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Objectives

Students should be able to:

  1. Perform an exercise concerning water transpiration in plants as it occurs under several different environmental stresses.

  2. Define the water properties of hydrogen bonding, cohesion, adhesion, water potential and osmotic pressure as it relates to water transpiration.

  3. Explain the physiology of the leaf stomata.

  4. Discuss the cohesion-tension model of water transport.

  5. Identify and describe the vascular tissues found in true plant roots, stems and leaves.

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Introduction

A plant's need for water is dependent on the condition and type of plant, environmental stresses and the availability of the water at any given time. Water is used by the plant for maintenance and growth, and it moves through the plant via vascular tissues. These tissues are conductive and water is literally "pulled" up the plant. This lab will concentrate on the processes of transpiration and water absorption as it occurs in some common local plants.

Large amounts of water move through a plant on a daily basis. The plant looses water by two basic processes. These processes are transpiration, which is the evaporation of water from a plant's surface (stomata); and guttation, which is the loss of water at the leaf margins (hydathodes). In vascular plants, water enters the xylem tissue of the roots through extensions of the root epidermal cells. These extensions are called root hairs. The movement of water into the xylem occurs by osmosis and this movement creates what is termed root pressure. Root pressure can only force water up a short distance. Since a plant can be tall, the root pressure is not great enough to force water up to the leaves of the plant, therefore, another mechanism must be in play also. This mechanism is defined by the cohesion-tension model of xylem transport. If the water lost by a plant through transpiration and guttation is not replaced then the plant will wilt and eventually die.

In this laboratory exercise you will microscopically examine the vascular tissues of plants and perform an experiment designed to measure the effect of various environmental conditions on the transpiration rate.

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Pre-Lab Questions (html version)        Pre-Lab Questions (Word version)

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Pre-lab Activities

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Pre-Lab Activity #1 - Set-up for Lab

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Download image of transpiration set-up

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Data Collection Sheet (Excel Spreadsheet - download and save)

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The Laboratory Activities and Data Collection

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Lab Activity #1 - Transpiration in Vascular Plants

Materials:

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Transpirometer, consisting of the following:

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1 mL pipette - cotton removed

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tubing

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2 split one-hole stoppers

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Ring stand

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2 clamps

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2 hook clamps
 

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Plant - provided by the instructor

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Pasteur pipette

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Petroleum jelly

Procedures:

  1. Attach the two clamps to the ring stand using the two hook clamps. Note these clamps are offset, the one for the pipette is lower than the one holding the plant sample.

  2. Place the 1 mL pipette, tip down into one end of the tubing.

  3. Place the pipette in one of the stoppers by opening the split and forcing the pipette into the space created by the hole.

  4. Place the other end of the tubing into the other stopper using the same procedure and the first stopper.

  5. Place the two stoppers with the pipette and tubing, into the two clamps and snuggly tighten the clamps.

  6. Fill the open end of the tubing with water until it rises up into the pipette.

  7. Insert the cut stem of the plant into the open end of the tubing and if needed seal it with petroleum jelly.

  8. Using the treatment type provided by your instructor, set up and expose the plant to those conditions. Click on the links below to view images of the setups.

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    Room conditions - plant is left at room conditions

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    Light - Set up a flood lamp 1 meter from the plant

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    Humidity - spray the leaves of the plant and cover it with a see-through plastic bag. Leave the bottom of the bag open.

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    Wind - Set a fan 1 m. from the plant, and turn it on low.

  9. Let the set-up equilibrate for 10 to 12 minutes

  10. At the end of the equilibration time, read the level of the water in the pipette and record it on your Data Collection Sheet. (time 0).

  11. Continue to read and record the water level in the pipette every 10 minutes for 30 minutes.

  12. Obtain data from the other lab groups.

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Lab Activity #2 - Graphing the Transpiration Data

Materials:

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Data Collection Sheet (Saved)

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Microsoft Office Excel 2003 or equivalent


Procedures:

  1. Obtain the transpiration data from the other lab groups

  2. Graph the data on the Data Collection Sheet using the Chart Wizard in Excel.

  3. Use this graph to answer some of the Post-Lab Questions.

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Post-lab Activities and Data Analysis

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Results and Analysis

  1. Fill in all data requested on the Data Collection Sheet and graph the data.
     

  2. Answer all Post-Lab Questions.
     

  3. Attach all materials to the Data Collection Sheet or Post-Lab Questions as requested.
     

  4. Return all materials to the area designated by the instructor.
     

  5. Clean up your lab station.

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Post-Lab Questions (html version)        Post-Lab Questions (Word version)

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