Potometer





A potometer is a device used for measurung the rate of water uptake of a
leafy plant shoot. The main reason for water uptake by a cut shoot is
transpiration.

By changing the surrounding atmospheric conditions. The effect on
transpiration of wind, heat, and humidity can be measured. Potometers are
notoriously difficult to set up, because air bubbles in the xylem of the
plant or in the apparatus itself will prevent the device from working porperly.

The Basic Design of a Potometer

Potometers come in a variety of designs, but all follow the same basic
principle. The device shown in the diagram is a Ganong's Potometer. It
consists of:

   * A length of capillary tube A bubble is introduced to the capillary. As
     water is taken up by the plant, the bubble moves. The ditance the
     bubble travels in a given time is determined by the rate of water
     uptake by the plant.
   * A resevoir. By turning the tap on the resevoir, the position of the
     bubble can be set at the start of the experiment. {Some designs of
     potometer use a syringe instead of a funnel with a tap).
   * A tube for holding the shoot. In the diagram the shoot is held in place
     by inserting a rubber bung in the tube. The hole in the bung { through
     which the shoot passes must be thoughroughly greased with petrolium
     jelly to keep it airtight.

Setting up

  1. Cut a leafy shoot from a plant {e.g. holly} and plunge it's base into
     water {try not to get any water on the leaves}. This prevents the xylem
     from taking up any air.
  2. Back in the laboratory, put the stem into a large sink full of water
     and carefully trim the shoot again, by cutting off the bottom under
     water with a sharp razor blade. Keep the leaves out of the water.
  3. Immerse the whole of the potometer into the sink. Move it about until
     all the air bubbles come out.
  4. Put the the shoot stem into the bung, grease the joint with plenty of
     petroleum jelly, then put the bung into the potometer.
  5. Make sure the tap is closed, then lift the whole ensamble out of the
     water.
  6. Leave the end of the capillary tube out of the water until an air
     bubble forms then put the end into a beaker of water.

Using a Potometer

Allow the bubble time to round the corner and start at the beginning of the
mm scale. Then time how far the bubble moves in a given period of time.
Repeat under different conditions and compare. The usual conditions to try
is placing the plant in a bright light, placing it by a fan ,and placing it
in a humid atmosphere.