Transpiration is the evaporation of water from the spongy mesophyll cells followed by the diffusion of water through the air spaces and out of the stomata.

Diagram showing transpiration in a leaf. Labelled are the Xylem vessels, Spongy mesophyll cells, and Guard cells. Arrows show the direction taken by the water.

Most of the water that enters a plant will exit the leaf into the atmosphere.

This provides a continuous stream of water which is known as the transpiration stream.

The transpiration stream is important because:

  • It provides leaves with a continuous supply of water (for photosynthesis).
  • It continually transports minerals from the roots to the leaves and other parts of the plant.
  • It provides plant cells with support from turgor.
  • It provides water for transpiration.

If water availability is limited, plants will need to reduce transpiration.

In order to do this, the stomata (mainly found on the underside of the leaf) close.

Factors affecting transpiration

FactorEffect on transpirationHow to change this factor experimentally
Surface area (not an environmental factor)Smaller leaves will contain less stomata causing a decrease in transpiration.Remove leaves.
Wind speedA higher wind speed will increase the rate of evaporation causing an increase in transpiration.Use a fan.
TemperatureA higher temperature will increase the rate of evaporation causing an increase in transpiration.Use a heater.
HumidityA higher humidity level will reduce the rate of evaporation causing a decrease in transpiration.Place a clear, plastic bag over the plant shoot.
Light intensityDarkness will cause the stomata to close causing a decrease in transpiration.Use a lamp.

If less water is being lost through transpiration the plant will absorb less water at the roots.