Our Transpiration Lab last year was a disaster in my humble opinion. Our key problem in putting together our potometers was getting an airtight seal of the tubing around the stem of the leaves. This year we solved the problem of the airtight seal by using a leaf with thicker stems--Pothos. I had read on the AP Bio forum about teachers using special clips to help keep the seal airtight, but didn't have enough time to find and order them before the lab. We added a binder clip to the tube where the stem came in. I'm not sure if the clip actually helped, but the connection there did remain airtight throughout the experiment.
I did the control conditions and got decent readings, but measured for 42 minutes, which turned out well, since I forgot about the 10 minute wait period to allow it to equilibrate (trying to do the lab from memory). Unfortunately, I have allowed my students to also be sloppy about reading directions since I often will give them verbal instructions to help them get going. Next year, I am planning to make it a practice to give minimal directions and let them read and figure it out more independently. If I want them to be independent learners, I need to help them in that direction. The lab group testing bright light also had movement of water in their potometer, but not our fan or high humidity group. None of them waited the 10 minutes before measuring.
I wonder though if our Pothos plant doesn't have a high enough density of stomata to get decent readings from the potometer in 30 minutes (all groups measured about 15.5 stomata per mm^2). I may try a different thick-stemmed plant next year.
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| I gave them some data afterward so they would still have practice making and reading graphs. |
I know we could move to the whole plant method of measuring transpiration, but we only meet for three block periods a week, Tuesday through Thursday, which makes it a challenge to take readings everyday for a week.
The directions the students use is a version adapted from the College Board's AP Biology Investigation 11 Transpiration lab. It really isn't an inquiry lab the way we do it, but I like the thinking and math that is required in this lab.
Here's the Transpiration Lab version I use with students (minus the links to our google sheet).
I found syringes that fit into the tubing and add colored water to help read the pipet. I also have found that holly bush leaves are perfect. They are woody enough to "stick" into the tubing and make a tight seal although I still have the students add vaseline and parafilm wrap around the stem, tubing, and ring stand rod.
ReplyDeleteI may have to try holly and colored water. Our syringes fit into the tubing, fortunately. I am planning to try the whole plant method next year, since I’ll see students every day next year, but I do want to come back to the potometers since I like to be able to collect all of the data for this lab in one day.
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