Name four factors that affect the rate of transpiration.
Reveal answer
Light intensity, temperature, humidity and wind speed.
Key words: light · temperature · humidity · wind
3(h) TRANSPORT · RECALL
3.7B Factors affecting the transpiration rate
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Why does high humidity slow transpiration?
Reveal answer
More water vapour outside → smaller concentration gradient → slower diffusion out.
Key words: gradient
Why does wind speed up transpiration?
Reveal answer
Removes water vapour near leaf → steeper gradient → faster diffusion.
Key words: gradient
What is meant by the term boundary layer?
Reveal answer
Layer of relatively still, humid air beside a leaf.
Key words: boundary layer
What is meant by the term humidity?
Reveal answer
Amount of water vapour in air.
Key words: humidity
Environmental effects: for Temperature, give the typical effect and the reason.
Reveal answer
Typical effect: Rate increases. Reason: Faster evaporation and diffusion.
Key words: Temperature
Environmental effects: for Humidity, give the typical effect and the reason.
Reveal answer
Typical effect: Rate decreases. Reason: Smaller vapour gradient.
Key words: Humidity
Environmental effects: for Wind speed, give the typical effect and the reason.
Reveal answer
Typical effect: Rate increases. Reason: Humid boundary layer is removed.
Key words: Wind speed
Environmental effects: for Light intensity, give the typical effect and the reason.
Reveal answer
Typical effect: Rate increases then may plateau. Reason: Stomata open; another factor can become limiting.
Key words: Light intensity
True or false, and why? “Wind closes stomata and always lowers transpiration.”
Reveal answer
False. Moderate wind usually removes humid air and increases loss; severe stress may trigger closure.
True or false, and why? “Higher humidity increases transpiration because more water is present.”
Reveal answer
False. It reduces the gradient for water-vapour diffusion.
True or false, and why? “Light directly evaporates water.”
Reveal answer
False. Its principal specified effect is often stomatal opening, while radiant heating may also affect temperature.
Why does a higher temperature speed up transpiration?
Reveal answer
Water molecules gain kinetic energy → faster evaporation and diffusion of water vapour out of the leaf.
Key words: evaporation
Why does brighter light increase transpiration?
Reveal answer
More stomata open for photosynthesis → more water vapour diffuses out.
Key words: stomata
Which apparatus measures the rate of water uptake by a leafy shoot?
Reveal answer
A potometer.
Key words: potometer