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6.4: Transpiration and Cohesion -Tension Theory

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  • When water evaporates from plant tissues, it is called transpiration. Ninety percent of water that evaporates from terrestrial surfaces occurs via transpiration--plants are the world’s greatest water filters! Water is absorbed by (most) plants through specialized organs called roots. The earliest plants, the bryophytes, don’t have roots. Instead, these plants rely on the absorption of water across the entire plant body and dispersal of this water by osmosis. For this lab, we will focus on the later groups of plants--the tracheophytes--that have specialized tissues for water absorption and transportation throughout the plant.

    In Plant Cell Types and Tissues lab, you learned about cell types and tissues. The image above is a specialized cell called a tracheid. What tissue would you find this cell in? Is that tissue simple or complex?

    Figure \(\PageIndex{1}\): A tracheid

    Based on your knowledge of root words, what does the term tracheophytes mean?

    Early plants have tracheids, while later groups of plants have an additional type of water conducting cell: vessel elements.

    Figure \(\PageIndex{2}\): Vessel elements in corn root

    The image above is a cross section through the xylem of a corn root. You can see large open areas (vessel elements) surrounded by smaller, more densely packed cells (tracheids).

    How would these two cell types differ in the ability to take up and transport water?

    Testing the Relationship Between Tube Diameter and Water Movement

    1. Put some water in a shallow dish or petri plate, at least enough to coat the bottom. Add a drop of food coloring and mix thoroughly. You can also mix the dye into the water before adding it to the dish.
    2. Obtain glass tubes of different diameters (capillary tubes recommended). Measure and record the diameter of each tube in the table below. Note: The diameter is the longest distance across the opening of the tube.
    3. Place the bottom of one of the tubes into the water, leaving space between the bottom of the tube and the bottom of the dish so water can move into the tube.
    4. Mark the height of the water on the tube with a pen, remove it from the water, then measure the distance from the bottom of the tube to the line you drew.
    5. Repeat steps 3 and 4 for each tube and record your data in the table below.
    6. Next to the table, make a graph that shows your results.

    Diameter of Tube (mm)

    Maximum height of water (mm)

    Is there any correlation between tube diameter and the height that the water traveled up the tube? If so, explain the relationship.

    Vessel elements are large-diameter conducting cells in the xylem, while tracheids have a much smaller diameter. How would this influence capillary action and adhesion? Use examples from the tube experiment to help explain your answer.

    Transpiration in Action

    Water moves through the dead water-conducting cells in the xylem much like it moves through a tube. Transpiration acts like suction from the top of the tube, but as you saw in the previous experiment, other forces aid in the movement of the water: cohesion, adhesion, tension, and capillary action. All of these forces work to pull water into the plant through the root hairs, into the xylem, and out through the stomata.

    You set up four plants at the start of lab. What were the conditions for each plant?

    Plant 1 -

    Plant 2 -

    Plant 3 -

    Plant 4 -

    Check on the plants and, before doing anything, simply observe the appearance of the bags. Describe your observations below.

    Turn each plant on its side and carefully remove the bags. Try not to let any condensation in the bag escape. Use a scale to obtain the mass of each bag. Note: if you used different types of bags, adjust your end mass measurements by subtracting the initial mass.

    Mass of bag (adjusted)





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