MATERIALS: At least two species of conifers should be used.  Use freshly cut branches larger than you need (see step A).  Place the cut ends in water and store in a refrigerator for several hours to acclimate them. 

ADVANCE PREPARATION:  Potometers (ring stand, clamps, rubber stoppers, tubing, and pipette) should be assembled in advance.  In class, students insert the twigs and fill the pipette and tubing under water to insure that no air bubbles enter the system.

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:  Major differences between summer and winter transpiration include: (a) the cold environmental temperatures in winter which reduce the overall rate of molecular diffusion and (b) the low metabolic use of water by conifers in winter.  (Photosynthesis probably does not occur in winter conifers.)  Wind should increase the diffusion gradient by removing water molecules faster, but decrease the temperature due to evaporative cooling.  Light should increase the temperature.  The following data were obtained using a conifer in winter.

STEP A: Twigs must be cut off branches under water, matching the diameters to fit snugly into the clear plastic tubing .

STEP B: The graduated pipette can be used to measure water loss in thousandths of a millilIter.  However, it is easier for students to attach a ruler and measure the decrease in water level in millimeters along the length of the pipette and express the rate as mm/2 min .

STEPS C-D: If students use the lamp before the fan, they must wait several minutes for the plant to cool down before continuing.

QUESTIONS 6-12 require higher order thinking skills.  We recommend that you discuss these questions in class before students write out their answer .

2. Conifers show a much lower rate of transpiration in winter than in summer conifers .

3. The twig exposed to the fan may show a slight increase in transpiration rate, or it may actually show a decrease (see question 8) .

4. Exposure to the lamp should increase the rate of cuticular transpiration (see question 9) .

5. The lamp in both cases results in increased transpiration rates.  The fan increased the transpiration rate in the summer conifer, but may cause an increase or decrease in the winter conifer .

6. All species of conifers should have very low transpiration rates, may differ according to the total surface area of the leaves .

7. The stomates of summer conifers are open during daylight hours so that more water is lost by transpiration.  In winter, the stomates and the only water loss may be by cuticular transpiration .

8. Transpiration rates may show an increase or decrease.  The fan may increase the transpiration rate because the moisture-laden air near the plant is removed creating a larger diffusion gradient. However there might be a decrease in the transpiration rate due to the equalization of the temperature of the leaf and the air, with a reduction in the diffusion gradient .

9. The 200 watt lamp increases the cuticular transpiration rate by increasing the diffusion gradient as a result of increased leaf temperature .

10. The warming effect of the lamp would be counteracted by the cooling effect of the fan; the transpiration rate is expected to show a smaller increase than when the lamp alone is used .

11. A deciduous twig used in the potometer would probably show no measurable water loss since it is leafless, thus there is no stomatal transpiration.  Water loss through the lenticels or stem cuticle would be measurable only over long periods of time .

12. Deciduous trees become dormant and shed their leaves in the autumn which reduces transpiration rates during the winter.  The waxy evergreen leaves of dormant winter conifers lose very little water by transpiration.  The thick cuticle retards water loss and the stomates are closed.  Retaining their leaves is not a disadvantage.  The survival advantages may include (1) longer growing season since conifers could carry on photosynthesis longer in the autumn and/or earlier in the spring, and (2) mineral and/or energy conservation since conifers do not have to produce a whole new set of leaves in the spring to begin photosynthesis.