TEACHER GUIDE

TOPICS:  Plant physiology, plant ecology

TIME:  One double class period

LEVEL:  High school and advanced placement

ADVANCE PREPARATION:  The woody branches must be put in the refrigerator for 24 hours prior to testing to allow them to thaw slowly, before exposing them to room temperature .

MATERIALS: Dry ice usually can be obtained locally from companies requiring cooling such as milk storage plants.  For best results, digital thermocouple thermometers should be sensitive to -60oC, calibrated in 0.1oC intervals, with #30 bare wire junction probes.  The switchbox allows the use of one thermocouple per sample .

SAFETY NOTE: Warn students that dry ice should not be touched since it can cause tissue damage by frost bite.  Keep it in an insulated container and place the flasks filled with ethanol on top or between two pieces of dry ice .

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: Students may think that intercellular water freezes at or near 0oC and that intracellular ice forms at somewhat lower temperatures.  For many northern species of trees and shrubs the first exotherm occurs at about -10oC, and is usually obtained after about 5-10 minutes of cooling.  The second exotherm occurs at about -35oC and may not be obtainable using the dry ice/ethanol bath because of the time involved and difficulty in achieving such low temperatures.  For some species the second exotherm may occur at temperatures so low (-60oC or lower) that it cannot be achieved using dry ice, but requires a liquid nitrogen system.

QUESTIONS 5-9 require higher order thinking skills.  Question 5 will require library work. We recommend that you discuss these questions in class before students write out their answers .

2-4. Answers will vary .

5. It is logical to expect that plants with the northernmost ranges will have the greatest freezing resistance.  Suitable reference books include To Know the Trees, Yearbook Separate No. 2156, Yearbook of Agriculture, 1949, or Golden Guide Book:  How to Know the Trees.  Many other references are available .

6. The thermocouple probe must be inserted into the vascular cambium because this is the living tissue of the woody plant.  The xylem below is dead tissue .

7. Solute concentrations of such chemicals as sugars and glycols act as biological antifreeze, depressing tissue freezing temperature .

8. Answers will vary depending on geographic location and local weather conditions .

9. Other kinds of organisms that are found outdoors in winter may be tested for freezing resistance, including soil invertebrates, aquatic insects, gall fly larva, etc.