Transpiration is the loss of water vapour from the leaves. This is obviously caused by evaporation. The lost of water causes a suction – or low pressure in the stem, which pulls the water up the xylem in a continuous flow known as the transpiration stream. It is like sucking water from a straw.
Role of Stomata in Transpiration 🌱
The reason why stomata is found usually on the lower surface of the leaves, is because it would lose too much water if exposed to direct sunlight. Too much transpiration can be bad and lead to excessive water loss. This is where the stomata come to help! When the guard cells are turgid from water, they bend outwards, opening the stomata, to let water diffuse out of the plant. When the guard cells are flaccid (usually at night or after water leaves), it closes the stomata, preventing more water from being lost.
Measuring the Rate of Transpiration
Everything must be placed in a sink whilst setting up to prevent formation of air bubbles. The shoot is cut at an angle to make it easier to push into the rubber tubing. Vaseline can be used to seal joints. A capillary tube is used to magnify the uptake. Time taken for the bubble to move along the scale is recorded.
Other things to keep constant:
- Size/Mass of Leaf
- No. of leaves
- Species of leaves
- Age of leaves
- Condition of leaves (starting amount of starch…etc)
Factors that affect Transpiration 🧐
How It Can Be Kept Constant
Increases transpiration because stomata open during the day and not at night.
High temperatures increase the rate of transpiration, by increasing the rate of evaporation of water as water particles gain more energy.
If the air is humid, this reduces the diffusion gradient. The opposite happens when the air is dry.
Increases rate of transpiration with faster air movements across the surface of the leaf. It removes any water vapour near the stomata.
That's the end of the topic!
Drafted by Joey (Biology)