Amazingly enough, plants actually breath. They breath very different in comparison to us but yet they still breath. Plant 'breathing' takes place through tiny microscopic pores located on the surface of their leaves called stomata. Through these pores water and oxygen are released but carbon dioxide is absorbed. This process of breathing is called transpiration and actually pulls and moves water through a plant from the roots all the way to the leaves. We have developed a very simple way to measure plant transpiration using a tool called a potometer. Actually, I am sure someone else developed it before we did but we came up with it independently... but anyway.
Next you will need the plant cutting that you are going to test on the potometer. Before you place the stem in the tubing you must first make a continuous column of water that goes all the way through the tubing and ends somewhere in the pipette. In order to do this you must submerge your tubing and part of your pipette under water. Make sure there are no air bubbles in the tubing and pipette. Then, by keeping the end of the tubing underwater insert the stem of your test plant into it. The stem cutting must be as fresh as possible! Make sure that only the stem portion of the plant cutting goes underwater. If the leaf portions of the cutting gets wet the plant will not transpire normally. When the plant stem is inserted into the tubing make sure it has a tight fit so nothing can escape or enter, and make sure there is no air bubble between the stem and the water. After all this is done you should have a column of water, with no air bubbles, that touches the stem and ends somewhere in the pipette. Tape this up somewhere like in the photo above and mark the starting point for the water in the pipette (the starting point will be where the water ends in the pipette).
After your potometer is set-up wait an hour or two and see how far down the water has moved the pipette to determine how much water has been transpired. The same volume of water that has been removed from the pipette has been absorbed and transpired out of the plant as it 'breathed'. This basic set-up can be used to make a variety of experiments such as comparing how much a plant transpired in the light vs. dark, different temperatures, comparing different species, and comparing different numbers of leaves. If you are doing a temperature of light comparison it is best to use the same plant species and maybe the same plant cutting. If you don't use the same cutting try to use the same number of leaves. Or if you are comparing different species try to make leaf area as equal as possible for each cutting as much as possible. A whole variety of tests can be used to demonstrate plant water use with this simple device.