Computer Interfacing

Teacher Information: Transpiration

Tips and Techniques

1. The Biology Gas Pressure Sensor is not waterproof! If ANY water enters the sensors, they will not function.

2. To help ensure that no air bubbles form in the tube at the cut end of the stem, allow water to sit at room temperature overnight in a beaker to allow any excess dissolved gases to escape.

3. If air bubbles do form it may be necessary to stop the experiment. To solve the problem, remove the plant and tubing from the two utility clamps and allow the plant to hang towards the ground with the other end of the tubing up. Carefully tap on the sides of the tubing to shake loose any bubbles so they will float to the water's surface at the other end. Once all bubbles are removed check the plants seal at the tube and fix any leaks by blotting the water with a paper towel and applying more petroleum jelly. Once this is done secure your plant back in the tubing and restart the data collection.

4. The plants must have a sufficiently long stem so that they can be inserted into the plastic tubing. The plant cuttings should have approximately the same number and size of leaves for each test.

5. Students need to work in cooperative groups and share their data. If you choose to have each group complete all four sections of the lab, have students rotate from one lab station to another.

6. A good source of plants are weeds, such as honeysuckle, which might be available close to the school. These yield excellent results. Alternatively, germinate bush bean seedlings for two to three weeks prior to the lab. Students can plant them in small pots and observe the shoot development, see the cotyledons, hypocotyl hook, and other aspects of plant development.

7. A slanted cut on the stem makes it easier to insert into the tubing and gives more surface area for water entry.

8. Since plant stems vary in size, tubing attached to the barometric sensors may not be the same gauge as the tubing required for the plant. Tubing of several different diameters may be needed. Air-line tubing comes in a variety of sizes and is available at aquarium supply stores. If you use this, each group will need two lengths of tubing Ð one that attaches to the Biology Gas Pressure Sensor and one that connects to the plant stem. Penn Plax sells a package of six universal size Air-Line connectors which will join tubing of different sizes. Often, tubing is available on reels in plumbing and hardware stores and can be purchased by the foot.

9. Students may incorrectly assume the curves they generate can be compared directly to the control or the other variables. It is good to discuss this with students and have them realize that the leaf surface area is an important variable that affects plant transpiration.

10. Another method of determining the surface area of all the leaves on each of your plant cuttings is to:

• Trace all your leaves on cm² lined graph paper.
• Count the number of square centimeters within the tracings and enter that as the total leaf surface area in Table 1.

11. The actual amount of water taken up may be very small and rate values may need to be expressed in scientific notation. Alternatively, you may want to have students choose a convenient set of units in which to express their answers. Surface area might be in m² or cm², and time might be in seconds, minutes, hours, or days.

12. The Barometer Sensor may be used in place of the Biology Gas Pressure Sensor. Although the resolution is equal, the Biology Gas Pressure Sensor's maximum pressure value (1.5 atm) is greater than the Barometer Sensor's maximum value.

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