II. Water Transport Mechanism

According to the transpiration-adhesion-cohesion-tension mechanism of water transport in xylem, water evaporating through the stomata produces a tension, or negative pressure, that pulls the water column up the plant. This column is maintained by the hydrogen bonding between water molecules (cohesion) and the hydrogen bonding between water molecules and the molecules that make up the cell walls (adhesion).

If the above mechanism is correct, then a twig that has been removed from the rest of the plant and attached to an artificial water column (as in a pipette) could pull water up the pipette. The rate of movement can be determined by adding a dye to the water at the bottom of the pipette.

Begin Experiment:

1. Place a transport apparatus (1 mL pipette with a piece of clear tubing on the larger end) under water using the large dish pan. Manipulate the apparatus until it is completely filled with water - NO air bubbles.

To do this, click on the yellow tubing to place it on the pipette. Next, click on the pipette to place it in the dish pan. It will fill with water.

2. Place the cut end of the juniper twig under water and cut off another centimeter to insure that there are no bubbles in the transport tissue that would disrupt water transport. Insert the freshly cut end into the above tubing while under water, making certain that NO air bubbles are trapped within the assembly. If the twig does not fit tightly in the clear tubing, wrap several strips of Parafilm around the stem before inserting it into the tubing. Attach the twig firmly with wire using the pliers.

Click on the juniper twig to place it into the dish pan. Click on the knife to cut off an additional cm under water. Next click on the twig again to insert it into the tubing. For our purposes, assume it fits snugly.

3. Add about 20 mL of red colored dye (Congo red) to the bottom of a small beaker. With your finger over the tip of the pipette, transfer the assembly so that the tip is now into the dye. Remember: NO air bubbles!

Click on the test tube to add the Congo red. Next click on the pipette/twig assembly to place it into the test tube.

4. Transfer the whole thing to the ring stand. Clamp it into place and turn on the light at the top of the assembly.

Click on your set up to complete assembly.

5. This same set-up is repeated with a juniper stem without leaves to use as a control.

Click here to see the control's set-up.

6. At periodic intervals, look for a movement of dye up the pipette. Click here to start the experiment.

NOTE: The movement of the dye observed takes approximately half an hour in real time.

Click here to see the set-up and results of this experiment from a laboratory class. Click on either beaker in the experiment to get a close-up of the final results.

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