Transpiration is the process of water movement through a plant and its evaporation from the aerial parts of a plant, like its leaves.

1. Water vapour in the air spaces of the leaf diffuses out of the leaf through the stomata
2. Water evaporates from mesophyll cells into the air spaces of the leaf
3. Water is pulled out of the xylem vessels into the mesophyll cells to replace that which was lost

The evaporation of water from mesophyll cells in plant leaves causes water to move into mesophyll cells from xylem vessels, which pulls a column of water up the xylem vessels.

Transpiration is a passive process - it does not require metabolic energy.

1. High light intensity - higher rate of photosynthesis so more stomata open to obtain more carbon dioxide
2. High temperature - more kinetic energy means water evaporates faster, and there will also be a higher rate of photosynthesis
3. High wind speed - maintains a steep water vapour diffusion gradient between the air spaces in the leaf and environmental air
4. Low humidity - maintains a steep water vapour diffusion gradient between the air spaces in the leaf and environmental air

Transpiration is a consequence of gaseous exchange because in order for plants to take in carbon dioxide, they have to open their stomata, which water vapour can diffuse out of.

A potometer is a device used to estimate transpiration rate. It does this by measuring the rate of water uptake of a leafy shoot, which is almost equal to the water loss by transpiration.

1. Fill the potometer with water
2. Cut a shoot underwater and fix it to the potometer
3. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap
4. Record the starting position of the air bubble
5. Start a timer and record the distance moved by the bubble
6. Calculate the rate of transpiration: volume of water uptake ÷ time taken