TOPICS:  Plant maintenance, photosynthesis

LEVEL:  High school. Because boiling alcohol is used, this activity is not recommended for middle school students, except as a teacher demonstration.

TIME:  One class period for procedure.  Advance preparation will take about 20 minutes two days before testing.

MATERIALS: Winter evergreen samples should be located and identified in advance.  Suitable evergreen species include:  Pine, Hemlock, Balsam Fir, Christmas Fern, Wood Fern, Boxwood, Rhododendron, Yew, Leatherleaf, Labrador tea, Goldthread, Pachysandra, Myrtle, Wintergreen, Holly .

PREDICTIONS: 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 correctly predict that light is necessary (but not sufficient) for a positive starch test in both kinds of plants at any temperature.  Students may also use their knowledge of how temperature affects enzyme action to conclude that photosynthesis is probably low or absent in evergreens at low temperatures.

NOTE: Available information indicates that winter evergreen leaves do not carry on photosynthesis.  Winter temperatures are unfavorable for enzyme activity. Available evidence also suggests that the stomata are closed. The survival advantage of evergreen leaves may lie in being able to photosynthesize longer in the fall and in having a photosynthetic mechanism immediately available when conditions again become favorable in the spring.  Evergreen leaves may be a mineral conservation strategy, as evidenced by their characteristic presence in many bog plants adapted for growth in a mineral-deficient substrate.  The structure of evergreen leaves may also play a role in water conservation resulting in reduced transpiration.  (Note that the preceding information applies to tracheophytes; other groups such as mosses are known to photosynthesize in winter.) See Plant Water and Energy Relations Teacher Background for more information. Thus the results are likely to be: no no yes no .

PROCEDURE: To save time and to preserve student motivation, you may wish to have a boiling water bath set up at the beginning of class to start boiling the evergreen samples as a teacher demonstration. Boiling the geranium leaves in step G for 5 min is sufficient to remove most of the color.

SAFETY NOTE: Never boil alcohol in a beaker directly on a hot plate or over an open flame.  Make sure the room is well ventilated with the windows open.  Flooding an alcohol fire with water will safely extinguish the flame.

QUESTIONS 5-12 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answers .

2. Answers will vary .

3. Photosynthesis occurs when leaves are exposed to light .

4. The winter evergreen samples should show no evidence of photosynthesis .

5.The experimental variable was light in both experiments.  Temperature was not a variable within either of the two experiments (see answer to question 8) .

6. The unmasked leaves exposed to natural conditions were the control group.  The masked leaves, where light exposure was manipulated, were the experimental group.  You must compare at least two groups exposed to different light conditions to determine if the experimental variable had an effect on photosynthesis .

7. The two experiments differ not only in temperature, but also in plant species tested. Any difference in response between the geranium and evergreen leaves cannot be attributed to temperature alone .

8. A logical case can be made either way for winter evergreen leaves exposed to warmer weather.  Since the leaves are green and the temperature is favorable to photosynthesis, students may think photosynthesis will occur in the winter evergreen. In fact, the waxy evergreen leaves are probably incapable of photosynthesis even under the most favorable experimental conditions due to changes in the leaf that accompany winter dormancy. Student answers will vary on how to test these ideas, but should include control and experimental groups .

9. A plant with evergreen leaves does not have to produce a whole new set of leaves in the spring to begin photosynthesizing.  This may be a survival advantage in terms of energy and minerals, and may allow photosynthesis to occur as soon as conditions become favorable.  Evergreen leaves may also photosynthesize longer in the autumn and begin earlier in spring than do those of deciduous plants .

10. Stomates allow gases to diffuse into and out of the leaf.  Plugged stomates mean that the leaf cannot absorb CO₂ needed for photo-synthesis.  However, the plant is able to conserve water since less is lost by transpiration .

11. Vascular tissue, xylem and phloem, trans-ports fluids vertically throughout the plant.  Students may logically conclude that if the fluids are frozen, water will not be available for photosynthesis in the leaves.  However, recent evidence suggests that water for photosynthesis may travel through cell wall micropores (see Teacher Background) .

12. Low temperatures slows enzyme action which decreases the rate of photosynthesis.