Transpiration Lab Take 3

Although I've been determined to be successful with the potometer in the AP Biology Transpiration Lab, teachers on the AP Biology teachers Facebook group have mentioned a different single leaf method. The set-up looked so easy, I had to give it a try.

Last night I stuck a leaf from the Golden Pothos we used in the transpiration lab this spring in the test tube of water, topped off with a few drops of vegetable oil.  After massing it, I stuck it in a sunny window.

Every hour or so, I'd mass it again. The mass did actually go down, so I decided to give the experiment a try with different environmental conditions.



I've read that results could be all over the place, so I was curious to see what happened.  I set up the leaves in a sunny room with a fan.

After my first read, the humidity leaf lost more mass than the sun one.  I wondered if the leaf in a bag in the sun was creating an extra warm "greenhouse" condition in the bag, and moved the bagged leaf out of the direct sun.

In the end, the humidity conditions leaf had a faster rate of transpiration than the leaf in the sun, but I wonder if part of it was my set-up.  Also, toward the end, we had friends over and were in the pool, so I wasn't paying as close attention to the leaves and their positions in the sun. =)

Before graphing the data, I calculated the surface area of the leaves and converted it from cm^2 to m^2. And since the volume of a gram of water is 1 mL, the graph could be in mL/m^2.  Here are the results.



I certainly think it's worth a try for this coming year for my class. I'll just need to pick a day that my students can come back in a lunch and the end of the day to get more reading, since this is a slow method. (But not as slow as the whole plant transpiration.) I think they can get enough reads during one school day.  It's easy enough that they could even redo the next day if their procedure needs revising.