Tracy Joint Union High School
The question this activity addresses is what affects the rate of transpiration in plants. This is a very simple way to measure a rate of transpiration utilizing very simple materials easily obtained by all teachers. This activity lends itself to the analysis of variables and other factors that might slow or speed up the rate of transpiration.
Notes to Teacher:
When getting pipets, it is necessary to remove any plugs in the pipet. It is also necessary to enlarge the end of the pipet to be approximately the same size as the suction end of the pipet. The easiest method is to take a collection to the woodshop and sand them down. Be careful and wear safety glasses.
Required of Students:
The students can be directed as to how to construct the apparatus or given little direction and allowed to determine what they will modify to adjust for sources of error. The later enables students to develop more research skills and explore. The apparatus need to be away from large drafts and not disturbed for three days. Students should take measurements three times a day.
Class time needed:
Approximately three one-hour periods.
In this lab you will be using a potometer to measure the amount of transpiration taking place from a small branch with leaves attached. You will take measurements over a three day period and explore various factors that might influence the rate of transpiration.
Ring stands and clamps
10 ml disposable pipette
1 holed stopper
Small branch with leaves
Clear fingernail polish
Set up the potometer as shown below.
Pick a branch that will fit in the one holed stopper that you have.
Cut it and clear off the lower leaves so that it can be placed down into the stopper. Remember to seal the areas where leaves were removed.
Place the stem into the rubber stopper so there is a tight seal. Further seal the stem near the stopper by melting paraffin on the junction between stem and stopper (Do not get the heat near the stem).
Fill the pipet and tube with water so the is completely filled with water. NO AIR AT ALL! Make sure that the cotton plug is removed from the pipet if it has one and that the tip of the pipet has been removed.
Develop a way to test the different transpiration rates between different plants or what might cause different rates of transpiration amoung a sigle species of plants.
Take some clear finger nair polish and paint the lower surface of the leaves you used in your potometer.
Wait until the polish dries and peel it away from the lower surface.
Place this on a microscope and look at the number and distribution of the stomata. Draw what you observe.
Calculate an approximate number of stomata on each leaf. Compare the various different leaves that the class has chosen and predict where they might live ( sun or shade).
Calculate an estimation of the water loss per stomata over the three day period. What factors may lead to fluctuations in your data?
Write up all findings in your databook.
Methods of Evaluation:
Check for hypothesis.
Check data tables and graphs of water loss per day
Check calculations ( Significant Figs.)
Students should have explored sources of error.
Students should have observed other groups results and combined with theirs to explore factors that reduce water loss.
Students can obtain a transpirometer from a local plant physiologist and measure transpiration rates and compare them to the results they obtained. Some models are very portable and can be loaned easily.