Transpiration Process

Transpiration is the loss of around 99% of the water that the plant absorbs from the soil to evaporation through the stomata. The flow of water through the plant is called the transpiration stream. 

Plants rely on a gradient in water potential to move water through their cells. Water flows passively from soil to air along a gradient of decreasing water potential. The gradient in water potential is the driving force in the ascent of water up a plant. 

A number of processes contribute to water movement up the plant:-

1. Transpiration pull.

- Water lost form the air spaces by evaporation through stomata is replaced by water from the mesophyll cells. The constant loss of water to the air creates a lower water potential in the leaf cels than in the cells further from the evaporation site. Water is pulled through the plant along a decreasing gradient in water potential. 

2. Cohesion.

- Water molecules cling together as they are pulled through the plant and they adhere to the walls of the xylem. This creates and unbroken column of water through the plant. The upward pull on the cohesive sap creates a tension (negative pressure). This facilitates water uptake and movement through the plant.

3. Root Pressure.

- Water entering from the soil creates a root pressure; a weak push effect for the water’s upward movement through the plant. Root pressure can force water droplets from some small plants under certain conditions, but generally plays a minor part in the ascent of water

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