Demonstration of Transpiration Pull
There are three mechanisms which facilitate movement of water from the ground through the stem to the leaves: root pressure, transpiration pull and capillarity. Transpiration pull occurs when water evaporates through the stomata. As water evaporate through the stomata, mesophyll cells draws water from the xylem of the leaves, which in turn draws water from xylem in the stem. This create a tension called transpiration pull.
To conduct experiments to demonstrate transpiration pull.
Beaker*, narrow tubes from a used ink pen, cover from syringe needle, GV stain*, super glue, and the stem from a plant with leaves attached.
GV stain skin and clothes.
1. Cut the closed end of the syringe cover to t the ink pen tube. Fit the ink pen tube into the hole in the syringe cover.
2. Seal the junction between the syringe cover and the tube with super glue so that air or water cannot pass through.
3. Cut a plant to get leafy shoots without roots.
4. Prepare coloured water using few drops of GV.
1. Fill the beaker with coloured water.
2. Fill the tube with clean water, making sure that one air bubble is created in the tube. Mark the location of the air bubble using pen.
3. Fix the stem into the open part of the syringe cover.
4. Transfer the ink pen tube-syringe cover-plant set up into the coloured water in the plastic container. Hold the set-up so that the tube does not touch the bottom of the container. Place near a window.
5. Observe the set up every 15 minutes and note the upward movement of air bubble through the tube.
The leaves and stem will draw up water, causing the air bubble to move up. The higher the rate of transpiration, the higher the speed of the moving air bubble.
Collect all the used materials, cleaning and storing items that will be used later. No special waste disposal is required.
Capillarity and root pressure are not enough to push water to the leaves. Root pressure facilitate movement of water to the leaves. However, the tran-spiration pull is associated with water loss. Therefore, transpiration pull facilitates drawing of water upwards, but results in a loss of water. The comelina plant has been tested and works very well for this experiment. The stem of the plant should tightly into the syringe cap.
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