TOPICS:  Plant maintenance, photosynthesis

LEVEL: High school and advanced placement. (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 min 24 hr before testing.

MATERIALS: Suitable evergreen species include:  Pine, Hemlock, Balsam Fir, Christmas Fern, Wood Fern, Boxwood, Rhododendron, Yew, Leatherleaf, Labrador tea, Goldthread, Pachysandra, Myrtle, Wintergreen, Holly. Use gooseneck lamp(s) and extension cord(s) to provide continuous illumination inside the refrigerator .

ADVANCE PREPARATION: Duplicate winter evergreen samples should be collected and identified in advance.  Failure to acclimate the leaves in the refrigerator for several hours prior to setting up the experiment may result in rapid wilting by some species .

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 reduced or absent in both plants at cold temperatures.  A logical case can be made either way for the winter evergreen sample at room temperature.  Since the leaves are green and room temperature is favorable to photosynthesis in summer evergreens, students may think photosynthesis will occur in the winter evergreen brought into the warmth of the classroom.  On the other hand, students may think that photosynthesis will be absent because (unlike the geranium) the evergreen sample has been cut from the plant and subjected to a large temperature increase during the experiment.  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. (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.) .

PROCEDURE: To save time and to preserve student motivation, you may wish to have two boiling water baths 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 5-9 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 and warm temperatures .

4. Temperature should have no effect on photosynthesis in winter evergreens.  The samples at both temperatures should show negative tests for starch .

5. No.  The winter evergreen samples in the refrigerator at 5^oC (conditions similar to winter thaw) and at room temperature showed no evidence of photosynthesis .

6. Stomata allow gases to diffuse into and out of the leaf.  Closed stomata 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 .

7. 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) .

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

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.