Deadlines

Teaching Events‎ > ‎

Conducting Pathways in Plants (Craig Marsden)


Title. Water Conducting Pathways in the Plant Shoot.

Principle(s) Investigated:

 A syringe is used to force air through a shoot held under water. The appearance of air bubbles shows continuity of the vessels. 

Standards :

5. 2. Plants and animals have structures for respiration, digestion, waste disposal, and transport of materials. As a basis for understanding this concept: 
a. Students know many multicellular organisms have specialized structures to support the transport of materials.  
e. Students know how sugar, water, and minerals are transported in a vascular plant. 

6.5.e The number and types of organisms an ecosystem can support depends on the resources available and on abiotic factors, such as quantities of light and water, a range of temperatures, and soil composition.

6.7; 7.7; 8.9 Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept . . . students should develop their own questions and perform investigations .

6.7.e Recognize whether evidence is consistent with a proposed explanation.

Materials:

Advance preparation and materials

  • Containers. Large, clear glass jars such as 500 ml beakers are needed.
  • Shoots. The stem diameter should be about 5 mm to ensure a good fit in the pressure tubing. The leafy part of the shoot should be small enough to fit inside the container without cutting off any leaves or branches.
  • Syringe. Use 10 cm3 plastic disposable syringes with 4 cm lengths of pressure tubing, 9 mm outside diameter, 3 mm bore attached to the nozzle.

Each group needs one each of the above items.

Apparatus

  • syringe with tubing, as described above
  • glass jar, as above
  • scissors, for cutting leaves 
Procedure
  1.  Fill the large container with water. 
  2.  Withdraw the syringe plunger to the top mark or beyond and fit the syringe to the cut end of the stem by means of the rubber tubing. 
  3.  Immerse the leafy shoot in the container of water and press the syringe plunger (Fig. 2). Keep the pressure as high as possible without breaking the seal between the rubber tubing and the syringe. 
  4.  Maintain the pressure for about 30 seconds and look for any signs of bubbles appearing from the stem or leaves. 
  5.  Cut all or most of the leaves transversely across in the mid-line (Fig. 1) to leave only the lower half of the leaf attached to the  stem. Repeat the experiment, maintaining the pressure for at least 30 seconds. 
  6.  Now cut the leaves off completely, leaving only the petioles(leaf stalks) attached to the stem and repeat the experiment. 
  7.  Finally, cut a piece of stem about 5 cm long. Fill the syringe with water, connect it to the twig, point the apparatus vertically upwards and very slowly depress the plunger to see if it is possible to force water through the stem (Fig. 3).
 

Fig. 1                                                                      Fig. 2                              Fig. 3



Student prior knowledge:
  • Meaning of the terms 'stomata', 'lenticels', 'petiole'.
  • Knowledge of basic water chemistry and ionic bonding.

Explanation

Video 1 gives a common misconception regarding the release of water through the stomata in the leaf surface of plant leaves. In this presentation, water is shown as a moving in a continuous chain from root to stomata opening. Leaf morphology does not support this model.

Water moves up through the plant stem inside xylem channels utilizing transpiration, adhesion, cohesion, and tension (TACT). Water moves from the leaf vein into the spongy mesophyll through bundle sheath membranes across a water potential gradient.The two components of water potential are 1) the differences in osmotic concentration (concentration of solute) between two regions and 2) the differences in water pressure (created because of the rigidity of plant cell walls).

Once into the spongy mesophyll, water primarily exists as water vapor. Spongy mesophyll cells utilize water vapor for cooling, turgor and for photosynthesis. Stomata open to allow the exchange of water vapor and O2 with "fresh air" containing CO2 necessary for photosynthesis (Fig.4). When stomata are closed water vapor is near saturation in the spongy mesophyll. When stomata open, gas exchange occurs and the water potential gradient is induced through evapotranspiration. 

Our experiment shows that no air is pushed through the conductive tissue of a complete leaf surface Because the water chain is broken inside the leaf. Air is however, pushed through the cut leaf veins and through the petiole. Water is pushed through the cut stem.(Figure 3)
Fig. 4   Water pathway in leaf                                    Fig. 5  Leaf morphology


Fig. 6   Stem cross section  with lenticels                   Fig. 7 Electron microscopy of Walnut leaf.


Questions & Answers:

    1. What conclusions are you entitled to draw from the results of this experiment, about the conducting pathways in the shoot?

There is evidence for a continuous pathway from the stem, through the branches and into the leaf petioles and midribs (and possibly into some of the smaller veins as well). This continuous pathway does not appear to include stomata or lenticels.

    2. How would a hot day affect the transport of water into the leaf if stomata remained open?

A warm or dry atmosphere outside the leaf would evaporate water more quickly, increasing the the water potential gradient and pulling water into the leaf more quickly.
   
 3. Does giving a plant extra water, watering the soil more frequently, assure that the plant will not wilt?

No. Plants wilt when the leaves cells lack turgor, due to lack of water, in the spongy mesophyll cells. Very dry conditions or hot sun directly on the leaf may cause low water vapor in the spongy mesophyll layer even though water is available to the roots.


Applications to Everyday Life: 

    1.Frequently mowed lawns use more water than low growing groundcovers because the cut surfaces of the leaf blade evaporate water directly from the leaf veins. There is no stomatal control of water loss.


    2. Like spongy mesophyll, evaporative coolers have a high surface area. The spaces within the cooling pads are cooled by the evaporation of water. 




    3. Green roof technology uses the cooling and water recycling effectiveness of plants to reduce the thermal sink efffect and water runoff of large urban roof areas.






Photographs: 

Videos: Include links to videos posted on the web that relate to your activity. These can be videos you have made or ones others have made.g

YouTube Video

Comments