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Crops Investigation #6

Goal:

Students will understand the xylem system and the mechanism of transpiration. 

Objectives:

Students will be able to:

Standards Met:

Overview:

There are three portions within this series of activities. First, students examine the location of xylem in roots, stems, and leaves through dissection and staining. Next, the students discuss the mechanism of transpiration. Extensions can be used to provide activities. Finally, mineral transport is discussed. 

Materials:

Resources:

Actions:

Prior to the lesson:

  1. The class has already talked about the major plant organs (roots, stems, leaves).

During the lesson:

  1. Use a series of questions to move the students to an understanding of the three main topics for today.
  2. "What are the functions of roots?"  As you have this discussion, two functions that should be noted are to provide water and minerals for the plant.
  3. "What do plants need the water for?"  As you have this discussion, three functions of water should be noted. One is for plant growth. Water is needed to keep the cells turgid and to increase of cell (and therefore, plant) size. Second is for photosynthesis. Water is one of the reactants for photosynthesis because it provides electrons to the photosystem chlorophylls that lost electrons when they absorbed light energy.  Third, water is needed for transporting minerals and other dissolved materials through the xylem by means of transpiration.
  4. "How do you get water from the roots to the leaves?"  This discussion should include an explanation of the physical xylem transport system. The xylem system is a continuous series of tubes connecting all organs of the plant. The xylem is located in different places in different plant organs. In many roots, the xylem is located in the vascular cylinder in the middle of the root. Give each student a carrot disk. Carefully bite into the carrot so that the outer ring of tissue is broken away from the inner circle. First, have the students taste the outer ring. "How does it taste?"  Now, taste the inner circle. "How does it taste?" "Why do you think that these two parts taste different?"  Discuss the names of these two regions. Make a drawing of the root cross section in your science notebook. Label the names of these sections. Record your data. Explain why the two regions taste different.
  5. In stems, the xylem is located in vascular bundles that are scattered throughout the stem. Sometimes you can see the vascular bundles if you cut a stem in cross section. Since they are hard to see, you can use a dye to selectively color only the xylem cells. If you put a dye (food coloring) in water and allow the plant stem to transport the water, then only the xylem cells will be stained.
    1. Students should place a small amount of food coloring solution (1cm is plenty) in a test tube.
    2. Make a fresh cut across the end of the plant stem using a sharp razor blade. Immediately, place the stem in the test tube.
    3. Allow time for transpiration to pull the water up the stem, then cut the stem in cross section to see the xylem cells among the other cells. (note: The time necessary will depend on the plant used. You can leave the stem in the test tube overnight and make the observations in the morning. If you leave it overnight, the flower petals will be colored.) Use dissecting microscopes, if they are available, for this examination.
    4. Students should make a drawing in their science notebook of the stem cross section and label the location of the xylem.
  6. In leaves, the xylem is located in the vascular bundles of the veins. The students should examine the size and variation in veins of the leaves provided. Celery stalks are modified petioles (part of the leaf). You can place celery stalks in food coloring, as you did for the stems. The veins in the celery are so prominent that you can peel the vascular bundles out of the stalk. Xylem movement in celery is very fast, so you will need only about 20 minutes for the food coloring to go about 6 inches. Students should make drawings of the leaves observed and the location of veins/xylem.
  7. After the students have an understanding of the physical xylem transport system, discuss the mechanism of xylem transport: transpiration. "Now that we understand the transport system that the water moves through, what makes the water go from the roots to the leaves? What is the mechanism for the movement?"  There are three key things to point out. The first is that transpiration is based on concentration gradients of water. Water is always moving to where there is less water. The second is that water is a special molecule because, since it is polar, it experiences adhesion and cohesion. Thus, it sticks to itself and it sticks to other molecules with great force. The third is that water moves based on pressure. When water evaporates from the leaf, this pulls water through the leaf, which pulls water up through the stem, which pulls water up from the roots, which pulls water into the plant from the soil.
  8. Use the online resource to review transpiration (transport system and mechanism).
  9. "What about minerals? How do they get from the soil into the plant? How do they get moving around inside of the plant?"  There are three important points of discussion. The first is that minerals must be dissolved in the soil water to be taken up by the roots. The second is that minerals are selectively taken up by the root. For a mineral to be get inside of the root, there must be a transporter for that mineral in the plant cell. Finally, minerals give in the xylem just like water does (because they are dissolved in water).

Extensions:

  1. The rate of transpiration can easily be determined during either (or both) of the food coloring experiments. To do this, you measure the distance that the dye traveled in the time period that was provided. (warning: This experiment requires that the students remove the stem (or petiole) before the dye reaches that top of the plant segment.) Starting from the end that went into the dye, have the students cut the stem (or petiole) into 1 cm long sections until the dye can no longer be seen. The number of sections is the distance traveled. Divide the distance traveled by the period of time that the organ was in the dye.
  2. Students can design experiments to examine factors that affect the rate of transpiration. Such factors might include: light, temperature, wind, length of the stem, diameter of the xylem, or humidity.

Assessment:

During the exploration, the students make and label drawings. They also record observations. If an extension is done, there are calculations for Extension 1 and an experimental design and/or lab report for Extension 2.

Reflection:

Please write a reflection after you teach this lesson to help us make improvements. Reflections should be e-mailed to j.okeefe@moreheadstate.edu and c.wymer@moreheadstate.edu. If you also complete student reflections, please include these as well.