Students often believe that only animals “breathe”, but all things exchange gases with their environment. It’s just that the process is not so obvious in plants. Elodea is a very common water plant that can be found in aquarium stores. As photosynthesis occurs, oxygen is produced as a by-product. Elodea releases bubbles of oxygen as it photosynthesizes. In fact, the number or volume of bubbles in a certain amount of time can be used as a rough measure of photosynthetic rate.
The measure is rough because oxygen dissolves in water so may not always appear as a gas. Moreover, most light sources produce heat, which causes the water temperature to increase, which in turn causes lower gas solubility, and thus may produce bubbles just by turning the light on. Finally, the size of a bubble is not constant thus counting the number of bubbles per unit time is only a very rough measure. The total volume of gas produced is a better measure but again falls victim to the other caveats. The best measure is to use a dissolved oxygen meter which unfortunately costs a fair bit of money (between $400 to over $1000). You can also try disposable dissolved oxygen tests (see the Sources section of the Water Analysis activity).
An important note is that freshly cut Elodea stems produce more bubbles than the leaves. That is because Elodea stems contain large intracellular air passageways. As oxygen is produced, the plant transports the oxygen away from the leaves towards the roots. Thus, a freshly cut stem will produce oxygen bubbles at an observable rate. The cells in the leaves are much more tightly packed together and provide greater resistance to the emerging oxygen gas than the stems. Thus, to maximize oxygen gas production in this experiment, cut the stem of the Elodea then place the plant upside-down in the test tube. For more information about why the bubbles emerge from the stems, not the leaves, see this article by David Hershey of the Mad Scientist Network.
An excellent resource with more information about the use of Elodea in this experiment can be found at the Clifton College Science School website. There you will find detailed information about Elodea, how oxygen is produced, experiments by Frost Blackman, practical advice and more.