TEACHER GUIDE

TOPICS:  Plant structure, plant growth, ecology

LEVEL:  High school

TIME:  10-15 minutes on 6 midwinter days over several weeks for data collection.  One class period for graph construction and discussion.

ADVANCE PREPARATION:  Dendrometers are NOT commercially available, and must be constructed out of 1 m pieces of steel tape (Roover's Tapewriter Tape #2883 available for about $13 per roll as Stock #83036 from Forestry Suppliers, Inc., P.O. Box 8397, Jackson, MS 39204-0397, 800-647-5368). Drill a hole in one end of the steel tape to attach a small spring. The spring should be stiff enough so that it can be pulled apart by hand with some difficulty.  Cut a shallow V-shaped notch near the spring and drill a series of holes in the other end.  Scratch a reference mark on the notch with ball point pen

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PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not suggested by students: Based on the information in the introduction, students should predict that sap freezes below 0oC.  The exact temperature is not important, but usually is from -5oC to -10oC (23oF to 14oF).   Whether or not your winter temperatures are low enough to cause freezing of the sap depends on your geographic location (latitude and altitude) .

PROCEDURE: This is a good time to introduce or review a lesson on tree identification.  (See "Identification of Trees by Their Winter Twigs" Activity.)

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STEP A: You may select trees on school grounds or assign students to put dendrometers on trees near their homes.  At least two trees are needed, one evergreen and one deciduous tree, in addition to the telephone pole control.  Four to six trees are recommended so that reliable generalizations can be made .

STEP E: You may wish to keep track of TV or newspaper weather forecasts to choose suitable days for data collection.

QUESTIONS 6-9 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answers .

2. The wooden telephone pole should show little or no increase or decrease in circumference, whereas the trees will clearly show changes in trunk size if temperatures go below about -10oC .

3. Different species will show differing amounts of expansion and contraction with air temperature.  Differences might be noticeable between evergreen and deciduous trees (softwoods and hardwoods) .

4. No, sap does not freeze at 0oC.  The graph does not show an increase in tree circumference at or near 0oC .

5. Answers will vary according to geographic location and severity of the winter weather.  When the sap freezes there is a marked increase in the circumference of the tree as ice crystals form.  For many northern species sap freezes around -10oC.  However, species vary in their resistance to freezing .

6. The telephone pole served as a control for comparison with the trees.  Although made of wood, it does not contain sap and therefore should show little or no difference in circumference with changing air temperatures.  Living trees contain sap which expands and contracts as it freezes and thaws .

7. Use of the dendrometers offers clear and direct evidence that trees resist freezing.  The solute concentration of the sap accounts for freezing resistance.  (See "Freezing Resistance in Plant Tissues Activity".) .

8. Tree sap freezes and thaws, yet the tree resumes growth in the spring apparently without damage.  Intercellular ice formation usually does not cause tissue death .

9. Winter kill in trees may occur during a severe winter, especially in cultivated plants, as evidenced by dead trees or dead branches in the spring.  Tissue death results from the formation of intracellular ice formation .

GOING FURTHER:  For most of the Northeast, trees break dormancy and begin spring growth sometime during April, depending on latitude and altitude.  Weekly data can be collected and graphed.