Investigating Transpiration

Question 1

What is a potometer?

Answer 1

Apparatus used to measure rate of transpiration in plant shoots by measuring rate of water uptake
(assuming the two are equal).

Question 2

Why is rate of transpiration not the same as water uptake?

Answer 2

Shoots use some water to make cells turgid.
Shoots use some water in photosynthesis & produce some water in respiration.
Some water leaks from the apparatus

Question 3

Why is grease used with a potometer?

Answer 3

Grease blocks stomata and prevents all
gases (including water vapour) from entering and leaving leaves.

Question 4

What are the precautions when using a potometer?

Answer 4

•Cut the shoot under water
→ to prevent air entering the xylem
•Cut the shoot at a slant
→ increases s/a for water uptake
•Seal all joints with grease
→ reduce leakage of water
•Dry leaves on the shoot before starting measurements
→ so rate of transpiration is not affected by humidity.

Question 5

Precautions in comparing shoots from different plants. (Potometer)

Answer 5

Use shoots from plants of the same age
→ for valid comparison
→ e.g. cuticle is often thicker on older leaves