Transpiration is the process by which water moves through a plant from the roots through the xylem to the leaves, where it evaporates from the underside of the leaf and escapes as water vapor through openings called stomata. An experiment is described that uses a potometer to measure the rate of transpiration by recording the distance a bubble moves in a capillary tube as water is carried up through a cut plant stem. The document discusses factors that influence the rate of transpiration such as humidity, light intensity, temperature, air movement, and mechanisms plants use to control water loss like closing stomata.

1. Transpiration
Lesson objectives
• All will be able to describe transpiration
• Most will be able to explain differences in
transpiration speed
• Some will suggest reasons why plants
compromise between gas exchange and water
loss

2. How do tall trees get enough water?
Redwoods are the tallest species of tree in
the world - the tallest was 120 metres.
How does a plant this size get water
from its roots to the branches at the
top?
What experiment could show that water
travels up stems?

3. Transpiration
Transpiration can be measured using a potometer.
A cut plant stem is sealed into the
potometer using a rubber bung.
An air bubble is introduced
to the capillary tube.
The distance the bubble
travels shows how much
water the stem has taken up.

5. 1. Record a definition of transpiration
2. Where do plants lose water?
3. Transpiration will often lead to water loss by the plant so
what is its purpose and what is the major advantage?

6. Transpiration
• Sort the statements out into the correct order
to show how water moves to the top of the
plant

7. Transpiration
Experiment Distance bubble moved after each minute in mm
Time in
1 2 3 4 5 6 7 8 9 10
min
A 2 3 5 7 10 11 13 15 18 20
B 13 24 38 49 73 75 89 114 118 152
C 5 12 18 24 30 35 41 47 53 58

8. Is the rate of transpiration constant?
Transpiration is a bit like a straw, pulling water up
the plant. Sometimes the pulling force will be
stronger and the plant will lose more water.
The speed at which a plant loses water is called
the rate of transpiration.
What environmental factors will affect
the rate of transpiration?
 humidity (amount of moisture in the air)
 light intensity
 temperature
 air movement (wind).

10. How does a plant control water loss?
1. Waxy waterproof layer (cuticle)
2. Stomata on the underside of the leaf (not
expose to sun)
3. Wilting. Leaves collapse reducing surface
area
4. Stomata close