In a weight potometer the mass of water lost is measured
by recording the loss in mass over a period of time
Four leafy shoots are used.
A: one left untreated
B: top surface
covered in vaseline
C: lower surface covered in vaseline
D: Leaves removed from shoot
E: CONTROL/ no shoot
Once the shoots are in the water, put cooking
oil carefully onto the surface of the water
Then weigh the tubes
and weigh them
again after 48hrs
RESULTS
Tube A lost the most mass
No stomata were covered so there
was nothing to prevent water loss
so the water loss was greatest
Tube D lost the least
mass (not including E)
There were no stomata for the water to
evaporate out of so water loss was minimised
and the plant did not lose much mass
The difference between tubes B and C...
Tube B lost more mass than C because the lower surface of
the leaf contains more stomata, so as it is covered in C it will
conserve more weight and have a lower change in mass.
Why was oil put over the surface of the water in the flasks?
To prevent any water
evaporating from the
flask, not the stomata
What is the function of tube E?
Control - to prove that it was the
varying leaves affecting their mass
What must be kept constant?
Volume of water, temperature
of water, species of leaf, light
intensity, humidity