright light.
- Temperature — higher temperature speeds up evaporation, increasing transpiration.
- Humidity — high humidity lowers the diffusion gradient, so transpiration is slower.
- Air movement (wind) — carries away water vapour, keeping the gradient steep and speeding transpiration.
Example
On a hot, dry, windy afternoon a plant transpires quickly. On a cool, still, humid morning it transpires slowly — a potometer can be used to measure and compare these rates.